Sanders Magtech 500w stereo power amp

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Sanders Sound Systems

The Electrostatic Specialists -Hand Crafted In Colorardo

On Behalf

We have 1 left (1 sold) of these awesome Sanders 500w stereo power amps available being sold-on-behalf. Had little use and in perfect condition, guaranteed to get any speaker dancing to it's maximum enjoyment factor.

The popularity and success of the original Sanders ESL - Electrostatic Amplifiers for driving electrostatic speakers led to many requests for a companion amplifier that was specifically designed for driving more traditional, conventional speakers. Two years in development, the Magtech amplifier is that amplifier, it will drive even the most demanding speakers with ease. 

The Magtech Stereo Power amplifier will deliver over 500 watts/ch into an 8 Ohm load, and over 900 watts/ch into a 4 Ohm load.  Momentary output into a 2 Ohm load exceeds 1200 watts/ch. The amplifier is completely stable and will not be damaged even when driving a 1 ohm load.

DESIGN PHILOSOPHY - Electronics have their lowest distortion and optimum performance at a specific design voltage.  If the voltage varies, the amplifier's performance will suffer.

An additional problem in amplifiers is that they require bias to eliminate crossover notch distortion and determine their class of operation.  The bias will vary as the voltage does, which will further reduce performance.

An amplifier's voltage will fluctuate wildly as dynamic music is played.  This causes the amplifier's distortion and bias to vary constantly and fail to meet its full performance potential.

As if all these problems are not enough, as an amplifier's voltage sags under load, the power it can deliver is greatly reduced.  If the voltage would remain stable, the amplifier could produce much more power.  Since most audiophile speaker systems require several hundred watts of power to avoid clipping and compression of the dynamic range, power is extremely important.


All quality, line-level electronics use voltage regulation in their power supplies to produce a stable voltage, regardless of load or the mains voltage.  Audiophiles would not consider using a source component that did not have regulated power supplies.  So why use amplifiers with unregulated supplies?

The main problem is heat.  Amplifiers operate at much higher voltages and currents than line level source components.  These higher voltages and currents forces conventional regulator designs to waste large amounts of energy, which wastes expensive electricity and causes the amplifier to get very hot.

Also, many regulator designs radiate RF (Radio Frequency) energy when switching high currents and voltages.  This RF gets into the amplifier's electronics and can cause instability, oscillation, and noise.  As a result of these problems, modern power amplifiers do not use regulated power supplies and fail to take advantage of the benefits available from doing so.

Sanders has solved these problems by developing a voltage regulator that is essentially 100% efficient.  There is no heat dissipated by the regulator system.  There is no high-power/high-voltage switching that causes heat generation or RF problems.

The regulator in the Magtech amplifier maintains a stable voltage regardless of load or reasonable changes in the line voltage feeding the amplifier.  It runs stone cold, produces zero RF energy, and is simple and reliable.

Unlike other amplifiers, the distortion in the Magtech amplifier is virtually unchanged regardless of power level.  The bias is stable regardless of load.

The regulator makes it possible to obtain a 50% increase in power over the same amplifier operated unregulated.  In its stereo form, the Magtech will deliver 500 watts/channel into an 8 ohm load and 900 watts/channel into a 4 ohm load.

The Magtech is built into the same chassis as the ESL Amp, so it is compact enough (17" wide, 5-1/2" tall, 14" deep) to place on a shelf or into a cabinet.  It is also light enough (54 pounds) to be picked up.  Like the ESL amp, it runs very cool and may be left on continually without concern for power usage. 

The Magtech amplifier also uses the same advanced technology that makes the ESL amp able to drive the most difficult loads without performance-degrading, protective circuitry.  The Magtech amplifier is the only amplifier on today's market that features a linear voltage regulator.  The result is a compact, yet extremely powerful amplifier that is ideally suited to driving the most difficult magnetic speakers.






Magtech - the only amplifier with a linear, voltage regulator (Patent Pending) 
Over 500 watts/channel into an 8 Ohm load, 
Over 900 watts/channel into a 4 Ohm load.  
Momentary output into a 2 Ohm load exceeds 1200 watts. 
The amplifier is completely stable and will not be damaged even when driving a 1 ohm load.


500 watts RMS per channel into an 8 ohm load
900 watts RMS per channel into a 4ohm load (note - will drive down to below 1 ohm)
Bandwidth:     DC through 100kHz
Class of Operation:     Class AB
Slew Rate:     500 Volts/microsecond
Input voltage required for full output:     2 Volts
Input Impedance:     50kohm (both balanced and unbalanced)
Gain:     26dB
Noise:     More than 110dB below rated output
Damping Factor:     Greater than 600 into an 8 ohm load
THD:     Less than 0.004%, 20 Hz - 20 KHz
IMD:     Less than 0.003%, 20 Hz - 20 KHz
Voltage:     Voltage is user selectable for use world-wide.
Dimensions:     430W x 140H x 406D mm
Weight:     25 Kgs


A Hidden Gem - with the Magtech, the MRTs were rock solid at uncomfortably loud levels, visual confirmation of what I was hearing-"No Distortion".
Don Shaulis

Pure Enjoyment on Quad ESL-2805s: -  its brute strength the Magtech showed its sensitive side by gently reproducing each layer as the whole it was meant to be and capturing the surprisingly long, barely audible fadeout that would be lost on most systems. It also faithfully captured Joni Mitchell’s voice, mostly clear as a bell but sometimes exhibiting a slight tobacco-induced haze. 

The Magtech did an excellent job of revealing the nuances in Emmylou Harris’s voice on “Can You Hear Me Now” from Stumble Into Grace [Nonesuch 79805-2]. Again there were layers to be revealed including backup singing and soft guitars. And again the Magtech seemed particularly adept at revealing layers without losing the connectivity between them. Other amplifiers might etch each layer independently or smear them irretrievably. The Magtech made me enjoy this and other music to a level I had not previously experienced. I heard more, but not because I was trying. I heard more because it was presented so effortlessly and without drama. The seduction I previously alluded to. 

For some reason there remain relatively undiscovered gems in the audio world. The Sanders Sound Systems Magtech amplifier is one of them.

Certainly their designer, Roger Sanders, has been around the block several times and mention of the brand name often brings recognition but not familiarity. Someone may have read about Sanders Sound Systems loudspeakers and amplifiers in a show report or magazine but not actually heard them. 

