Interview with Robert Lee (Acoustic Zen) by Chip Stern - 6Moons

Between the two drivers, is one handling low frequencies and the other both midrange and bass as in a 2.5 way design?

No. It is a true D'Appolito-style array. Both 6.5" drivers are wired in parallel and handle 30Hz to 3000Hz. This style design employs two low-frequency drivers with a tweeter in the center. Such a configuration creates a larger sweet spot. It gives you greater low-frequency extension along with two-way cohere

Tell me about your woofers. How does an underhung driver differ from a well-hung driver?

Underhung means that it has a very short voice coil in a large magnetic field gap. The drivers are top grade and so are the magnets. The voice coils are very narrow and always move in a linear manner within a magnetic field regardless of excursion. Additionally, a traditional long voice coil moving in a short magnetic gap presents a high variable impedance to the amplifier.

Can you break it down in some more detail?

With a short voice coil in a long magnetic gap, as the music signal flows through the voice coil, the coil and the cone moving together produce the sound wave. A long magnetic gap structure offers a much wider linear magnetic flux density, alternately pushing and pulling the short voice coil back and forth. Since the magnetic force on the coil is constant and completely symmetrical with the motion of the piston assembly, the result is a sound wave with extremely low distortion and coloration, allowing it to respond much faster to a transient signal and to stop faster after the drive signal has ceased.

Compare that with the large signal behavior of high-excursion overhung transducers. Because the coil extends beyond the control of the magnetic flux field, it causes very high harmonic and transient distortion that can reach 5 - 20% whereas the underhung driver maxes out at 1%. Because of the very complex magnetic structure and complications involved in voice coil placement and fabrication, the underhung voice coil is difficult and expensive to fabricate.

What are the speaker cones made of?

They use a ceramic-impregnated fabric in the center with ceramic coating on either side to form a sandwich. These drivers are custom built for Acoustic Zen to my specifications. The ceramic coating makes for a strong, dense, rigid surface area that will not break up nor produce a lot of ringing. The cone is very stiff and light and as a result, very fast. It responds immediately to a signal and returns to its original position very quickly. This minimizes frequency overhang, colorations and distortions.

Why a Neodymium magnet?

An underhung driver's voice coil requires an extremely strong magnetic field. If you are using traditional magnets, the size and weight of the motor would be excessive. Neodymium allows us to maximize magnetic coverage while controlling weight and size.

Why did you choose a ribbon tweeter and how does your circular design differ from other approaches?

I don't want to get too technical here. My circular ribbon tweeter design, unlike a traditional dome or cone tweeter, presents a purely resistive impedance that's completely linear in the audio range. It also has an essentially linear phase response, which contributes to an immediate and precise response to any transients in a complicated music signal. This dramatically reduces distortion and coloration.

I am using the thinnest diaphragm possible, a 0.01 millimeter Kapton diaphragm. That's just about weightless, with a 95% covered aluminum circle of conductors across the entire vibrating area positioned between super strong Neodymium magnets. Some people use metal ribbons which can result in very harsh HF response. Kapton is not only extremely light, it is also heat resistant, hence the thermal behavior of the voice coil will not translate into audible distortions. My tweeter's larger circular membrane not only provides much higher power handling and a wider frequency response, it also eliminates the offsets between horizontal and vertical dispersion that are common with narrow ribbons.

What kind of crossover are you using?

We employ a symmetrical 18dB/octave 3rd-order high and low pass.

Some people believe that a simpler crossover gives greater sonic purity - that the more complex the crossover, the less natural the sound.

A 1st-order network would have been impossible in this system because of the overlap of frequencies. You have to use a very complicated audio circuit to filter down the frequencies that will cause issues with both impedance curves and distortion. Because our crossover point at 3000Hz is fairly high, we don't want the tweeter to handle midrange frequencies or vice versa. We want the woofers to cut off almost immediately. Because of the kind of speed and accuracy we wanted to achieve in the low frequencies, a 6.5" driver offered the best balance of linearity, frequency extension and speed. Larger drivers would cause other problems when paired with a 1.5" circular ribbon tweeter.

What was your thinking in terms of the shape of the cabinet where it gradually narrows toward the back?

The shape of the cabinet and the reduced size of the acoustic chamber/line subtract by almost 90% the back waves, standing waves and other distortions reflecting back to the drivers. The shape of the cabinet and the time-aligned vertical driver array reduce driver diffractions while providing a good acoustic center which improves soundstaging.

The sides of the cabinet are 1" MDF while the mid/woofer front baffles add up to a total of nearly 2" depth. Between the baffle and the cabinet face, there is an additional layer of decoupling damping material to absorb driver motion.

Tell me about the transmission line.

The use of a transmission line greatly increases low-frequency extension and minimizes the impedance peaks we see in ported alignments.

