MAC Autoformers?

@bifwynne , Well, since you asked, the answer is ’distortion vs actual frequency response’.

All amps make distortion. Some distortions (IMD and higher ordered harmonic distortion) are more audible than others. Of the two mentioned, the latter is what makes solid state amps bright and harsh, and does that to tube amps as well, although they are not as harsh as solid state because there is simply less of this form of distortion.
In a zero feedback amp, there is respectively (solid state or tube) even less. That’s why they sound smoother. The problem is, solid state amps have non-linear capacitances built into the junctions of the output devices. This capacitance is responsible for higher ordered distortions (brightness and harshness) that still affects a zero feedback solid state amp.

Since the ear uses the higher ordered harmonics to sense sound pressure, it is more sensitive to these harmonics than most test equipment, since it has to cover a 120-140db range! This is an insidious, inconvenient truth that the audio industry does not like to face. This is further complicated by the Fletcher-Munson loudness curves, which place many of these upper harmonics in the most sensitive region of the ear’s response.

That makes things particularly tricky.


The reason zero feedback tube amplifiers exist is that a good number of designers have come to recognize the problem of higher ordered harmonic distortion. One fairly easy way to sort that out is to avoid using feedback, since it is known to add such distortions of its own in the process of otherwise suppressing distortion. So this means that the designer has to use other means to suppress distortion.

This results in an amplifier with a fairly high output impedance.

The thing is, if you chose the speaker carefully, the result is **far** more neutral than is possible with the conventional approach (its not subtle- you can hear it right away, whether an audiophile or not). The reason this is possible has to do with two salient facts: first, no speaker made anywhere at any price is flat. Second, the ear converts distortion into tonality, and has tipping points wherein that tonality *is favored* over actual frequency response.

The latter fact is why two amps on the bench can have identical frequency response but one will sound bright and the other will not!

I’ve found over the years and decades that if the speaker requires the amplifier to employ feedback to get flat frequency response out of the speaker, that the result has no chance whatsoever of sounding like real music! At best it will sound like a very good stereo (this bit is tricky; many people are happy with the status quo of a good stereo, because they think getting it to sound like real music isn’t possible!). Its this difference where I have drawn the line, and why over 42 years we’ve avoided feedback.

**That** is why bother with a zero feedback tube amp. Sorry for the derailment.

excellent explination on the higher order harmonics.... Musicians know this stuff in and out and can appreciate harmonics big time since you tune string instruments for intonations,harmonics etc.... the music we hear or think about is usually more the many order harmonics instead of just a plain note. think Foriear transformans math and wave theory. same for light the eye likes vs the actual Frequency, back to the amps, they should not enhance and particular range or require feedback to quell a too bright range-Just increase current/voltage flatly from 0-30K. design your differential signal work in the pre and equilizer if you want. this can be seen on a RTA live. think when you tune your guitar on how super high the lead sounds when its only a few 1000hz,
 

A friend brought over a circulotron amplifier and we connected it to my Quad 63s I was immediately overhelmed with boomy one note bass. Knowing the Quads and most ESLs have high impedance in the bass I hooked up a generator to the amp and voltmeter to the speaker and found a very sharp 8dB peak at 50 Hz, the cause of the boomy one note bass. The treble was down several dB for the same reason; the non flat impedance curve. 

Cone speakers have similar impedance peaks at resonance in addition to peaks and dips in the entire curve due to multiple drivers and their crossovers. This cannot be ignored unless you really like the modification to the frequency response. Very few speakers have flat impedance curves as this is hard to achieve and still get good frequency response. I know of no speaker maker who designs for high output impedance amplifiers. 

One other point. The proper marriage of the ESL and OTL was the KLH 9 speaker and the Futterman amplifier. It was a good marriage for two reasons. The KLH 9 was a 16 ohm speaker that did not go below 12 ohms and the Futterman was a high damping amplifier. The Futterman, like all OTLs love a high impedance load and the KLH was. Now we have ESLs that dip to an ohm which no OTL likes.

I know of no other combination of ESL and high output impedance amplfier that is good one. 

@ramtubes
Roger, you might want to read this:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
If the Quads were close to the wall behind them, they will often have one-note bass. This often happens when a solid state amp is employed; the customer will note that there is no bass on account of the higher impedance of the speaker into which the amp can't make power, and so will move the speaker around, eventually finding out that if they move it closer to the wall behind it, they can finally get some bass. But this is not how *any* planar is supposed to be set up.

ESLs don't respond so well to voltage rules, depending on the speaker- the Quad ESL57 and 63 being pretty good examples- also the Sound Lab ESLs, AudioStatic and even Martin Logan (if you use a set of ZEROs to allow for their low impedance).  The reason is, unlike box speakers, the impedance curve is not also a map of its efficiency, which is pretty much the same across its bandwidth (on account of its impedance being based on a capacitor rather than a driver in a box with resonance). This is typically a 9 or 10:1 change in impedance! If the amplifier output power varies with this curve, the result will be too much highs and not enough bass.

The solution we've found with our customers using Quads is to have them pull the speakers further out into the room, so there is at least 5 feet behind the speaker to the wall. In this way the bass normalizes and customers report that the Quad is playing bass quite well.

I know of no speaker maker who designs for high output impedance amplifiers.

I can name a few- Coincident, Classic Audio Loudspeakers, DeVore Fidelity, Audiokinesis, Lowther, PHY, PureToneAudio and many more. Essentially, any speaker that works with an SET is working with an amplifier of high output impedance.

We are quite a ways off-topic; if you would like to discuss this further it would be a good topic for another thread.