MAC Autoformers?

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.

Ralph,
I did read your white paper before entering this discussion. Lets just agree to disagree. Peter Walker designed the 57 to be used with an amplifier with a damping of 20 and even specified the series inductance. To say that these speakers will play with a damping factor of 1 is not fair to the speaker, no matter that some people like a widely altered frequency response. How can we discuss little differences in distortion when the frequency response has been so altered to make the speaker unreconizable?

Thats a nice little list of expensive speakers that represent a vanishingly small part of the market. I agree a single driver speaker like the Lowther will work just fine with your amplifier. 

As an antique radio collector I have given a lot of thought to how early SE amps (the 45 in particular) got relatively flat response without feedback. In that case the driver was a single cone in an open baffle cabinet so the impedance rise was not so severe and relatively flat. These early radios sound pretty good. However that does not represent modern popular speakers. 

ramtubes
Ralph,
I did read your white paper before entering this discussion. Lets just agree to disagree. Peter Walker designed the 57 to be used with an amplifier with a damping of 20 and even specified the series inductance. To say that these speakers will play with a damping factor of 1 is not fair to the speaker, no matter that some people like a widely altered frequency response. How can we discuss little differences in distortion when the frequency response has been so altered to make the speaker unreconizable?

Hear hear, good to see someone else thinks the same, about amps becoming tone controls with wild speaker impedance curves. Low output impedance (damping factor) and current are the only fix to make the amp stay reasonably flat.
Or I suppose you could have an "inverse tone control" to counter it, but the amp in question will start to gag itself very early in volume level.

Cheers George