Why only one set of speaker terminals?

I am using our Acoustic Zen Satori speaker cabling for hookup wire which is 10 AWG/6N zero crystal copper. That is far better cabling than most people have going from their amplifier to their speaker. I really do not want for people to alter these relationships. I have already fine-tuned the frequency response and tweaked all phase adjustments. If somebody employs a bi-wire setup, it could alter the overall balance so I prefer a very simple, direct connection.

If you have unlimited funds and unlimited size, you can make your loudspeaker do anything you want. Then you can end up with a Wilson Audio Alexandria, which was pretty bloody awe-inspiring when I heard it in a tuned room at Innovative Audio in New York. But why the hell shouldn't it be? It costs $125,000/pr, weighs several hundred pounds and is taller than Wilt Chamberlain. There are no practical limitations. I found that your speaker had some of the best attributes of a good two-way and a floorstander. Still, at its $4,300 price, there have to be some compromises. What were the relative trade-offs you had to address? The Adagio does more things well than most speakers I've heard at its price yet naturally, it won't have the frequency extension of a Vandersteen Quattro with its built-in subwoofer. Nor does it have the immense soundstage and ambience retrieval of the Dynaudio Confidence C1. However, both are more specialized speakers and both are significantly more expensive.

That is a good point and I would like to say this. There are thousands of speaker manufacturers worldwide but so far I have identified less that ten who employ underhung drivers. I think that underhung drivers are the best solution to reduce harmonic distortion in the bass. No one ever talks about the huge amounts of THD in the low frequencies, especially when you are playing things loud. Most drivers create from 10-12% THD when you listen at over 10 watts. If you are listening to 10 watts through your speaker system, you get 5% harmonic distortion. So what are you listening to? When people design amplifiers, they rate them at 0.05% harmonic distortion. Meanwhile even at modest volume levels, your low-frequency drivers put out 5% or greater harmonic distortion so nobody can listen at even low levels and achieve true purity with low distortion, never mind concert levels.

That's why I selected to use both the underhung driver and a ribbon tweeter of my own design. The advantage of ribbon tweeters is their low distortion. The drivers in the Adagios result in what I believe to be the lowest distortion loudspeaker system in the world - at any price. People are so concerned over distortion figures in their amplifiers and CD players and would never dream of accepting the levels of distortion their speakers routinely put out. Most audiophiles remain unaware of this simple truth.

Even when speaker makers achieve good phase coherence, they are still using high distortion drivers for low frequencies that impact the quality of the signal. When listeners hear the resolution and low distortion achievable by underhung drivers, I predict many other manufacturers will follow suit and employ them. It's like the use of single crystal copper which Acoustic Zen helped popularize in cables. Now many manufacturers employ single crystal copper and silver.

The only company doing something like I am doing now (using ribbon tweeters and underhung drivers) is Wisdom Audio. And their top-line speakers are far more expensive. Thiel has been at the forefront in this arena but they feature a different style of tweeter/midrange. So the Adagio is really a very unique loudspeaker product. At only four feet high, it is a very good speaker for the average rooms most people actually have. Unless you use a very, very large room, the Adagio should be just right for most real-world listening environments. People have been taught that they should get a really large speaker that has the potential to excite plenty of reflections from wall to wall; to excite bass frequencies and room nodes; to produce all manner of colorations and frequency cancellations. Not the Adagio. It is perfect for 15-20 foot rooms.

Without neighbors getting most of the bass [laughter]. Obviously, there are speakers that will do certain things better, yes?

I am already working on a subwoofer that will match the Adagios perfectly, handle all frequencies below 60Hz and go down cleanly to 18Hz. I think the only thing the Adagios cannot do is give you that punch and pressure and power below 30Hz. Yet you can still hear the bass clearly all the way to the bottom of the piano and when the low frequencies stop, the drivers stop instantaneously. There is a minimum of uncontrolled motion and overhang. That is another advantage of underhung woofers.

Your bass drivers do stop on a dime, with no discernible overhang. And the tweeter is not bright at all.

Other things that are very important to me as a designer are the phase and impedance curves. I think those are far more important than ultimate SPLs. You want those curves to be as flat as possible so you don't place a terrible strain on the amplifier. My phase curve is essentially flat from 10Hz to 40kHz. And the nominal impedance of the Adagios is 5 ohms. It is most flat from 100Hz to 10kHz, almost a straight line keeping within that 5 ohm impedance. You don't have different frequencies displaying wildly fluctuating impedances. Impedance is like resistance. So if different frequencies display different levels of resistance, the amplifier reacts and distortion is the result. As audio designers, we must reduce distortions wherever we find it to make the experience of listening to music more involving. That is the ultimate goal.

Robert Lee (Acoustic Zen) and Chip Stern (6 Moons)