AM AUDIO RS Passive preamplifier fully balanced transformer (4) type.

AM 01 PA RS
NZ$ 3,995.00 ea (incl. GST)
AM Audio

The ultimate purpose is to be able to reproduce 'genuine music' not just detail

New

"you will find yourself with one of the best sounding preamps available. Add to that the exquisite build quality and spectacular design and you're looking at a great deal with the AM-RS"......  Maarten van Casteren (TNT UK AUDIO).

To some audiophiles, a Passive Preamp sounds odd and mysterious but to some experienced audiophiles, passive pre-amplification may represent a pure and simple method of obtaining high fidelity. Anyway, what is a Passive Preamp? 

Simply, a passive means no active circuitry, no connection to AC wall power, no internal power supply. Nowadays, at the request of convenience, the need of remote control function has narrowed the concept of passives.

In today's modern era 'no power on the analogue circuit' is probably a more precise way to describe a modern 'Passive Pre. 

Passive Magnetic - is a high level type of Passive Pre. Magnetic refers to the use of multi-tapped attenuation transformers instead of resistive volume control devices. However, design and manufacturing of this type of transformer is quite a complicated task.

With Audio Music also being a high end audio transformer supplier, which has dedicated many years of research and manufacturing tomaudiophile grade transformers. Housed inside refined high quality aluminium alloy chassis, with our knobs AMR-S presents one of the finest sounding and value for money Passive Preamplifiers in today's market period!

Why Choose A Passive Preamplifier?
For the audio fundamentalist who pursues a flawless neutral sound without any contamination from their devices, the passive is the preamplifier of choice. Theoretically, all that's required from a passive preamp is source selection and a volume control via signal reduction. Ideally, a well designed and built Passive is supposed to link the source and amplifier with necessary controls over the signal. Any altering, rendering and amplification would be considered detractive, however, as a qualified link device, it should also be a good match electronically both to source device and amplifier.

Why A Passive Magnetic Preamplifier?

Entry level Passive Preamplifiers usually refer to resistive passives which suffer impedance mismatches and concomitant frequency response aberrations depending on settings and cable load. Traditional Passive Preamplifiers impose large impedance values with additional resistance in part of accomplishing attenuation. They require the source to provide the power to drive the volume control's resistance. 

By comparison, transformer-based attenuation with its electromagnetic coupling does not rely on the addition of resistance. The transformer itself has very high impedance across the audio band.

Accordingly, very little of the source's output power is lost and only the very low winding resistance appears in series with the source prior to the preamplifier's output. Thus the source only needs to drive the load and cables connecting to and from the Passive Preamplifier. However, as the Passive Magnetic Preamplifier's attenuation is increased (preamplifiers in most systems end up cutting the signal by 20dB or more at all times), the output impedance rapidly falls to very low values. This in turn translates into greater improvements in its ability to drive a load or cable. 

In fact, a transformer simply takes the drive ability of the source and passes it through with only a? slight decrease over the source when any significant attenuation level is used.

The use of transformers dramatically improves on the ability of the Passive Preamplifier to drive cables (and loads) compared to traditional designs. In addition, the use of transformers will actually improve the ability of the source to drive its cables as well, which explains the experiences of experienced audiophiles who have found the use of the Passive Magnetic Preamplifier to be a substantial improvement over using no preamp at all.

The AMR-S is fully transformer-balanced input to output design. the standard transformer windings are copper but silver versions are available. Both RCA (unbalanced) and true XLR (balanced) inputs and outputs are equipped or optional. RCA solution should be sufficient for most serious systems. However for the discerning audiophile an XLR or combination of XLR and RCA inputs and outputs for convenience can also be specified.

