tube amps and electrostatics

Ralph and Al, the discussion begs the question of how can one know whether an ESL was voiced to be driven by a tube versus SS amp. Obviously Sound Lab ESLs were designed to be tube friendly, ergo Ralph's comment that his amps have been coupled with Sound Lab ESLs for years. Is it the same old answer -- call the manufacturer and ask?? Or try to determine the type of amp that was used by the manufacturer to display the ESLs at shows??

That's easy. Most of the ESLs intended to be driven by transistors tend to have very low maximum impedances. For example MLs tend to be 4 ohms in the bass. When you figure that the typical ESL varies by 10:1 in impedance from the bottom octave to the top, that means that the MLs should be about 0.4 ohms at 20KHz, and a number of them are.

However by use of a set of ZEROs you solve this problem, as I have mentioned before. Let's take the example of the ML and its 0.4 ohm impedance at 20KHz. The ZERO has taps that set up the amplifier load at 16 ohms, assuming that the speaker load is 4,3 or 2 ohms. If you are using the 4 ohm tap that 0.4 ohm load will look like 1.6 ohms. If the tube amp employs negative feedback it will have no trouble making this work with flat (linear) bandwidth. If the tube amp lacks feedback, you might have to use the 2 or 3 ohm tap, and it will still work quite well.

So really what it comes down to is whether the tube amp makes enough raw power that might be suitable to drive the speaker. IME, the Accoustat is a good example. Some versions of the Accoustat, as mentioned earlier in this thread are high impedance, meaning a little OTL like our M-60s can drive them fine across the entire band. Other Accoustats have that dreaded low impedance and require the use of the ZEROs. A good friend of mine had a set of Accoustats like that, and he used our M-60s with a set of the autoformers. The amps made plenty of power- and the setup was in a room 17' by 24', and we never clipped the amps.

The point here is that one must not confuse efficiency with the impedance. The ZERO is a problem-solver here- it allows almost any tube amp to effectively drive almost any ESL, provided the amp makes enough power in the first place.

Rodman99999, Thanks, I think I see what you mean.

Ralph and Al, the discussion begs the question of how can one know whether an ESL was voiced to be driven by a tube versus SS amp.

Bruce, altho' you've directed the question to Ralph & Al, just my 2 cents: since ESL behave like (giant) capacitors which have impedance inversely proportional to freq, wouldn't it always be true that ESLs are almost always voiced with tube amps (not saying that you cannot or should not use a BJT-based power amp if you want to)? Barring those like Sanders Sound Lab ESLs where the impedance is deliberately kept low so as to use a s.s. power amp.

I recieved an email from Rodger. (below in parentheses)

Roger Sanders ESL and amp guru is not one to get into heated debates on forums, this is why he has presented his white paper on the subject as simply as he knew how, so it can do the talking for those who can understand it.
So if this is wrong by anyone, a white paper should be done by the "one" who has the so called "knowledge" to oppose it. So he can also be then be also laid bare for all the audio community to view, like Roger has done with his white paper.

"Hi George,
I completely agree with your assessment of the tubes vs. transistors controversy among audiophiles.
Transistor amps are the only type that can produce linear frequency response and high output when driving ESLs. But getting audiophiles to understand and believe the technical reasons why this is so is difficult. You have your work cut out for you -- but I support you 100%.

I hope that posting my white paper will help some audiophiles gain a better understanding of amplifiers. I do not have the time to post on forums. And in any case, I am not interested in getting into arguments with audiophiles. So I will not be making any posts.
However, any interested individual may feel free to contact me directly if he has any questions."

Great listening,
-Roger"

Cheers George

Autoformers cost more than many speakers themselves. That's a factor to consider in the equation that can't be ignored.

Ralph & Bruce & Mapman, thanks very much for the nice words.

Frogman, great post! I agree completely. Richness, body, and dimensionality are terms I find myself using frequently, I believe in the same sense as you are using the terms fullness and image density. And although I can't formulate a meaningful technical explanation, and I suspect that a good explanation would be a complex one involving a multitude of factors, my perception over the years has been that there is a tendency for those qualities to often be compromised in systems that are designed and assembled in a manner that results in flat frequency response being a leading priority.

Best regards,
-- Al