I found the Sanders Magtech amplifier somewhat by accident myself. I was searching for an amplifier for my Quad ESL-2805 loudspeakers and I found the Sanders ESL Mark II amplifier designed specifically to handle the more complex load of electrostatic loudspeakers (ESLs). The Sanders Sound Systems website explains that ESLs are driven by voltage in contrast to magnetic speakers that are driven by current. All this occurs because a magnetic speaker is a resistive and inductive load while ESLs are a capacitive load. Since most amplifiers are designed to drive the more common magnetic speakers, Roger Sanders recognized the need for an amplifier more appropriate for driving both his own and other brands of ESLs.
Hang in there; I will get to the Magtech amplifier shortly. A little more information on the ESL Mark II amplifier is warranted here since it is the foundation on which the Magtech amplifier is designed and built. While they are designed with different loudspeaker demands in mind, neither amplifier is limited in its final application. Roger Sanders has assured me the ESL Mark II amplifier can perform very well on magnetic speakers as well as on ESLs. He further advises that the Magtech amplifier is the best amplifier for either application but the edge over the ESL Mark II amplifier is not as significant on electrostatic loudspeakers. 
To achieve optimum performance Roger has eliminated protective circuitry that causes the harshness frequently associated with solid-state amplifiers. One key to eliminating protective circuitry is the design of an output stage that is so powerful it is never stressed. The ESL Mark II uses twenty large output transistors capable of delivering 7,000 watts (64 volts RMS) into an ESL. The amplifier can drive a 1/3-ohm load. 
But there is more to eliminating protective circuitry than just “Tim the Tool Man Taylor” amounts of power. An amplifier must also be designed to prevent thermal runaway and maintain a stable bias at the appropriate level regardless of power-level demands or constantly varying load demands from loudspeakers at different frequencies. Both the Mark II ESL and Magtech amplifiers use Thermal Trak transistors. Thermal Trak transistors have built in temperature sensors to adjust bias much more quickly than conventional amplifiers. Most conventional solid-state amplifiers have temperature sensors mounted on the heat sinks, which results in inaccurate readings due to the time delay. The result is an unstable and inaccurate bias and the possibility of overheating, which ultimately demands protective circuitry. 
The above features are also incorporated into the Magtech amplifier, which is essentially a Mark II ESL amplifier with the addition of a linear voltage regulator. Line-level electronics typically use voltage regulated power supplies but regulated power supplies would run too hot for the larger power requirements of amplifiers. To solve this problem Roger Sanders has developed (patent pending) a voltage regulator that he claims is essentially 100% efficient. In addition, the regulator does not employ switching that could create radio frequency problems. Roger further claims the regulator maintains a stable voltage regardless of load or reasonable changes in the line voltage. This enables the maintenance of stable bias and unchanged distortion levels. 
After living with the Magtech amplifier for a couple months I spent some more serious time rereading the information on the Sanders Sound System website. A well-written white paper can always sound convincing to me. But what resonated with me as I explored the technical design of the Magtech was how much it confirmed my subjective observations. 
At first blush Magtech amplifiers are not that exciting. Seduction is more their style. One reason the Magtech amplifier does not call attention to itself is that it does not excel in (or exaggerate) any specific area. But rather, it performs equally well at all frequencies and never shows strain. Consistency was what I had written in my listening notes. Performance was uniformly good from top to bottom. The figurative light bulb was burning brightly above my head. Finally, something I read in a white paper could be confirmed by my actual experience.
It is easy to get Quad speakers to sound good. They are legendary for coherence and an engaging midrange. It is more difficult to get them to sound their best throughout their frequency range. I have tried various tube amplifiers on the Quads and found tube amplification provided extremely enjoyable sound but only within certain frequency range limits. The Quads are not as easy a load as one would be lead to believe by how good they sound with tube amplification. The Magtech further revealed the tube shortcomings (which I had already noticed) by expanding the peak performance range of the Quads. The “sweet region” grew to the limits of the speakers themselves. Meaning they suddenly didn’t have extended bass but what they did have was tighter and more refined. The top end was not more extended, just smoother and less brittle. The Magtech amplifier did not create new loudspeakers; it just let them be all that they could be. 
The amplifier also exhibited a rich tonal density with good soundstage depth and imaging. Performers and instruments were not exaggerated in size. Extended harmonic reproduction made piano and stringed instruments more realistic. 
While the Magtech is a solid Class-A/B design it provides more power and is more efficient than most A/B designs. It draws approximately 40 watts at idle and up to 2000 watts at full power. The Magtech does generate heat but significantly less than many other Class-A/B amplifiers. The heat sinks are small but effective. I measured the idle and operating temperature using a thermal probe. Operating temperature varied with how hard the amplifier was working. The outside edge of the heat sinks averaged about 82°F at idle and ranged from 89-95°F while operating when ambient temperature was 66°F. The low surface temperature makes Magtech amplifiers kid and kitty friendly.
Its size and weight (55 pounds) give it serious attitude but it will easily fit on an audio rack and one healthy male can handle it and bask in a prideful testosterone rush of accomplishment. If you want to feel even more manly, Magtech amplifiers are also available in a monoblock version that produce 1600 watts into an 8-ohm load and 2000 watts into a 4-ohm load. Although they have both XLR and RCA inputs, the stereo Magtech operates in single-ended mode while the monoblock version operates in balanced mode. 
I personally found the Magtech amplifier (black faceplate) quite attractive. The entire amplifier has a solid look and feel to it including the thick top and small heat sinks. Fit and finish are excellent. The faceplate was simple but somehow elegant. This is all in keeping with Roger Sanders’ more pragmatic attitude toward design. His intention is to design equipment that brings the best sound in realistically sized and priced packages. I quote Sanders from his website: “I believe in designing and selling products of real value for a reasonable price.” I applaud that attitude. Too many times I have seen equipment that is only available to the 1% with the biggest wallets, listening spaces, and egos. Ego aside, I don’t fit the other two criteria. I very much appreciate the more practical approach.
Sanders further stands behind his products (both amplifiers and loudspeakers) like no other manufacturer I am aware of. Sanders Sound Systems offers a 30-day, in-home, risk-free trial. That includes the unbelievable offer of free round-trip shipping anywhere in the world for any Sanders equipment. If that is not enough craziness, Sanders products also carry a lifetime warranty for the original owner. Sanders Sound System products are manufactured in Conifer, Colorado and sold both directly and through selected dealers. 
Pure Enjoyment on Quad ESL-2805s:
A consummate artist but not performer, Joni Mitchell was uncomfortable on stage but a master in the studio. She applied multiple layers to “The Tea Leaf Prophecy (Lay Down Your Arms)” from Chalk Mark in a Rain Storm [Geffen 24172-2]. Despite its brute strength the Magtech showed its sensitive side by gently reproducing each layer as the whole it was meant to be and capturing the surprisingly long, barely audible fadeout that would be lost on most systems. It also faithfully captured Joni Mitchell’s voice, mostly clear as a bell but sometimes exhibiting a slight tobacco-induced haze. 
The Magtech did an excellent job of revealing the nuances in Emmylou Harris’s voice on “Can You Hear Me Now” from Stumble Into Grace [Nonesuch 79805-2]. Again there were layers to be revealed including backup singing and soft guitars. And again the Magtech seemed particularly adept at revealing layers without losing the connectivity between them. Other amplifiers might etch each layer independently or smear them irretrievably. The Magtech made me enjoy this and other music to a level I had not previously experienced. I heard more, but not because I was trying. I heard more because it was presented so effortlessly and without drama. The seduction I previously alluded to. 
No One-Trick Pony:
The Magtech had so much available power I turned down the output of my Laufer Teknik (formerly Nova Physics Group) Memory Player to protect my Quad ESL 2805 loudspeakers. When I rotated my Apogee Stage loudspeakers into service I increased the output slightly but still maintained it at a reduced level. I found the Magtech amplifier was equally at home on electrostatic and planar-magnetic speakers. 
Although Apogee Stage loudspeakers are nominally rated at three ohms they can dip much lower. The Magtech appeared to have no care in the world; easily handing everything I threw at it. I was particularly impressed that at loud levels I could detect no discernable movement of the Apogee’s midrange tweeter ribbon (MRT). Lesser amplifiers can clip and cause the ribbons to dance with reckless abandon. Slight MRT motion is either a sign of imminent amplifier clipping or the speakers are being over-driven. With the Magtech, the MRTs were rock solid at uncomfortably loud levels. Visual confirmation of what I was hearing-no distortion.
The Sanders Sound Systems Magtech amplifier deserves more than just name recognition. It is well worthy of consideration in any system where its power would be appropriate for the loudspeakers in use. But don’t take my word for it. Find out for yourself by taking advantage of the unprecedented 30-day, in-home, risk-free trial offer.

Call Terry on +64 21 880 884 or vist his website: 

Roger Sanders' new speaker system strikes me as being a remarkable achievement, - I left it on for months, it was always quiet, it drove the crap out of anything I connected to it, and it seemed to have no distortion
Phillip Holmes

Please note: since this review Roger updated the ESL amp to 360w/ch plus other modifications and introducded its bigger brother the Magtech @ 500w/ch, the outcome is for even great enjoyment and power to drive any speaker. 

The Sanders Sound ESL Amp is actually closer sounding to a powerful big-ticket tube amp than a “normal” 60wpc transistor amp. That’s what makes me consider the ESL Amp an enormous success. Outside of the user friendly qualities that Roger has designed into the amp, it does a tremendous job of amplifying a signal without changing its character. Considering the reasonable price, I think it’s an amazing product. If you need power, and most of you need more than you realize, you must hear it if it fits your budget.

Extended review:
I consider this more of an addendum than a proper review. I had the ESL power amp for use with the 10B speaker system. The 10B comes with electronic crossover and built-in bass amp, but you had to supply your own amp for the electrostatic panels. I had some tube amps on hand, but the load presented in the highs would cause high frequency roll-off. The impedance drops to 1.6 ohms at 20 kHz. I had an amp from Plinius, but the combination sounded bright. I wasn’t expecting it to be bright and couldn’t figure out why. Roger suggested it was possibly an oscillation caused by the wrong kind of speaker cable and an unhappy capacitive load for the big Plinius. I tried some video coax for speaker cabling since it was shielded and of very low capacitance. I still had the same tonal balance. I wanted to hear the same combination I had heard at CES 2009, so I contacted Roger about sending the Magtech Amp to complete the system.
Outside of a powerful tube amp (at least 120 watts), I can’t imagine a better amp to drive an electrostatic loudspeaker than the ESL Amp. Roger thoroughly addressed every pitfall of driving ESLs, and did so with a transistor design. It would’ve been easier for Roger to design a powerful mono tube amp of generic design, say parallel push-pull sweep tubes for 200 watts. Tube amps don’t oscillate when driving the very high capacitance loads presented by ESLs. They just lose steam in the highs. In the end, Roger’s ESL Amp has flat frequency response, is several orders of magnitude more efficient, generates almost no heat, is housed in one chassis that can be situated almost anywhere and can be left on all the time.
What’s more important for this article is how it sounds with other speakers. To wit, how does it sound driving the incredibly inefficient and low impedance Magnepan 2.6R? I’ve had these speakers for years, and after modifications (updates), still enjoy the sound. On the other hand, I always felt as if I needed WAY more power.
The ESL is rated at 2000 volt-amps, which is similar to a “normal” 1000-watt amplifier, see here for an explanation. The volt-amp rating applies to capacitive loads. With magnetic speakers, the ratings are more typical, though still quite powerful:
· 300 watts/channel into 8 ohms
· 600 watts/channel into 4 ohms
· 1000 watts/channel into 2 ohms (momentary/dynamic output)
If this isn’t enough, there is the mono version that delivers 800 watts into 8 ohms and 1200 into 4 ohms.
I found out from Roger that the ESL has since been upgraded and uprated:
“The latest version is:
- 360 w/c @ 8 ohms,
- 700 w/c @ 4 ohms,
- 1000 watts at 8 ohms
- 1600 watts at 4 ohms.

