Does anyone care to ask an amplifier designer a technical question? My door is open.

Agreed that polypropylene is best for the money. Improvements on polyp are incremental and absolutely not worth it - unless you are a hobbyist who wants to see how far he can go, just because. 

What I see in Plitron (theoretically) is two things: first, the two plates of the ESL are driven by the same core, so they can be precisely balanced; and second, they offer 75:1 step-up instead of 250:1. Since I committed to 75:1 with 25V rails, going back to 250:1 would involve re-engineering the protection circuits.

Rather than that, I would try to build high voltage amps to drive the Quads directly. Wish me luck - and long life!

What I hear (subjectively) is that the Plitron 2905's are much brighter than the remaining stock 2905, without sounding harsh. Piano on vinyl sounds quite similar to the instrument upstairs on the modified units. The unmodified unit sounds mellow but imprecise compared to the Plitronned. In fairness I must add that this was accomplished with the help of nichrome wire and a small inductor to equalize primary impedance over the frequency band, as per the Vandersteen data sheets.

My DC offset used to be 50mV or less. What it is now is a good question, one that you have prompted me to ask. Thank you.

@georgehifi 

Hey George, did you see the review of the Cary SLI-100 in the lastest issue? Why would they be so dumb to send an amp that is going to look so bad on JA's well documented test procedure? 

Read Dec, pg 91. Read pg 153 for Cary's response. In there their response they say measurements are not everything and an amplifier should sound "intoxication and emotional". "A degree of second order harmonic texture gives the midrange a certain musical.... and expands the soundstange". Pure marketing drivel. Where is Dennis Had when we need him?

The whole argument about 2nd harmonic distortion being "benign" totally ignores Intermodulation distortion which is far worse and always higher. If you are listening to a small group, singer, solo guitar a little THD isnt bad. It just creates the octave. My question is "what is the 2nd harmonic of Beethoventh's 9th:". In a full orchestra, with chorus. there is no fundamental to do much with. Some people must think the amplifier has the intelligence to go find the fundamental note and create the octave. Amplifiers arent that smart. They just amplify a time/voltage varient signal. Its not like they are following the score.  :)

All the specs on this amp are horrible, shameful, disgusting, who designed this? Please go do something else.   By John's standard this 100 watt amp reaches 1% THD as 3.2 watts, 3% at 22 watts, finally reaching its 100 watt rating at 10%. 

You all get this in addition to poor damping, noise and rising distortion even at one watt on both ends of the frequency range. 

If you want a comparson look at the review of an RM-200 II, an amp at virtually the same price.

@terry9

What I see in Plitron (theoretically) is two things: first, the two plates of the ESL are driven by the same core, so they can be precisely balanced; and second, they offer 75:1 step-up instead of 250:1. Since I committed to 75:1 with 25V rails, going back to 250:1 would involve re-engineering the protection circuits.
Rather than that, I would try to build high voltage amps to drive the Quads directly. Wish me luck - and long life!What I hear (subjectively) is that the Plitron 2905’s are much brighter than the remaining stock 2905, without sounding harsh. Piano on vinyl sounds quite similar to the instrument upstairs on the modified units. The unmodified unit sounds mellow but imprecise compared to the Plitronned. In fairness I must add that this was accomplished with the help of nichrome wire and a small inductor to equalize primary impedance over the frequency band, as per the Vandersteen data sheets.My DC offset used to be 50mV or less. What it is now is a good question, one that you have prompted me to ask. Thank you.

Terry, all of this is intended to help, so have a seat and let me inform you of some things going on. These things may alarm you. You may like what you have created better than the original, however you may not if you go back. Here are the problems I encourage you to look into... deeply, with meters, oscilloscope, numbers.

1. 50 mV offeset on 0.1 ohms is 1/2 amp, a considerable problem for both the Plitron and your amp. 500 mA is a significant portion of your 1200 ma idle current. Later we can talk about how saturated your core may be.

2. There is NO advantage in having the two stators driven from the same core. I can almost promise you the leakage inductance on either side of the center tap of the Plitron is far from equal. It would almost have to be unequal if the thing is done in two winds. You do know how torrids are wound? Great videos on YouTube for those who want to see.

3. The two QUAD transformers are likely more matched at high frequencies than the two halves of your torroid. You can easily confirm this by driving them from 10K-20KHz with a dual trace scope, any low level is fine. I believe you will see the two stator drives rise and fall opposite to one another as you sweep the audio range. See at what frequency this starts. Dont judge those transformers on how small they are and how they look. Nor judge the Plitron on its looks, size or marketing. 800 pf in the specs is a red flag for me. That is 45 uF on the primary, which is like 1/4 ohm at the top frequencies. 

4. I have built many DD amps and I am listening to a 5200 volt DD now on my ESLs. Perhaps we could talk about a kit for you.I stil make small production runs if you are interested. DD is the way to go then all that other stuff like parallel capacitance, leakage inductance, core saturation all disappear with the transformers which have always been the weak link of ESL speakers.

5. I think you mean the Vanderveen data not Richard Vandersteen.
I sincerely doubt there is anything special going on in that torroid. I would have to say that a torroid is a flat out wrong way to make an ESL stepup. I make my own, but not that way. One has to reduce the capacitance not increase it. Torroid winding is the highest capacitance on earth.

If you do all this, you will learn a lot. I have been building ESLs since 1976. If you dont know who Harold Beveridge was I suggest you do a little research.Peter Baxandall wrote an entire chapter (in some large general speaker book) just about the QUAD 57 and 63. Well worth reading. I have his chapter in my file and refer to it often. After reading it many times I think Baxandall had a lot to do with, or entierly did the electronics while Walker had the concept of the rings and delay line. Baxandall gives a compete model of both drive systems.

Perhaps I shouldn’t say which speaker is the work of art.

Please take all this as education, Im just reviewing your good work and adventure so far. I admire you getting as far as you have.

BTW 5,000 volt amps are not so difficult as what you have gone through so far with your SS amps. They dont need protection circuits and I wonder why you are still using yours?

Do you live anywhere near SF, CA?

Great posts in here!

i get my info directly from Steve McCormack,
im watching this convo like a hawk.
thak you

Thank you for your interest, Roger. I take your comments in the helpful spirit in which they were given.

You have motivated me to look again at the stock Quads, and think about driving them with my current amps. These amps have proven themselves to be very stable over the years, and don't really need protection circuits in the speakers. I think. So I just may continue to do without. To be clear, I am not using the Quads' protection circuits.

Near to SF is relative. Emotionally nearer than geographically. I did post-graduate work at UC Irvine, but then moved back home. I am now 1000 miles away. But what's that to a man with a thumb, as we said in the 60's.

I would be interested in a DD amplifier, but I am somewhat frightened of the high potentials. The one thing I know about super-KV work is that it's qualitatively different from sub-KV work. Perhaps you could point me to your website?