Features

Specifications

Reviews

Features

  • As a serious high end Passive Preamplifier, the AM passive is essentially unbreakable. There are no coupling capacitors to age or blow up, no resistors to fail. The AM units are true isolation transformers, with separate primary and secondary windings. Gain in most preamps is redundant when the majority of amplifiers are driven to full output by the source voltage. Superior amplifiers don't need assistance from preamps except for the bare functionality of attenuation and input switching. If you already own a high level amplifier, the AM will be the best match to make your amp shine. With full bandwidth, regardless of setting and no impedance problems, this is a passive that doesn't sound flat, boring and bereft of life, especially used in balanced mode. 
    • Totally Passive design with zero distortion and no compromise.
    • True balanced input/output design with four transformers which isolates components, breaks ground loop problems and reduces noise.
    • Dual mono, dual volume switches.
    • Dual 0/6dB switches option.
    • Seiden high quality 33 position stepped attenuators.
    • 3 RCA inputs 3 XLR inputs (as standard).
    • RCA outputs XLR outputs.
    • Total number of inputs (3) XLR or RCA on each of the three inputs.
    • Point to point wiring with Teflon coated solid core copper wires.
    • Silver wired is optional.
    • Full Heavy duty refined aluminium alloy faceplate and knobs
    • Beautiful Acrylic made body for enhanced cosmetics and isolation.

Specifications

The use of 4 transformers attenuation is now with 33 volume steps. The selector is of exceptionally high quality.
True balance input/output design with four transformers attenuation which isolates components, break ground loop problem, and reduce noise. 
33 step volume attenuation with remote control based on 33 transformer output terminals. 
3 single-ended or RCA input, 3 balanced or XLR input .
2 single-ended or RCA output, 2 balanced or XLR output .
Point to point hand wiring with Teflon coated solid core copper wires. 
Full Heavy duty refined aluminium alloy body, and faceplate. 
6dB passive gain, so low gain amplifiers can be driven to their maximum efficiency by components that otherwise might require active boosting. 
Dimensions: 340W x 300D x 110H mm 
Weight: 8.5 Kg net / 11 Kg shipping weight 

Reviews

you will find yourself with one of the best sounding preamps available. Add to that the exquisite build quality and spectacular design and you're looking at a great deal with the RS.
Maarten van Casteren

REVIEW SUMMARY: The build quality is truly exemplary. Very, very impressive. The TVCs are wound by AM themselves, and the whole preamp is made from a single block of acrylic, with spaces for the transformers, switches and wiring routed out. The top and bottom are made out of thick sheets of stainless steel. The result is amazing, with a solidity that I haven’t experienced before. There’s absolutely no ringing or even any movement possible. The preamp sits on three stainless steel conical feet, again absolutely stabile. The whole look and feel of this preamp is completely unique and it really deserves to be shown off at the top shelf of your rack.

Typically the best active preamps can still beat a transformer preamp on bass performance and drive but NOT the AM-RS I think, and if there's an active pre out there that is seriously better, then I'd like to try it.

EXTENDED REVIEW: Yet another transformer preamp, I know, I know. I apologise, but they do happen to be one of my favourite types of audio gear, and this one looked so special, I just had to try it. Actually, it not only looks great, but it’s also a fully balanced design which is something that I haven't experienced before.

All transformer preamps I have tried until now had a single transformer for each channel. You could call them single ended for that reason, but in the world of transformer volume controls things are not that simple. That's because a transformer is capable of converting a balanced signal into single ended and the other way around. In pro audio they are used precisely for this purpose all the time, so saying that a transformer is either single ended or balanced is basically wrong: they can be both. That's not to say that any transformer will always be able to handle both single ended and balanced signals. For proper balanced signal handling the transformer will need a centre tap, making sure that both sides of the output are fully identical relative to ground. But, actually most balanced power amps aren't that picky and will happily accept a 'pseudo balanced' output from a transformer without centre tap. On the input side things are even simpler: a centre tap is rarely necessary there.

My trusty Django, which uses a pair of the famous S&B TX102 transformers, has a full complement of both single ended and balanced inputs as well as outputs. It will mix and match between these without problems, and the only thing to remember is a little switch at the back to select the appropriate output mode, which will only change the grounding. But, obviously, the Django only has a single set of transformers and the balanced connections lack the centre tap. This has never been a problem for me, and I have used the Django for over 7 years now with all sorts of sources, power amps and active loudspeakers. It always worked fine, single ended as well as balanced.