I need to update my website! The amp you tested was the original 300w version.”
For the sake of saving time, and touching on the technical issues, please refer to the “ESL Amp White Paper” on the Sanders Sound website.
I can say that none of the statements are snake-oil ramblings, or marketing hyperbole. I left it on for months, it was always quiet, it drove the crap out of anything I connected to it, and it seemed to have no distortion (that I could hear).
There’s no reason to spend a thousand words to describe the music I listened to with the ESL Amp, but if you haven’t read the 10B speaker review, then do so. Whatever I say about the sound of the 10B goes for the ESL Amp. If the ESL Amp weren’t good, the 10B is transparent enough that it would’ve told me it stinks (which it did on cables, amps, tubes, cartridge set-ups….you name it; if you get it wrong, you’ll know with the 10B).
What I do want to point out is that this is a primo quality amp for driving power-thirsty Magnepans. Until Roger’s ESL, the most powerful amps I had tried were 150-watt mono tube amps. They could play “loud”, but sounded forced and flustered when trying to recreate the sound and fury of the Beethoven 9th, Saint-Saens “Organ Symphony”, Der Ring Cycle, or anything by Shostakovich. By extension, this meant that I couldn’t rock-the-house with tube amps. No balls when listening to AC/DC, Metallica, The White Stripes, “The King James Version”, “Still Harry After All These Years”, and a lot of good stuff from Basie and Ellington. Sure, they could play Beethoven’s 1-8, any jazz quartet/quintet, “Kind Of Blue” and most pop music. Growing up around music, and having spent years in concert halls, I can tell you that music is often felt as much as heard. Regardless of what it might do to your hearing, a tam-tam at 50’ while the symphony is playing Stravinsky takes your breath away. It requires monumental amounts of power to try to recreate something like that, and 150 watts won’t cut it with the ESL 10B.
The ESL Amp delivered the goods when it came to reproducing musical peaks without strain. There’s a hot-rodding saying that applies here: there is no substitute for cubic inches. Even the “tricks” like supercharging are just making the engine hold the same amount of air that a larger displacement normally-aspirated engine would use. With speakers I’ve used, even the high efficiency designs, the use of a healthy power amp allows me to listen to all kinds of music, not just audiophile recordings of pretty girls. The intermodulation distortion of under-powered systems is something that you won’t hear until you get an amp of proper power. Once your ear becomes accustomed to hearing the IM distortion and compression, you can’t go back. I’ve heard 98dB single-driver speakers that compressed and got muddy, and not at what I consider realistic levels. That might mean an 833-based amp instead of a 300B for a horn. Maybe 150 watts for moderately efficient speakers. With the Maggies, it must mean that at least 600 watts is the minimum to sound relaxed. I’ve heard what a system can do with 3,000 tube watts driving 93dB efficient speakers, and the power was only sometimes needed. The ability to play peaks with no effort means less listening fatigue. Less IM and compression means more enjoyment.
That’s not to say that just any old, powerful amp is going to be “mission accomplished”. The mantra that power corrupts and absolute power corrupts absolutely is well applied to amplifiers. Few are the powerful amps that sound transparent. Many are the powerful amps that sound downright unmusical. Until the ESL, I’d heard only a couple other designs that had the openness and transparency of the tube designs I like.
I think one of, or maybe the thing, that makes the ESL Amp sound the way it does, is that it doesn’t use coupling (DC blocking) capacitors in the signal path. The only transistor amps I’ve heard that I liked were direct-coupled/DC-coupled (no coupling caps). Further, like the other transistor amps I’ve liked, it is balanced/complimentary/push-pull all the way through. This contributes to a dead-quiet nature of the ESL Amp.
Roger differs with my opinion and had this to offer: “Truth be told, the main reason that the ESL Amp sounds good has nothing to do with capacitors. It is due to the fact that it has no protective circuitry that ruins the sound when in operation. The output stage of the ESL Amp is so massive that it can never be taxed when driving difficult loads, so I can eliminate the awful audio effects of protective circuitry.”
Overall, the ESL Amp is one of the most neutral power amps I’ve had the pleasure of hearing. It’s very transparent. If you have something amiss, you’ll be able to hear it. Recordings have their sound amplified intact. There is no “sterile transistor sound” or pseudo-tube romanticism.
Macro-dynamics are very strong. Bass drum whacks are first rate. The plucked string is also very good, but not quite as jumpy as a SEDHT amp. There is some penalty for the extra devices and complexity, that seemed to be one of the trade-offs here: a penalty on microdynamics. Likewise, this amp isn’t as immediate as simple tube amps. It still has immediacy, but not on the level of the finest tube products. I could say the same thing about really big tube amps: not as much immediacy or micro-dynamics—very simple amps can do some things quite well.
Soundstage width of the ESL Amp is first rate, imaging well outside the speakers and placing a solid center image. It’s not as deep or dimensional as some competing thermionic designs, but it’s as good as any transistor amp I’ve heard.
Many of the stereotypes of tubes-versus-transistors have more to do with execution and design than any inherent weakness of the transistor (or the tube if that’s where you stand). At this level of performance, I am splitting hairs. The Sanders Sound ESL Amp is actually closer sounding to a powerful big-ticket tube amp than a “normal” 60wpc transistor amp. That’s what makes me consider the ESL Amp an enormous success. Outside of the user friendly qualities that Roger has designed into the amp, it does a tremendous job of amplifying a signal without changing its character. Considering the reasonable price, I think it’s an amazing product. If you need power, and most of you need more than you realize, you must hear it if it fits your budget.
As a follow-up to the review, I spoke with Roger about his other amps, and his response is in the following:
Note that I now have my Magtech amplifier in production. It uses the same outstanding amplifier design as my ESL Amp, but incorporates a linear regulated power supply that is virtually 100% efficient. As a result, it produces much more power than the ESL Amp (500 w/c @ 8 ohms) even though it is in the same chassis.
Because the internal voltages are not modulated over a range of 30% (typical of conventional amps that do not have regulated power supplies), the distortion remains extremely low at all times (0.0012% typical).
There are virtually no amplifiers on the market that have regulated power supplies (only the Krell monoblocks at $100K are regulated). And the Magtech is the only amp that uses a linear supply that is essentially 100% efficient (patent pending). It would drive your Maggies even better than the ESL Amp.
Sounds like Roger isn’t sitting still (pun intended).
The Sanders ESL amplifier can form the heart of a fine reference audio system
Ron Nagle

What can I say, practically all the clean power one could ever want, capable of driving any audiophile loudspeaker, surely the last amplifier you will ever buy? Understand that I believe the ML Vista electrostatic hybrid loudspeaker serves as an excellent test for an amplifiers ability to drive any type of loudspeaker. Most certainly, this amplifier represents the current here and now edge of technology applied with precision without becoming hard or sterile sounding.

Let me premise what follows by saying I am impressed by what I have learned so far. These ESL amplifiers spring from the mind of an audiophile and not just a numbers-crunching paradigm driven techno type. Having said that Mr. Sanders is an interesting story unto himself. Back in 1974, he authored an article in Speaker Builder magazine about electrostatic speakers. Then in 1976, this was followed with an article on amplifiers designed to drive them. In 1980, he wrote about the construction of an electrostatic speaker with a curved diaphragm and he was the first to develop this curved profile. You might remember that a similar Curvilinear electrostatic radiating surface is the hallmark of all of the loudspeakers made by the MartinLogan Company. He is best known for his contribution to the state of the art in his book, "The Electrostatic Loudspeakers Design Cookbook" published by The Audio Amateur in 1993.
In 1997 Roger Sanders joined Raj Varma to form the Innersound Company. Their products were a hybrid loudspeaker with a cone woofer and a flat electrostatic panel and the Innersound ESL amplifier designed to drive their loudspeakers. Both products received a lot of good press in the audiophile articles written at that time. Then in 2003, Innersound moved from Georgia to Boulder, Colorado. The year 2004 Roger Sanders left and in 2007 he formed Sanders Sound Systems now located in Conifer, CO. It is important to note that Mr. Sanders has managed to not only continue to make and improve his ESL amplifiers but after a gap of three years, he has resumed production of his innovative electrostatic loudspeakers. The now discontinued Innersound loudspeakers and the new Sanders sound systems loudspeakers abandoned the curved electrostatic panels he developed explaining that the curved surface introduces a new set of problems not inherent with flat electrostatic panels.
Facts Of Life
It just may be that you need a very special amplifier to get the best performance from an electrostatic loudspeaker. Electrostatic loudspeakers present an amplifier with a very varied and difficult load more complex than conventional cone driven loudspeakers.
The designer of a dedicated ESL (Electrostatic Loudspeaker) amplifier has to consider many factors, not only the crazy impedance shifts but also the fact that as frequency increases electrostatic speakers morph into something that looks like a giant capacitor. Basic electronics textbooks will tell you that at high frequencies capacitive current leads voltage by 90 degrees. To make an obvious point such an amplifier must be able to supply enough voltage and current to more than match any demands made by frequency changes and dynamic swings in music. Most importantly, it is necessary to supply this power in an evenhanded manner (low distortion) at all frequencies. All of these factors mandate that the power source be absolutely rock solid stable under any conditions. Normally I would mention only a few salient amplifier stats and leave the rest to the specifications I list at the end of an article. However, some of the specifications that describe the Sanders ESL Amplifier are damn amazing. I wouldn't want these languishing as an unread end of the article after thought. All the same I would like you to understand like must audiophiles I am really not into measurements because experience tells us they usually do not predict how an amplifier will sound, having said that good specs are generally not a bad thing.
Amplifier Nomenclature
It is important to understand that the Sanders Sound Systems amplifier under review is a different and much improved version of the earlier Innersound design. The chassis is now aluminum and is of a heavier gauge construction. The balanced XLR connectors are Neutrik, and the RCA connectors are Cardas. The more powerful-shielded toroidal power transformer is massive and fills about 75 percent of the chassis interior. This larger transformer has increased the power rating by 60 watts it is now rated at 360 watts per channel into 8 Ohms. A redesigned power supply circuit board now eliminates all wire except for a 2-inch piece connecting it to the speaker binding posts. The low-level input circuitry is redesigned and uses higher-grade transistors. More powerful and more linear bipolar output transistors are now used; this in combination with improved circuitry lowers harmonic distortion from 0.03% in the Innersound version to a vanishing 0.008% for the revised Sanders amplifier.
In this newer ESL amplifier there are 18 bipolar hi-power transistors in each channel connected in parallel they have a combined power rating of 4500 watts at 10 MHz. The peak current rating is 135 Amperes. The term damping factor refers to a ratio of amplifier output impedance to loudspeaker load impedance, this computation gives you some idea of the control exerted at bass frequencies. These select hi-capacity power transistors present a total parallel output resistance of only 0.01 Ohm. This translates into a very high damping factor of 800 into an 8-Ohm load. The power rating listed is 360 watts per channel into 8 Ohms and 600 watts per channel into 4 Ohms and the amplifier is stable down to 1 Ohm. The rated power is 1700 VA per channel into capacitive loads with both channels driven from 20Hz to 20kHz. The distortion is listed as 0.03% from 20Hz > 20kHz. Noise less than 100dB below rated output. Last but not least the specifications are for a linear class A/B amplifier. The definition of a linear amplifier is: an amplifier where the output signal is a replica of the input signal and the output is directly proportional to the input. There are many more specifications I could list but you get the idea.
There are two ergonomic difficulties that I encountered. The owner's manual suggests that you could leave the amplifier powered on and this would eliminate stress by not cycling the amplifiers power on and off. Naturally, this makes a lot of common sense but I have a tube preamplifier and it needs to be powered down when not in use. In addition, it is not a very good idea to turn on my tube preamplifier while it is plugged into this power amplifier. There should be a way to mute the amplifier output and some indication that it is muted. My last concern is that the power switch is located on the back panel and if you have the amplifier stacked on a shelf in an equipment rack (like I have) than it would be hard to reach.
Raison D'etre

Hooked on air, the kind that surrounds me in sound. It is the something that makes music appear on a gossamer fabric of film that energizes the air around. I have owned Quad ESL 63 electrostatic speakers for many years. I love the way they make music that flows effortlessly and naturally in continuous wavelets free from mechanical artifact. While having never stopped trying to squeeze the very last drop of music out of my Quads, over the years I replaced all the panels and upgraded the grills than installed a mod squad resistor capacitor network plus mounted them on dedicated Gradient woofers. So you may understand my abiding interest in Roger Sanders ESL amplifier made specifically for electrostatic loudspeakers. The ESL amplifier arrived just as I was in the process of evaluating a pair of MartinLogan's brand new Vista loudspeakers. This $3995 electrostatic hybrid is the only loudspeaker in the extensive MartinLogan line that does not have a built in 'Class D' woofer amplifier. It stands 57 inches high 40 inches of that is a narrow line source electrostatic panel and below in a ported enclosure is an 8inch metal cone woofer. Common sense will tell you if you want to use these loudspeakers to evaluate audiophile electronics then you best had better not use their built in amplifier to complicate the musical mix. At this point I would love to go off on a tangent and tell you what a mess most switching amplifiers make of high frequencies but that is best left for another time.
Amplifier Trials

Using my Marantz DV8400 Universal disc player feeding an Audio Research SP 9 MK3 preamplifier signal from this then routed to my power amplifiers via three meters of Wire World Eclipse 2 unbalanced cables, this was the unchanging reference front end. My in house loudspeakers are called, Strata Mini by These 4 way loudspeakers have a built in woofer amplifier so even though I listened to them they are not part of this report.
Initially I powered the ML Vista loudspeakers with a PrimaLuna Prologue 2 this is a 40wpc KT88 integrated amplifier. The bass was less defined and not as agile when compared to the mid range and treble speed but overall it was not at all hard to listen to. Next up was my feature filled OutLaw RR2150 receiver. This 100-wpc receiver has bass and treble controls and a separate EQ control to select bass turnover frequencies. With this EQ control, I was able to limit the amount of bass at a frequency of 55 Hz confirming that without this bass cut the bass sound was, as the British would say a bit "plumy". Next in line was my much-modified 250 wpc Hafler 500 amplifier. The main advantage here was that it has the ability to pump a lot more power into the Vista loudspeakers.
At this point what I heard was a lot closer to the musical truth. The Hafler seemed to be able to supply the power necessary to inject life into the performance by way of dynamic contrasts. The Sanders SEL Amplifier arrived at this time and in a manner of speaking it placed all the other amplifiers into a different musical context. What I had been doing was not just switching amplifiers but sampling music through a variety of tinted lenses. My analogy is that in retrospect some of the amplifier characteristics I noted were subtle colorations not inherent to the music. None of these was unpleasant but none of these was quite accurate. In retrospect, there was a suggestion from the tube amplifier of a subdued shade of mauve. The Outlaw amplifier tended more toward a shade of pale blue and the Mosfet Hafler was a pleasant slightly fuzzy light beige. Forgive my imaginative rainbow of a description but that is the best way to convey my subjective impressions.
Aural Aspects
What do you hear when you have all the clean music power you need and limitations have been pushed to a place that is practically unapproachable. Metaphorically speaking, we can experience this (borrowing a worn phrase) as a clearer window on the performance. With the Sanders ESL amplifier driving the MartinLogan Vista loudspeakers the cone bass drivers were now held in a tight grip and became nearly as articulate as the electrostatic panels. (Note: 90 percent of the audio power produced is used to drive the loudspeaker at frequencies below 200Hz) At a manufacturers specified frequency of 20 kHz the Vista loudspeakers become more capacitive and the resistive load drops down too a very difficult 1.3 Ohms.
Right now I can't think of a better recording to illustrate the articulation and speed of the ESL amplifier than a wonderful version of "You Were Always On My Mind" by Willy Nelson. I found it on a Sony (CD A21562) it is from an album titled Willie Nelson Yours Always. This is a wonderfully engineered and detailed studio mix with transient speed and studio ambience that highlights the ESL amplifiers resolving ability. At two minutes and forty seconds into this cut there is a brief faint sound of a person in the distant background whistling along with the melody. Meanwhile Willy's vocal resides front and center in all his nasal glory amidst the glisten of steel guitar strings. Backup vocals are etched and clearly delineated and recede in layers from the immediate left center stage. In a strange and unexpected way the metallic reverberation of the guitar strings complements the country twang of Willy's voice.
If you want to test low frequency tonal integration than I can think of no better way than to play "Adagio d' Albinoni" as performed by Gary Karr and Harmon Lewis. This was originally on a Japanese Firebird label but it may be available via the Cisco Music catalog (GCD8003). Recorded in a cavernous Japanese cathedral it is a duet of a large sonorous pipe organ and Karr's centuries old Amati bass fiddle. It is not enough to just rattle low bass it is another thing to get all the wooden warmth and rich sad sounding harmonic overtones of this bass instrument correct.
The reverberations and echo of the low register organ pipes holds the woofer for a moment on a deep sustained rumble. The resinous bass bowing sighs and breathes a mournful moan that tugs at your heart. The sound is absolutely organic and for a time you forget that it's not flesh and blood calling out to you. Every contributing element of my system and music selections was clear to hear. If I had to pick my best CD it would more than likely be The Look Of Love by "Diana Krall"; it is the Verve [589-597-2] SACD release. She appears on this sound stage warm and made of flesh and blood with a subtle pause and a moist intake of air she seems as real as life. Moreover, if I wasn't happily married and I could just get my; but that's another story. I would like to mention one last recording even though it's not strictly a reference but rather a fun recording I enjoy very much. It is a gathering of famous artists brought together to celebrate the Queen of England on her Golden Jubilee. Recorded live out doors in front of Buckingham Palace it is ever so clean spacious and dynamic with ESL power applied to the Martin Logan Vistas. You can find it on Virgin [7243 8 12833 25] and it is called Party At The Palace. Did you ever hope to hear Brian Wilson performing "GodOnly Knows" backed by The Royal Academy Of Music Symphony Orchestra? Of course there is much more to commend it, like Rod Stewart, Elton John, Paul McCartney and Eric Clapton to name only some of the talent on this disc, it is wonderful music that is wonder filled! 
What can I say, practically all the clean power one could ever want, capable of driving any audiophile loudspeaker, surely the last amplifier you will ever buy? Understand that I believe the ML Vista electrostatic hybrid loudspeaker serves as an excellent test for an amplifiers ability to drive any type of loudspeaker. Most certainly, this amplifier represents the current here and now edge of technology applied with precision without becoming hard or sterile sounding.  One side of my reviewer's brain craves exactitude while the other half soaks in a soft bed of rose petals. What to do?
Logic tells me accuracy is undeniably essential to what I need to know. The Sanders ESL amplifier can form the heart of a fine reference audio system. It could serve as a tool to evaluate every type of loudspeaker or even the performance of each separate driver. Our you might use it's purity to hear deeper into a recording and dissect the way it was recorded and mixed down. However, there is a way to have your cake and eat it too. Using its neutrality there are many options that were not available before. You could go upstream and swap in softer sounding interconnects or feed it from a tubed preamplifier, like my Audio research SP9 MK3. Oh! Lest I forget, Mr. Sanders makes a dual mono version of this amplifier and a (what the hell could he be thinking) 1000 watts per channel stereo amplifier! With the permission of the Editor, the boys down at the IRS and my spousal partner I would like to provide a new home for this much deserving amplifier. 
…….Ron Nagle 
This amp is a keeper, absolutely no doubt about it
Sir Sanders Zingmore - audio forum member
A bit of background
A few months ago, after searching high and low for an amp to drive my Sonus Faber Auditor M's I had a flash of insight; the amp may not have been ideal but it was really that the speakers were not for me.
A long and enjoyable quest finally saw me buying my Zingalis which I have waxed lyrical about in other posts and some fellow SNAers have heard at my GTG
Trouble with the Zingali's is that they are very revealing of weaknesses elsewhere in the system. The dreaded upgrade bug had bitten. At first I felt that my DAC was the weak link in the chain and was tempted to spend a LOT of money upgrading. An expensive gardening project put paid to that but also made me pause and think about amplification.
In my initial quest for an amp for my Sonus Fabers, For those not familiar with Sanders Sound, Roger is best known for his electrostatic speakers and the amps he designs to drive them. He also makes Magtech amps for 'traditional speakers. His designs are innovative and although I don't fully understand the technical stuff, Roger very patiently and promptly responded to every email I sent over many many months. I never felt pressured and I certainly felt like I was being educated by someone who has been in the business for a long time and really knows his stuff. His approach is very much no-nonsense and I imagine quite controversial as he doesn't seem to play by the usual rules that encourage audiophiles to spend extraordinary amounts of money. I like this approach a lot. (The same service is now available through Terry at Audio Reference in New Zealand).
It took me a long time and many questions to decide to go ahead with this trial - I didn't want to audition unless I was really ready to keep the amp if I liked it.
Anyway, a few days ago one of his Magtech amps arrived at my door. This thing is powerful - 500w into 8 ohms, 900 into 4 ohms
So am I crazy driving 97dB speakers with 500w of solid state power.............. Not on your life!
This amp is a keeper, absolutely no doubt about it :confused:. It disappears. It gets out of the way and lets the music through. Not sure what else to say about it cos I've spent long enough writing and I want to go back to listening :)
Sanders Sound Systems website

Many audiophiles have asked if the regulator in the Magtech is truly 100% efficient as claimed.  The purpose of this paper is to describe how it works so that you can see that it is, in fact, super efficient.  It actually does run cold and truly solves the heat problems of conventional regulators that prevent their use in power amplifiers.