Then I discovered the Audio Music R-S. It doesn't use a single transformer per channel, it uses two. This preamp really is fully balanced, with a separate TVC (transformer volume control) per side of the balanced signal. That’s balanced volume control done properly, a fact that is proudly shown on the top of the R-S, where 4 transformers stick out of the housing. This separate handling of both sides of the signal prevents them being summed and then split again, although one could argue that this can be done in a transformer without any penalties anyway. What it does mean is more headroom, I assume, as the load is now split between two transformers.

Obviously, using double TVCs also means using a double set of switches for both input selection and volume control. All of this makes the wiring inside quite complex, and perhaps for that reason it has separate volume knobs for left and right. The only other controls, beside input selection, are two small switches at the top towards the back. They allow you to increase gain by 6dB. My Django also has a similar gain switch, which I have to admit I have never used. Actually, I've almost never had the volume beyond half way up, so I suspect that the additional 6dB isn't necessary in most cases.

One thing the Django has is a switch to lift the ground, essentially breaking any direct electrical connection between input and output. This can break ground loops, and is one of the unique features of transformer preamps. Except that the R-S does not have one. The documentation online mentions the fact that ground loops will be broken as one of the big advantages of transformer preamps, but when I checked I found that ground is simply connected from all inputs to all outputs in the R-S, without any option to break this connection. I didn’t have any trouble during the review period, but still think this is a bit of a missed opportunity.

Just like the Django, the AMR-S features single ended as well as balanced inputs and outputs. But when using balanced input with a single ended output, only a single pin of the balanced signal is used: the single ended output is simply taken from the transformer connected to the hot pin of the XLR input. Of course this will work just fine, and even sound OK, but it certainly is not a proper conversion in the real sense of the word. The same could be done with a simple lead that only connects RCA to the hot pin. Although this will serve fine for secondary sources, if your main source is balanced I suppose that you'd like to use the full signal from that source, and not just half of it.

The situation gets worse when using single ended input with balanced outputs. Again, only the hot pin of the output XLR connectors is now being driven. This is not a safe way to drive a balanced amplifier, so AM does not recommend this configuration. That still means there's a certain risk involved with the fully balanced setup: if you have single ended secondary sources, like a TV or an MP3 player, it would be tempting to also connect them to your main system through one of the RCA connectors. It is difficult to say what would happen: some power amps will work without any problems like that, but others might cause trouble. I don't think it's a very good idea, to be honest, and I didn't even try it out. I wouldn't recommend it either.

All this leaves the number of possible configurations rather limited in my opinion. Using this preamp with a single ended power amp is a bit wasteful, as you are now effectively only using half the preamp, even with balanced inputs. Using it with a balanced power amp, or balanced active speakers, makes use of all transformers, but now you are limited to fully balanced sources only and have to remember to never use a single ended source. Personally I think it would have been much better to simply limit the input and output options to balanced only, and market this preamp as such.

Build quality is truly exemplary. Very, very impressive. The TVCs are wound by AM themselves, and the whole preamp is made from a single block of acrylic, with spaces for the transformers, switches and wiring routed out. The top and bottom are made out of thick sheets of stainless steel. The result is amazing, with a solidity that I haven’t experienced before. There’s absolutely no ringing or even any movement possible. The preamp sits on three stainless steel conical feet, again absolutely stabile. The whole look and feel of this preamp is completely unique and it really deserves to be shown off at the top shelf of your rack.

There's one additional control at the back, which I only noticed at the end when I was taking the photos. It is a potentiometer, but without a knob attached, so I assumed it was set in the factory and not meant to be fiddled with by the user. But then, when I returned the preamp to Iain Borthwick, who imports them for the UK, he told me this was a control to set the output impedance. As far as I know this should simply be as low as possible and you do not need a control for that, but Iain demonstrated the effect to me in his rather nice demo system. The effect was clearly audible, but the best sounding setting was with the control all the way down, so basically out of circuit.

My final point concerns the double mono volume control...... this way you also get a balance control, which is an undeniable advantage. Some people might also find the lack of remote control a problem, but I personally don't care about that, certainly not with a preamp that has such a nice, solid feel to its controls.