So how does the Magtech regulator work?  I'll explain, but readers will need to understand the basics in order to appreciate the problems and solutions involved.  Since the technical expertise of readers varies, I will cover the basics.  I apologize in advance if some of what I am about to say is review for some readers.
First, what exactly is "efficiency" as applies to a voltage regulator?  Efficiency is the amount of energy put into a system compared to the amount of energy that you get out of it.  Since energy cannot be destroyed and must be accounted for, any losses in efficiency will be reflected as waste heat somewhere in the system.  
Or to put it another way, any heat that is produced by the voltage regulator is a loss in efficiency and results in less power being fed to the electronics than would be the case if the regulator was not present.  The exact efficiency percentage can be calculated based on watts in compared to watts out or watts of waste heat produced.  
In the Magtech's voltage regulator, you will not find any waste heat.  It will pass virtually all of the watts put into it on to the amplifiers.  
To see why, it is necessary to understand exactly how a power supply operates.  Only then will it be possible to see how the Magtech's voltage regulator works and how it can be so efficient.  
The purpose of a power supply is to produce smooth DC (Direct Current) at specific voltages to drive the downstream electronics.  A basic, linear power supply consists of three sections, each having different types of output characteristics.     
The first section is the power transformer.  This converts the mains voltage to the voltage(s) required by the downstream electronics.  The output is AC (Alternating Current) in the form of a sine wave.  
A sine wave is a smooth wave form without any harmonic structure with alternating positive and negative polarity.  There is one positive and one negative wave per mains cycle (60 Hz in North America, 50 Hz in the rest of the world).
The second section is a bridge rectifier.  This consists of four diodes.  Diodes are electric check valves that flow current in only one direction.  These diodes flip the phase (polarity) of alternating waves by 180 degrees so that all the waves are in phase.  Therefore the output from the bridge rectifier will be pulsating DC sines at twice the mains frequency. 
The third section is capacitance.  This usually takes the form of a bank of large, storage capacitors.  A capacitor bank acts like a rechargeable battery in that it can store a lot of electrons and release them when needed.  
The practical difference between a rechargeable battery and a bank of capacitors is speed.  A capacitor bank can be charged and discharged virtually instantly, which is necessary to meet the sudden large current demands of an amplifier.
The main purpose of the storage capacitors is to smooth out the current flow from pulsating DC to continuous DC.  The storage capacitors are often called filter capacitors since they "filter out" the DC pulses from reaching the downstream electronics.  If there were no filter capacitors, an amplifier would make a very loud hum come from the speakers.  
The storage capacitors are also needed to help the power transformer deliver enough peak current to reproduce dynamic peaks that require more current than the transformer can deliver.  Think of the current that is required to drive the woofer at that moment when a bass drum is struck . . .
The current required by a Class B amplifier is directly proportional to the energy in the music.  So at idle (no music), no current is needed or used.  Very loud music will require an equally large amount of current to drive the speakers loudly.  
It is this huge difference in current that causes the large voltage changes in the rails (the power supply output voltage) you find in most amplifiers.  The difference in the rail voltage between idle and full power in most amplifiers is around 30%.  This massive voltage drop causes the distortion, the bias, and the output capability of an amplifier to be modulated by the music.  
An electronic circuit's distortion can only be optimized at a specific voltage. Any variation of voltage will result in increased distortion.
Class AB amplifiers are a bit more complicated than Class B amplifiers as they require a constant bias current that requires some power.  The bias will be optimized at a specific rail voltage.  Therefore, the bias will change directly with changes in the rail voltage.  
But the biggest issue is that an amplifier's power will fall as its rail voltages fall.  So unregulated amplifiers suffer significant performance degradation as the music modulates their power supply voltage.
The rail voltage fluctuations caused by amplifier load are only part of the problem.  The mains voltage is not stable either.  
The mains voltage will vary depending on the load on the power grid and the load on the house wiring.  High load conditions can cause the mains voltage to vary by 10% or more.
For example, compare the electrical load and usage in the middle of the night to early evening on a hot summer day.  At night people are sleeping so they are not using electrical equipment and the temperature is cool so air conditioners are not running much.  
In the early evening, everybody is home from work, dinner is being cooked, electric washers and clothes dryers are operating, air conditioners are maxed out, people are using power-hungry electronics like big TVs, the lights are on, the water heater is running, etc.  So the load on both the grid and home wiring is great.  
And when do you listen to your music system?  Of course, when power demand is the highest and voltage is the lowest.  Murphy is hard at work here.
And there is even more bad news.  The amplifier itself can severely tax the capacity of your house wiring to which it is attached.  A powerful amplifier can draw all the power that is available from your wall receptacle, which is limited to about 2,400 watts on a 20 amp circuit.  This will drop the voltage on that line by several percent -- this is in addition to the losses on the grid and in your home from other power uses.
Furthermore, the mains frequency has a big effect on the output of a power supply.  This is because a transformer's power is determined by the current it can deliver in its power pulses multiplied by the frequency of those pulses.  
This means that a transformer can deliver about 20% more power when operated on a 60 Hz mains than it can when operated on a 50 Hz mains.  Therefore, an amplifier with an unregulated power supply will lose up to 20% of its power supply power when operated on a 50 Hz mains.
All this is further complicated by the fact that the relationship between voltage and power in an amplifier is not linear.  Power varies by the square of the voltage.
Power is the product of volts times amps.  Ohm's Law says that one volt will drive one amp through one Ohm of resistance.  If you do the math, you will come to realize that the power of an amplifier is determined by the voltage that it can drive into the loudspeaker (assuming it can also deliver the current required).  
The formula for calculating amplifier power is the amplifier's RMS output voltage squared and then divided by the speaker's impedance.  As an aside, impedance and resistance are the same thing.  Resistance applies to DC circuits while impedance is used for AC circuits.  This is because the impedance often varies with frequency in AC circuits but there is no frequency in DC circuits.  For calculations, you may use impedance and resistance the same way.
To determine the voltage, the formula is the square root of the product of watts times Ohms.
Using these formula, you can see that for an amplifier to drive 100 watts into an 8 Ohm speaker, it will have to produce 28.28 volts and deliver about 3.5 amps.  Now what happens if we drop the power supply voltage by half?  The voltage will then be 14.14 volts and the current will drop to 1.76 amps.  
How much power will the amplifier now drive into the speakers?  It will be just 25 watts.  This is a huge loss.  
So you can see that the typical 30% loss of rail voltage in an amplifier results in a very large loss of power -- about 50%.  If you add an additional loss of mains voltage due to heavy house wire loading, you will lose another big chunk of power.
When you add all the above factors together, you can see that an amplifier's performance is severely degraded by power supply voltage fluctuations and that eliminating them will produce substantially better amplifier performance in terms of power, distortion, and optimum bias levels.  So why don't amplifiers have voltage regulated power supplies?
The problem is that the poor efficiency of conventional voltage regulators results in vast amounts of waste heat.  Most amplifiers run very hot and adding large amounts of waste heat to an already hot amplifier is intolerable.  It is also expensive in terms of both hardware and electricity usage.  So it is very rare indeed to find any amplifier that is fully voltage regulated.
So exactly how does a voltage regulator work and what makes it so wasteful and hot that using it is impractical?  The most common type of voltage regulator is called a "down" regulator.  This means that it pulls down the power supply's voltage so that it remains stable under the worst case conditions.  
For example, all quality preamps are voltage regulated so that their power supply voltages will remain stable all the way down to a mains voltage of around 90 volts (using a 120 volt mains).  Only if the mains falls below 90 volts ("brown-out" conditions) will the regulation be insufficient and the power supply voltage will start to fall.
The power supply will be driven by a 120 volt mains most of the time, although it might be up to perhaps 125 on occassion.  The difference between 120 and 90 volts is about a 30%.   
Let's assume that the preamplifier's electronics operate on 12 volts.  The electronic engineer will design the power supply to deliver at least 30% more voltage than that (typically about 18 volts).  He will then add a "down" regulator to pull the power supply voltage down to 12 volts, which is about the voltage that the power supply would produce using a 90 volt mains.  So for any mains voltage between about 90 and 125, the preamp's power supply voltage will be stable at 12 volts.
The regulator actually works by placing a variable load across the power supply in the form of a power transistor that is shunted across the output.  A power transistor can be thought of as a very fast-acting, variable resistor whose resistance can be changed electronically.  By monitoring the rail voltage, the electronics can adjust the resistance of the transistor to alter the voltage.  
As the mains voltage rises, the electronics will reduce the resistance of the loading transistor, which will draw more power and drop the power supply voltage.  As the mains voltage falls, the electronics will increase the resistance of the load transistor, which will reduce the power used by the transistor and allow the voltage to rise.
Of course, the action of the electronics are nearly instantaneous, so there is no significant rise and fall of the rail voltages with changes in the mains.  The voltage will remain rock stable to within a tiny fraction of a percent.
A down regulator is very inefficient.  This is because it operates by feeding a voltage through a resistance.  This causes a voltage drop by converting some of the power supply's current into waste heat.  
Remember the above concept because it is extremely important.  To repeat -- anytime you apply voltage across a resistance, there will be a voltage drop.  The loss of current causing the voltage drop will result in waste heat.
The circuitry in a preamp uses only a tiny fraction of an amp (typically just a few milliamps).  So the power involved will only be a fraction of a watt or so. 
If you waste 30% of a watt in a voltage regulator, the heat produced and the electricity wasted is insignificant.  So nobody cares about the efficiency of down regulators when used in small-signal devices.
But now let's look at power amplifiers.  Just how much power do we need to regulate?
The typical Class AB amplifier is about 50% efficient.  Why?  Because it applies its power supply voltage to its output transistors and these act as variable resistors that control the voltage being applied to the speaker.  So once again, we have the issue of producing waste heat because we applied voltage across a resistance.
This means that for every watt that the amplifier feeds to the speaker, a watt will be injected as heat into its heat sinks, and two watts will be drawn from the mains.   A powerful amplifier like the Magtech will produce 500 watts per channel into 8 Ohms.  With both channels operating at full power, 1,000 watts will be fed to the speakers.  It also means that about 1,000 watts of waste heat will be fed into the heat sinks, and 2,000 watts will be drawn from the mains.  
The Magtech's power supply will produce 2,000 watts continuously, so a regulator must be able to control a minimum of 2,000 watts of power (and more to be conservative).  The regulator must be able to regulate at least 30% of the rail voltage in order to eliminate fluctuations in voltage due to the variable music demands.  In addition, it must be able to handle more than that to account for voltage variations in the mains and 50 Hz operation.  
All together, we are looking at regulating about half of the power supply's voltage.  This is a daunting task for a down regulator because it means that under worst-case conditions (maximum mains voltage, 60 Hz mains, and with the amp at idle), the regulator will have to dissipate half the power supply's voltage (and hence half its power) as waste heat.  
That means that the regulator would produce 1,000 watts of waste heat.  This would turn the amp into a room heater and require truly massive heat sinks.  It would waste enormous amounts of electricity, be very large, and the heat would cause failures of parts over time.  You should now be developing an appreciation of why amplifier power supplies are not regulated!
Although down regulators are very simple and easy to add to circuits, they are just not practical for use in high power circuits due to their inefficiency.  But there are other types of regulators, which are more efficient.  These are the "up" regulators.
An up regulator requires two power supplies.  These have different voltages where one is set for the worst case voltage and the other is set for the best case voltage (say 120 volts and 90 volts for example).  
The two power supplies are connected together by a power transistor whose resistance can be varied to allow more or less of the high voltage power supply to be added to the low voltage one.  This allows the high voltage supply to bring the voltage "up" and prevent it from falling based on load or mains voltage.  The rail voltage can therefore be kept constant between the two extremes by electronically controlling the coupling transistor.  
The big advantage of an up regulator is that it only has to handle a percentage of the total power supply voltage (in this example, 30%) instead of all of it.  Therefore the losses and waste heat are only a fraction of those produced by a down regulator.  But it requires two power supplies, is more complex, and more expensive than a down regulator.  
An up regulator still wastes far too much power and produces too much waste heat.  So it is still impractical for use in all but very low-power amplifiers.
The next general type of regulator is not a linear regulator like the types I have been describing.  It is the switching regulator.  
A switching regulator is rather complex, but I'll simplify its operation for clarity.  A switcher fundamentally places a transistor in series with the output from the power supply.  This transistor is then switched on and off at a high frequency to feed power to the electronics.  
The transistor oscillates at a fixed frequency and its "on" time is varied so that it feeds a percentage of the power supply's current to the electronics based on their need.  By feeding a capacitor bank, a switcher can adjust the current flow to produce a stable voltage.
Switching power supplies are very efficient (although not 100%) because their transistors are used in only the on or off state.  They are not partially turned on like the transistors in linear supplies where a significant resistance is presented to the voltage that produces waste heat.  
However, a transistor does not change state instantly.  There is still a small percentage of switching time during which the transistors are changing state and resistance is present.  So they still produce some waste heat, although this is relatively small, can be tolerated, and therefore switching power supplies can successfully be used in power amplifiers.
But there are big problems when using switching power supplies in high power applications.  The main one is noise -- both electrical and mechanical.  When switching high power and voltages at high frequencies, radio frequencies are produced.  These emissions can adversely affect associated audio electronics and cause instability, oscillation, noise, and general misbehavior.
Powerful switchers also make mechanical noise because there is physical vibration of the switching transistors due to the high currents involved.  Switching power supplies are vastly more complex than a simple, 3-part, linear supply and therefore the reliability of switching supplies can be a problem.
There are also many technical problems when designing switching power supplies that make them quite difficult to make work satisfactorily.  I won't get into any more detail about this, but rather simply point out that because of all the problems, it is extremely rare to find a linear amplifier with a switching power supply.  They do exist, but are not 100% efficient and are not a practical solution for the voltage regulator problem in power amplifiers.
Of course, I have just outlined the basics at this point.  There are many variations on the theme that are beyond the scope of this paper.  But you should now have enough information to appreciate the solutions that follow.  
So how can the efficiency problem of high power, regulated power supplies be solved?  Well, the answer came from thinking outside the box.  Specifically, since the heat is produced by applying a voltage to a resistance, the solution had to come from figuring out some way to eliminate doing so.
There is a way.  But it could not be done by regulating the continuous DC from the output of a power supply's capacitors because voltage is always present there.  The solution had to be done by figuring out a way of regulating without having voltage present.  That sounds crazy and impossible, but it can be done.  
What about the output from the rectifiers?  This is pulsating DC.  While the peak of each pulse is at high voltage and power, the voltage at the end and beginning of each pulse is at -- ZERO!
If the regulating power transistor operated only when the voltage was at zero, then there would be no current present, none would be wasted, and no waste heat would be generated.  But how can that regulate the voltage?  Here's how:
The Magtech uses two power supplies as you would in an up regulator.  I call the low voltage one the "ride" supply.  It is exactly like the power supply in a conventional, unregulated amplifier.  
The second power supply is the "boost" power supply.  It has a higher voltage and current rating than the ride supply and can add massively more power to the ride supply when needed.
The ride supply voltage is set for "easy" operation under optimum conditions, i.e., when the mains voltage is at maximum and the amp is at idle.  Under these conditions, only the ride supply drives the amplifier circuitry and the boost supply is just on standby.
Note that for the Magtech amp, this is the "easy" condition when the regulator does nothing.  By comparison, this is the toughest condition for a down regulator because it has to drag down the power to the worst case level and dissapate massive amounts of power and heat when doing so.  
But in the Magtech, this is the voltage that is desired and that the regulator will maintain.  Under these easy conditions, the boost supply is not needed.
When significant power is required, the rail voltages will start to fall.  This is detected by the power supply's monitoring circuitry, which then switches on the coupling transistors to connect the boost supply to the ride supply.  The additional power provided by the boost supply prevents the rail voltage from falling, thereby regulating it.
The key to efficient operation lies in the way that the coupling transistors are operated.  First, they are either fully switched on or fully turned off.  This means that they have either infinite resistance or essentially none.  This prevents them from putting any resistance in the circuit that would cause them to dissipate heat.  
Secondly, digital control circuitry is used to monitor the rectifiers' wave form and cause the transistors to switch states (either on or off) at the exact point where the DC pulses cross the zero voltage point.  This is important because even though transistors change states very quickly, they do not do so instantaneously.  So there is some resistance during the change of state.  This is the same problem that causes switching power supplies to be less than perfectly efficient.
If the transistors changed state while the power supply voltage was applied to them, there would be waste heat generated.  By only allowing state changes at the zero voltage points, there is no waste heat.  
Now if you are observant and thoughtful, you might comment that this does not sound like a very good regulation scheme because the two power supplies are either at maximum voltage or minimum voltage because the regulator operates as an all-or-nothing affair.  Your thinking is good, but you are overlooking an important feature in the Magtech's power supply.  
The digital control circuitry constantly monitors the pulsating waves from the regulator and the rail voltages.  It will then make a decision to turn the coupling transistors on or off at each zero point to add as many or few pulses as required to hold the voltage constant.  
Under heavy load, the coupling transistors would remain on (possibly even continually), letting most or all of the pulses through.  Under light load, they would only be switched on occasionally to let a few pulses through.
While it is true that the regulator has a maximum resolution of 120 pulses per second, each pulse has to charge up a very large bank of capacitors (80,000 uF). Doing so takes time and much current.  Therefore, even though each pulse has a lot of current and energy, it can only make a very small change in the capacitor bank's voltage.  
The electricity and voltage in the capacitors are analogous to the water in a swimming pool.  You can dump a large, 55 gallon drum of water into the pool (a pulse from the boost power supply), but it won't change the level of the water in the pool (the voltage in the capacitors) very much.
By adding more or less pulses as needed, the regulator can maintain a stable voltage to within 0.2 volts.  By comparison, without the regulator, the power supply's voltage would vary by more than 50 volts.  Which would you prefer?
You can now see why the Magtech regulator produces no heat and is virtually 100% efficient.  Technically, I can't claim that the regulator is absolutely 100% efficient because nothing is perfect and there is a very tiny amount of resistance in everything, including the coupling transistors when they are "on."  
But the resistance of the transistors is less than one Ohm, so they still do not get even warm when operating.  Furthermore, they only operate when the amplifier is working fairly hard, so the regulator isn't even active when the amplifier is at idle or at very low power.  
In some ways, the Magtech's power supply is like a switcher in that its transistors are either on or off.  But there is no specific oscillation involved as in a switcher.  Also, it is relatively simple and operates very little and at low frequencies, so its reliability is outstanding (no failures have ever occurred).  And because it never switches under power, there is no noise or radio frequency problems with it.  
In short, the Magtech's power supply is unique and solves all the problems of other regulators that have prevented power amplifiers from being regulated -- something they badly need even more than other types of electronics.  The Magtech regulator's circuit, and particularly the digital control technology involved is the subject of a patent, which currently is pending.
The Magtech amplifier modules are the same sophisticated ones used in the ESL amp that are capable of very high power, the ability to drive 1/3 Ohm loads, can handle the most difficult loads (as presented by electrostatic speakers), and need no protective circuitry that ruins the sound of many solid state amps.
When the ESL amplifier modules are combined with a practical voltage regulator, the result is an amplifier with seemingly unlimited power, virtually unmeasurable distortion, and the ability to drive even the most difficult loudspeakers with ease.  The Magtech offers a truly new level of performance in amplifiers.
Sanders Sound Systems website
Conventional amplifiers have serious problems when forced to drive (ESL) Electrostatic loudspeakers. Roger Sanders was the first to develop an amplifier specifically designed to drive these unusual speakers:


Electrostatic loudspeakers (ESLs) are very different from conventional magnetic speakers and place unusual and difficult demands on the way amplifiers deliver power to them.  A magnetic speaker presents a mostly resistive load to an amplifier, while an ESL appears mostly as a capacitor.
Conventional amplifiers have serious problems when forced to drive electrostatic loudspeakers. Roger Sanders was the first to develop an amplifier specifically designed to drive these unusual speakers:
Resistors dissipate power as heat.  So the voice coils of magnetic speakers get hot as they use up the current the amplifier sends to them.  A capacitor stores an amplifier's electrical energy instead of dissipating it as heat.  Therefore an ESL doesn't actually "use" power like magnetic speakers.  ESLs are sometimes called "wattless" speakers because of this.  Their behavior is highly reactive, which means that they send the electrical current back to the amplifier when the musical signal reverses polarity.  Amplifiers tend to be unstable with reactive loads.
A watt is a measurement of power.  It is the product of volts times amps.  Volts is a measurement of the pressure or "push" behind the electrons flowing along a conductor.  Amps (a short form of "ampere") is a measurement of the flow of electrons along a conductor.
Amplifier power is measured in watts, which is fine when working with magnetic speakers.  But an ESL doesn't operate on watts, it operates on voltage.  Therefore, an amplifier's wattage rating can be very deceptive when evaluating its ability to drive an ESL.
To take an extreme example, let's look at two amplifiers, both rated at 100 watts.  One has a 1 volt power supply that delivers 100 amps of current.  The other produces 100 volts at 1 amp.  Although both amplifiers generate 100 watts, the one with the higher voltage will drive an ESL to much louder levels than will the low voltage one.
Resistance in AC (alternating current) circuits is called impedance, because it often varies with frequency.  A resistor has essentially constant impedance, so the impedance of a magnetic speaker will be nearly constant, although there will some variation due to crossovers and resonances.
In a capacitor, the impedance is inversely proportional to frequency.  So an ESL will have a high impedance at low frequencies (perhaps several hundred), and a very low impedance at high frequencies -- typically around 2 Ohms.
ESLs are voltage operated devices.  The higher an amplifier's power supply voltage, the louder it will be able to play an ESL (assuming it can also deliver sufficient current).
Because high voltages are not needed for magnetic speakers, and because high voltage parts are expensive, conventional amplifiers often lack sufficient voltage to drive ESLs to truly loud levels.
When an amplifier runs out of voltage, it clips (called "voltage clipping").  This results in distortion and compressed dynamic range.  Depending on the amplifier and its behavior when clipping, the music will take on a wide variety of non-musical qualities.
If an amplifier is clipping, it really doesn't matter how well-built the amplifier is, or how impressive its design philosophy -- it simply won't sound as good as an amplifier that isn't clipping.  Therefore, the most important amplifier specification is its power rating. When observed on an oscilloscope, most audiophiles are amazed at how often their favorite amplifier is clipping when playing music moderately loudly.  Modern speakers require several hundred watts/channel to loudly reproduce today's highly dynamic music without distortion.
ESLs have a legendary reputation of being able to produce effortless and crystal-clear sound, with magnificent resolution of subtle inner-detail.  A clipping amplifier will destroy these qualities.
Sanders Sound Systems’ ESL amplifier operates at high voltage (plus/minus 92 volts).  This will drive most ESLs to "ear-bleeding" levels with voltage to spare.  The result is electrostatic sound that retains its totally effortless and clear qualities at any tolerable listening level.
Most transistor amplifiers require protective circuitry to prevent their output transistors from being damaged when they attempt to drive low impedance loads at high levels.  In high quality amplifiers, this circuitry switches the power on and off to the output transistors very quickly.  This causes voids and voltage spikes to be added to the sound which is one of the major causes of the harsh sound often heard in overloaded solid-state amplifiers.  In fact, it is this and the introduction of large amounts of odd-order harmonic distortion from voltage clipping that is the cause of the dreaded "transistor sound" -- not the use of transistors per se.
Sanders Sound Systems's ESL amplifier has such a massive output section that it does not need any protective circuitry.  It can drive loads below 1 Ohm without damaging its output transistors.  Since it has so much voltage and current capability that it virtually never clips, it doesn't exhibit any "transistor sound."
An amplifier must deliver more current as the impedance of the speaker decreases.  This requires a larger power supply and output devices that can pass large amounts of current.  Such parts are costly, so modestly-priced amplifiers are only designed to drive relatively high impedance loads -- like 8 Ohm speakers.  Better amps use superior parts and can handle 4 Ohm loads.
But few of even the best amplifiers can handle the very low, 2 Ohm impedance of an ESL well.  Many otherwise fine amplifiers find themselves unable to pass sufficient current through their output stages to drive an ESL at high frequencies.  This is known as "current clipping", and results in poor high frequency performance.  Tube amplifiers are particularly bad in this regard due to their relatively high, 4 Ohm output impedance.
The Sanders Sound Systems Electrostatic Amplifier ("ESL amp") solves this problem by using a massive output stage.  Each output transistor is capable of delivering 250 watts -- and there are eighteen of these per channel.  As a result, it can deliver a staggering 135 amps of current with a combined power rating of 4500 watts per channel!
The output impedance of an amplifier must be lower than the impedance of the speaker, or current clipping will result.  With so many output devices, the output impedance of the ESL amplifier is virtually zero.  Current clipping simply is no longer an issue.
As previously mentioned, "power" in the usual sense, does not apply to ESLs.  But it is useful to try to make comparisons to get an idea of what can be expected for a purpose-built ESL amplifier.  Also, many of the features that make the ESL amp so effective for ESLs also work splendidly with magnetic speakers.  So it is worth rating an ESL amplifier using conventional power measurements.
The term "volt-amps" is used instead of "watts" when evaluating an amplifier's ability to drive the capacitive load presented by an ESL.  Volt-amps is still the product of volts x amps (as is watts) but the difference is that they are not necessarily being delivered simultaneously.  Another way of saying this is that the voltage and current are out-of-phase with each other.
When driving a resistor, the voltage and current flow together.  In a capacitor, the current leads the voltage by 90 degrees.  This out-of-phase power delivery drives transistors out of their safe operating area.  In conventional amplifiers, it can cause output transistor failure and/or the premature triggering of protective circuitry.  This can cause the amplifier to deliver only a small fraction of its rated power and cause harsh sound quality.  The ESL amp's output stage is so robust that it can drive out-of-phase loads with ease, and since it needs no protective circuitry, there is nothing to ruin the sound quality.
Sanders Sound Systems’ ESL amplifier can deliver more than 2000 volt-amps per channel into an ESL.  That means it will act like a conventional amp rated at more than 1000 watts per channel.
When driving magnetic speakers, the ESL Stereo Power amplifier will deliver over 400 watts/channel into an 8 Ohm load, and over 780 watts/channel into a 4 Ohm load.  Momentary output into a 2 Ohm load exceeds 1200 watts. The amplifier is completely stable and will not be damaged even when driving a 1 ohm load.