Sound

I started using the AM S-R with my OPPO 105 universal player and my Unity Audio 'The Rock hifi' active speakers. Both of these feature balanced connections, so I could benefit from the fully balanced design of this preamp. The sound was an instant success: solid, sweet, powerful, clean and natural, just what you would expect from a good TVC preamp. Especially the bass was impressive, with excellent depth and power. Used like this, I have to admit this preamp is serious competition for the Django with better bass and equal sound quality for the rest of the spectrum. That makes it a seriously good preamp, one of the best I know.

The solidity of the sound is the defining characteristic of the R-S. Top quality TVC or AVC (autotransformer volume control) preamps tend to be very good at this, much better than resistive passive preamps, but typically the best active preamps can still beat a transformer preamp on bass performance and drive but NOT the AMR-S I think, and if there's an active pre out there that is seriously better, then I'd like to try it. Most telling is that the bass performance of the OPPO when connected directly to my active speakers is not better than when used with the AMR-S, which is very impressive indeed. The direct connection still wins in terms of impact and dynamic expression in the mids and upper range, but even there the difference is small.

Used single ended the difference with the Django becomes smaller, to the extend that it is difficult to decide which one is better. When using the R-S with the balanced outputs of the OPPO and the single ended inputs of the Rock loudspeakers, the sound lost some of its weight and impact compared to the balanced outputs, but the difference wasn't very big. It is a perfectly fine sound, but the knowledge that you are only using half of the balanced output of your source, and only half of your preamp makes me a bit uncomfortable, but for secondary sources this would be perfectly fine. Thing is, a single TVC preamp would work just as well in those circumstances, or perhaps even better as it will utilise both sides of the balanced input.

Conclusion

This is a completely balanced design, and although it can be used in other ways I don't think that makes much sense. Only when all your sources as well as your power amp are balanced will you get the benefit from the double TVC design of the Audio Music RS. If you ca use it fully balanced then you will find yourself with one of the best sounding preamps available. Add to that the exquisite build quality and spectacular design and you're looking at a great deal with the RS.

an Audio Excellence award is therefore entirely appropriate.
MARTIN COLLOMS

REVIEW SUMMARY: Unquestionably, and from the very beginning it was clear that this was a very high quality level control. The sound has a tidy, capable character, free of any emphasis, almost perfectly neutral. It has a sense of calm, of inner peace, and an almost ‘creamy’ timbre.....This beautifully made and finished double mono transformer volume control is of excellent quality in fully balanced mode, and still sounds very good in single-ended mode, an Audio Excellence award is therefore entirely appropriate.

EXTENDED REVIEW: Amplifier maker Audio Music is the brainchild of Am Fang of Guangzhou, China. His company was originally founded in 1993, and his creations were marketed under the StereoKnight brand for many years, primarily in the USA. A falling out between partners led to a complete reorganisation and the introduction of similar but improved products under the Audio Music brand in 2013. UK distribution is via LW Audio, a relatively new distributor founded by Iain Borthwick who feels that Chinese sources can offer remarkably high standards of performance at unexpectedly reasonable prices.

This  AM-RS passive control unit is just one of a number of amplification components from the company, all superbly built, excellently finished, and with a strong bias towards valve technology – AudioNote UK is a major source of inspiration. Construction consists of a very heavy anti-vibration core milled from solid acrylic, encased in heavy alloy cover plates. Inside is found top class Japanese Seiden 34-position attenuator switches with ball detents and double arm gold-plated wiping contacts, alongside no-expense-spared, high-nickel transformer-core attenuators. These are screened, mu-metal potted and connected up with single-strand Teflon-insulated point-to-point hard wiring, with no printed circuits or plug and socket connectors. In fact this unit is about as hard-wired as it gets. 

With double mono construction and a +6dB gain switch (albeit with reduced performance), the unit has just a little gain at full level when used in single ended mode. The signal paths which are balanced use four transformers. A double-mono 31-step control was found to have slightly unusual progression of volume steps, typically of 1-1.5dB resolution in single-ended mode. Maximum attenuation is a modest 44dB.

 In some cases the maximum attenuation may not be sufficient to set a low enough whisper quiet sound level if using the combination of a loud source, a well modulated music track plus a sensitive power amplifier. An eye therefore needs to be kept on system matching for a given set of audio components, perhaps also taking into account the loudspeaker sensitivity. 

Its earlier StereoKnight incarnation included a more complex version where the whole unit was relay operated and it came with a remote control handset. There was also a fully silver-wired option, but the current copper wire version is said to exceed the quality of the previous models. 

While a single-ended input will appear at the balanced output XLRs, and a balanced input can also be output from the single-ended connection (all done without any switches in the signal path, aside from the Seiden input selector), there is a catch. The transformer windings and connections are not arranged such that it is universal (ie so that a single-ended input will appear as a balanced output). Instead only one phase of the balanced output will be exercised with a single-ended input. Therefore in both theory and practice the best performance is available when used in fully balanced mode. However, I still liked it in single-ended mode, so high was its intrinsic quality.

Sound Quality 

Unquestionably, and from the very beginning it was clear that this was a very high quality level control. The sound has a tidy, capable character, free of any emphasis, almost perfectly neutral. It has a sense of calm, of inner peace, and an almost ‘creamy’ timbre. 

Somewhat belying the perfectly flat frequency responses found during the lab tests, the tonal balance here was judged to be very slightly mellow, but with very good bass extension and definition. Image focus was quite exceptional and image depth was close to excellent. 

Dynamics seemed very slightly curtailed, and while fine tuning the rear panel load matching control helped make some adjustment in this area, it was not found to address this aspect of performance completely. Conversely, adjusting this control did help optimise the sense of good rhythm and timing, which can fall off if the setting is too low. That said, while this control unit’s performance is undoubtedly very good on rhythm, we felt that we could not extract the last possible degree of this particular quality with the AM-RS. 

There was a moderate loss in ‘converting’ from single-ended to balanced operation, and likewise from balanced to single-ended mode because the internal design is fully balanced, and the single-ended option only makes use of one signal phase. (A single-ended input does not arrive as fully balanced at the output.) It sounds best in fully balanced mode scoring a reference level 135 marks, falling to a still very good 120 when working SE-to-SE. 

Lab Report 

A pre-set user-adjustable trim potentiometer on the back panel allows for fine tuning for unusual source or load impedances, and has a small effect. You can see it working on pulse signals, where it partly trims the rise time and overshoot on fast pulses, the effect varying with the attenuation and loading chosen. Set too low, it can become over-damped. 

The control has 31 increments, and measured in single-ended mode these were from 0.8dB steps at the top of the range, then increasing to 1.3dB by -10dB, then to1.5dB at an indicated -15dB (here attenuating 18.5dB). I measured 2dB steps by -20dB, and then typically 1.25dB steps to a maximum of -44.3dB, which is not considered a very deep attenuation. 

Channel balance was excellent, within 0.05dB over the control range. Also excellent was the frequency response: for a 600ohm source and with the level set at -6dB, the response was perfectly flat in the audio range, reading 10Hz to 85kHz at -0.5dB. (With a 20ohm source it was just -0.2dB at 10Hz.) High frequencies extended very well to 125kHz and showed a minor 2dB rise at 120kHz. 

As expected, at -6dB (a rather small attenuation) the output impedance still somewhat reflects the source impedance, and it measured 150ohm from a 600ohm source and a correspondingly lower 7.5ohms from a 20ohm source. These passive controls get into their best working range several dB below full level. Thus a single-ended input power amplifier with a low sensitivity might not be suitable unless the planned sources have a higher than usual output level. 

At useful lower volume settings the output impedance is very low: for example at -20dB, from a typical 50ohm source, the output from the control unit will be sourced at less than 1ohm. Lots of headroom is provided in the generous transformer cores: for example with a worst case 6dB of gain lift and with 22V output, the 1kHz distortion was less than 0.07%, and of low order harmonic order. At 2.5V output it was just 0.004%. There is a little more distortion at very low frequencies eg 0.22% at 20Hz (unity gain), then falling to 0.1% by 40Hz and 0.015% by 100Hz. At 20kHz it hit an all time distortion low of 0.0001%! 

Conclusions 

This beautifully made and finished double mono transformer volume control is of excellent quality in fully balanced mode, and still sounds very good in single-ended mode. an Audio Excellence award is therefore entirely appropriate.