When driving magnetic speakers, the ESL Monoblock Power amplifier will deliver over 360 watts/channel into an 8 Ohm load, and over 700 watts/channel into a 4 Ohm load.  Momentary output into a 2 Ohm load exceeds 1000 watts. The amplifier is completely stable and will not be damaged even when driving a 1 ohm load.

Many modern amplifiers are extremely inefficient.  Enormous amounts of their power, in fact most of it, is wasted as heat.
Audiophiles who prefer to leave their amplifiers on continually have discovered that a large, inefficient power amplifier can add over $100 per month to their electric bill.  In a ten-year period, their amplifier could cost them $12,000 to operate!  To produce so much waste heat, some of these amplifiers even require special mains wiring.
At Sanders Sound Systems, we believe that the use of such amplifiers is absurd, unnecessary, and environmentally irresponsible.  Proponents of these amplifiers claim that inefficiency is necessary to keep distortion at very low levels.  Many years ago, this was true.  But with modern technology, it is possible to make amplifiers that are extremely efficient while still maintaining vanishingly low distortion levels.
Sanders Sound Systems’ ESL amplifier has un-measurable distortion levels (less than 0.01%) from 20 Hz to 20 kHz up to the onset of clipping.  It does this while dissipating only three watts at idle and actually idles cold to the touch.  It may be left on indefinitely without concern for electricity usage.
The weird phase angles and high reactivity of ESLs tends to cause conventional amplifiers to become unstable.  The Sanders Sound Systems ESL amplifier is specifically designed to be unaffected by reactive loads.
The result is an amplifier that is completely stable under all conditions -- even at turn-on where no muting circuit is needed or used.  There is no "pop" or "thump" at either turn-on or turn-off and the amplifier switches on instantly.
Many of today's best amplifiers are so large and heavy (over 100 pounds) that one person cannot lift them.  They cannot be placed on a shelf or in an equipment rack.  Some are so big that they are split in two parts a "mono-block" for each channel.  It often is difficult to find a place on the floor to put them.  Many spouses are less-than-pleased about having such large amplifiers cluttering their living space.
To produce large amounts of power for driving resistive loads in highly inefficient amplifiers, it is necessary to use outrageously large and heavy power supplies and huge heat sinks.  So it is not surprising that such amplifiers are immense.
Despite its vast output potential, Sanders Sound Systems's ESL amplifier weighs 52  pounds and is sized scarcely larger than a full-sized preamp.  Its dimensions are 17" wide x 5.5" tall x 16" deep(43cm x 14cm x 40.6cm).  The Monoblock version of the ESL amp is the same small size as the stereo amp, and even two of them will fit in most racks.
Sanders Sound Systems has achieved this remarkable compactness by using a chassis made mostly of lightweight machined aluminium, and using the power supply to drive speakers instead of having its power converted to waste heat.
Because the ESL amplifier generates so little heat, the heat-sink requirements are greatly reduced.  Additionally, high-efficiency heat-sinks are used that make it possible to further lessen the weight and size of the amplifier.  Nor is this compactness achieved at the cost of having a noisy cooling fan.  The amplifier is completely silent.
The quality of construction is typical of the finest components made today.
Resistors are high reliability, precision metal film, 1% tolerance.
Capacitors are of the highest quality and none are used in the signal path.
Connectors are either gold or rhodium plated.
Both balanced and single-ended connectors are standard.
The finest quality, WBT binding posts are used. These are widely spaced and positioned at a 45 degree angle for ease of use and speaker cable routing.
There is no wire in the signal path save for a 2-inch piece connecting the circuit board to the output terminal.
The amplifier runs very cool.
The amplifier is completely modular for easy servicing or upgrading. The amplifier uses so little power at idle that it can be left on continually for optimum performance and longest life without concern for power usage or taxing your room's air conditioning system.
A massive output stage using 18 transistors makes the amplifier extremely rugged and reliable. It will drive the most difficult (low impedance and reactive) loads without strain or damage, which is 
why it is ideal for driving electrostatic loudspeakers.
The amplifier is completely stable and will not be damaged even when driving a 1 Ohm load.
Colour - Customer's choice of black or silver anodized machined-aluminum faceplate with black chassis.
Although called the "ESL amplifier", the same features that make it ideal for driving difficult electrostatic loads also make it ideal for conventional, magnetic loudspeakers, which are easier to drive. It is not limited only to electrostatic speakers. It is particularly good at driving planar magnetic speakers that have very low impedance and require powerful amplifiers.
The mains voltage is user-selectable from the rear panel, so the amplifier can be used anywhere without having to be modified at the factory for different voltages.
I'm convinced.
Review by cmk - Asylum forum member:

One of my key goals was to try to re-create life-like dynamics and impact. How else do you get the sense of a real performance in your room if you cannot feel it? The start of a note should be incisive to give this impact. With this amp, I did not feel any lack of power, compression, driving 93db/w speakers of course made it an easy load. Nevertheless, the Baltics are very reveiling of up stream components so I never felt anything was amiss. Instrumental decay lingered on till the black silence of the "end of track". 

Exrended review of Sanders Sound Systems ESL amplifier Amplifier (SS):

I happen to chance upon this amp one afternoon at the local dealer's showroom, driving a pair of Vandersteen 2CE SigIIs. Initial impressions were that it was physically reminiscent of Coda amps, which was a good start point, I had always liked Coda's purity of treble. Siting down for a brief listen was a "mistake", I was taken back by the amp's ultra clean treble, free of hash, vocals had a palpable presence, and the bass was just amazing, tight fisted control, deep and tuneful. Up till then, I had never heard the 2CEs with such performance, and as a previous owner of the 2CE Sig, I knew what they sounded like. 

Upon reaching home, I was haunted by the pure sounds from the Vandies, so pristine and pure. I had to get a home trial/demo to confirm if it was really the amp that was responsible. 
Carrying it back, I heard something rocking internally and thought something had come loose. It turned out to be the ultra big transformer, and was told that I could tighten the screw below. After I fired up the unit, it worked fine, so I left it at that. 
Having used tube amps for sometime (the more linear sort), I was used to having some body in the music and felt that accurately rendered a performance. Putting the ESL amp in place, it had the same body but gave some further insight into the music, little details popped out, largely due to the dark background that now surrounded each performer. 
On the Cabasse Baltics, the treble took on greater clarity, cymbals ringed without sounding harsh or distorted. In the all important midrange, where tonal accuracy is paramount, instruments had body, presence, impact was heightened, and decay seemed to be lengthened. Each instrument seemed to occupy a distinct place, while the soundstage extended beyond the speaker and room boundaries. But the tone of each instrument could be heard as never before...I mean I could tell if it was a grand piano or an upright playing. The sounds were similar to really good SE tube or SS amps, think Lamm and Dartzeel and you get an idea of what I mean. 
Bass in my system is reproduced by the powered Thor sub, fed in parallel with the main speakers via Anticable speaker wires. Here I also noticed a distinct tightening up of the entire bass region. Bass notes went deeper, with greater definition and weight. 
One of my key goals was to try to re-create life-like dynamics and impact. How else do you get the sense of a real performance in your room if you cannot feel it? The start of a note should be incisive to give this impact. With this amp, I did not feel any lack of power, compression, driving 93db/w speakers of course made it an easy load. Nevertheless, the Baltics are very reveiling of up stream components so I never felt anything was amiss. Instrumental decay lingered on till the black silence of the "end of track". 
Does it sound like a tube or SS amp? Well neither, and both. It was the purity of tone so typical of SE tube amps in the treble and mids, while it also possess the tight fisted control of the bass typical of SS amps. Yet the whole musical reproduction sound coherent and altogether musical. This amp is different from typical power amps going before, its neither hot - like class A amps, but sounds cleaner than a typical class A amp. I would suggest a read of the Sanders' white paper on their ESL amps to understand what he has done. I'm convinced. 
Sanders Sound System website
Many customers ask about how we can run our amplifiers so cool. The answer is that we can use very low bias current because we use modern, highly linear transistors that do not require high bias current in order to have vanishingly low distortion. Many audiophiles have assumed that hot, high-bias operation results in "warm" sound, and so they are surprised that our amplifiers sound so good with low bias. They wonder how bias affects sound.
The purpose of bias is to reduce amplifier distortion. Bias has no effect on frequency response, and it is frequency response that defines "warmth", "richness", and "fullness" in sound. In short, changing the bias will not add any "warmth" to the sound; it will only add warmth to the amplifier chassis. 
Different amplifying devices have different transconductance curves, whether they be tubes or transistors. Transconductance is where a certain amount of input voltage results in a certain amount of output current. There is a ratio between the input voltage and output current. This should be a constant at all power levels. Of course, no amplifying device is perfect, so the ratio is NOT constant. Distortion will develop to the degree that a device deviates from a constant transconductance ratio.
Typically, very small input voltages will not produce the same transconductance ratio as input voltages that put the device in the middle of its transconductance curve. In fact, the ratio will be zero at very small voltages, because the device won't conduct at all until a certain threshold is reached -- this can be seen as "crossover notch distortion" on an oscilloscope. Likewise, when a device nears its maximum power, the ratio changes as it can no longer deliver more current as the input voltage increases ("clipping").
The purpose of the bias current is to put the amplifying device into its linear operating range, and this will vary greatly depending on the type of device and its particular operating parameters. Typically, tubes are very non-linear and require quite a lot of bias to reach reasonably low distortion levels. Power MOSFETS are only slightly more linear than tubes, and the best bi-polar transistors are far better than either tubes or MOSFETS.
Before the discovery of negative feedback, some engineers even biased the tubes to the center of their transconductance curve (Class A operation). This was the only way to get distortion down to reasonable (around 3%) levels, but had the penalty of extreme heat generation and rapid tube deterioration.
Transistors vary widely, with power MOSFETs being the less linear than bi-polar types. So generally, MOSFET amps will need more bias to reach low distortion than bipolar types. And there are very different transconductance behavior with different transistor designs in the same class, so bias may vary widely.
Keep in mind that all modern, well-engineered amplifiers use negative feedback (NFB) to reduce distortion to levels far lower than can be achieved with bias alone. An amplifier's distortion is a combination of both bias and negative feedback. Keep in mind that for most humans, the threshold of distortion detection is around 3%. If very special test tones are used, some people can hear about 1% distortion. No test has ever shown that a human can hear distortion levels below 1% under any circumstances.
Sanders Sound Systems amplifiers use a very modern, sophisticated, and expensive type of bi-polar transistor made by Motorola that combines very high power capability with an amazingly linear transconductance transfer function. As a result, we are able to reach virtually un-measurable distortion levels with only a trace of bias.
Without bias or NFB, a typical amplifier will have high distortion levels (perhaps 10% or so, depending on the amplifying device). With enough bias applied to move the device into the linear area of its transconductance curve, the distortion will drop to around 3%, or even lower with very linear devices. The bipolar transistors Sanders Sound Systems uses can reach distortion levels of 0.08% without the use of NFB. Then very little NFB is required to reduce the distortion to non-detectible levels (less than 0.01%).
Some audiophiles remember the days when huge amounts of global NFB was used in amplifiers and this caused problems with TIM (transient intermodulation distortion). A few audiophiles still believe that NFB is undesirable because of TIM. But this is no longer true. Engineers now use NFB around the local circuits, use only a small amount of global feedback, and compensate it properly. The result is that NFB is now free of problems and has no adverse affects on an amplifier's performance. This is a good thing because it is impossible to make a low-distortion amplifier without the use of NFB.
Now let's look at the Sanders Sound Systems ESL amplifier without any NFB and see what effect bias alone has. With no bias at all, crossover notch distortion is present and the distortion is unacceptably high. With just enough bias to turn on the transistors, the distortion suddenly drops to around 0.2%. If we keep turning up the bias, we can reach a minimum distortion level of .06%. This is remarkably low distortion for an amplifier without any NFB. But this much bias requires a continuous power dissipation of more than 70 watts of power.
But why should we waste all that power? If we turn the bias down to just 3 watts, the distortion only climbs to about 0.2% -- still well under the 1% human distortion detection threshold. But we prefer lower distortion levels. So by adding just a little NFB, we can cause the distortion level to become un-measurable (less than 0.01%), and there is only 3 watts being wasted as heat.
In summary, there is no magic. Sanders Sound Systems amplifiers sound completely clean because they have very low distortion and lots of power. How this is achieved has nothing to do with its frequency response. We use a combination of very good transistors, low bias, and low NFB to achieve this performance, while keeping the amplifier running cool. Adding more bias will not change the sound of the amplifier in any way. 


A complete Sanders system comprising Sanders 10 ESL speakers, 2x Magtech 500w amps & Magetch Preamp