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Old 9th October 2012, 09:17 PM   #21
maudio is offline maudio  Netherlands
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yes that would be ideal. But unfortunately an esl is not behaving like a resistor, but like an almost perfect capacitor.

Placing resistors in parallel might make things a little easier when it comes to stability of the feedback loop of your amp, I see no other advantage. And the price is that the amplifier has to deliver even more power which is only wasted to heat in your resistors. Things are hard enough as it is. And for unstable amplifiers there are much better solutions.
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Old 9th October 2012, 09:57 PM   #22
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I'm sure that is good advice for somebody. But in my universe (which includes actually building and running the amp for two decades), I'll take a nicely behaved resistor load any day.

Ben
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Old 9th October 2012, 09:57 PM   #23
jcx is offline jcx  United States
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Class A direct drive for large ESL isn't impossible - just daunting - may need to run a dedicated 240 V line for the power

push-pull can be done with a "SRPP" derived modulated CCS

Class AB does require hi V drive for the upper follower device if quasi-comp n-channel output is used since they are the higher V rated parts - xfmr gate drive does seem like an option but could limit loop bandwidth
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Old 10th October 2012, 09:42 AM   #24
maudio is offline maudio  Netherlands
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Quote:
Originally Posted by jcx View Post
Class A direct drive for large ESL isn't impossible - just daunting - may need to run a dedicated 240 V line for the power
everything can be done, it's just not practical... Imagine getting rid of 3500 W of heat in a safe way. You need a lot of components to distribute that heat. It is almost impossible to achieve sufficient isolation between power fets and heat sink, so all fets need their own separate heat sink and all heat sinks have to be hidden within the enclosure as they will not be safe to touch. You need a huge enclosure. Forced ventilation must be used and a lot of it, causing noise disturbing your superb music experience. Your electricity bill will skyrocket, maybe not such an issue in the USA but it is here in Europe where energy prices are several times higher.

Quote:
push-pull can be done with a "SRPP" derived modulated CCS
Will in an ideal situation reduce dissipation by a factor 2, still a lot of heat. Ideal situation means purely resistive load and we don't have that. It is not easy to get SRPP to work well over sufficient bandwidth with a reactive load and even if you do get it to work the reduction in dissipation will be a lot less than a factor 2 due to phase shift issues.

Quote:
Class AB does require hi V drive for the upper follower device if quasi-comp n-channel output is used since they are the higher V rated parts - xfmr gate drive does seem like an option but could limit loop bandwidth
Bandwidth issues seem solvable to me. But I see a much bigger problem in the parasitic capacitances between prim/sec windings of the transformer. The full output swing is present over this capacity. And if the voltage is high enough even a few pf will cause enough current flow to mess things up. As a result you to loose the ability to drive the upper follower accurately enough. Don't ask me how i know
Same applies for the use of optocouplers btw.

Last edited by maudio; 10th October 2012 at 09:45 AM.
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Old 10th October 2012, 05:48 PM   #25
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Hello maudio,
do you mind revealing some thoughts on how to include a step up in the amplifier feedback loop ? Mr. Wright (Dayton Wright,look at XG 10 Mk III error_correcting-ESL) followed this solution stating incredible low THD numbers...
Concerning parasitic capacicty problem with optocouplers for galvanic insulation of gate signal in a dd amp, you are - as usual - right up the point : take for example 1pF with 1kV of AC Voltage drop - current is the same as 1nF with 1V AC drop...
regards, Philipp
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Old 10th October 2012, 07:20 PM   #26
jcx is offline jcx  United States
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common plastic DIP optoisolators may not survive multi kV for long - you need specialty products

transformer gate drive isolation amp with DC to low MHz bandwidth probably requires modulation/demodulation with >10 MHz FM, or PWM thru the xfmr

at that circuit complexity level you might as well add non-switching geometric mean Class AB circuitry too

few pF pri-sec is hardly noticible in a drive current budget that includes nF panel C
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Old 10th October 2012, 07:45 PM   #27
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Originally Posted by invisible force View Post
Hello maudio,
do you mind revealing some thoughts on how to include a step up in the amplifier feedback loop ? Mr. Wright (Dayton Wright,look at XG 10 Mk III error_correcting-ESL) followed this solution stating incredible low THD numbers... Philipp
I'm not sure what I said or didn't say before. Here's what I do say:

Starting with a biggish drive signal followed by a mild 1:10 step-up transformer (instead of the usual 1:75) might be a mighty good way of reducing transformer distortion and easily feasible. Sometimes a hybrid solution is a pretty good solution, even if not elegant.

I don't know if Wright had any feedback from the secondary side but it might be more feasible than with the challenges of getting a feedback-worthy signal from the far side of a 1:75 transformer.

My direct-drive amp (which drove a bunch of resistors and, by the way, a little parallel capacitor AKA the ESL load) did have total feedback.

I seem to recall that a bit of amp distortion sounds better than a bit of transformer distortion, given a choice.

Ben
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Old 10th October 2012, 08:12 PM   #28
maudio is offline maudio  Netherlands
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Originally Posted by invisible force View Post
Hello maudio,
do you mind revealing some thoughts on how to include a step up in the amplifier feedback loop ?
we are getting a bit offtopic, excuses to the TS. I'll try to keep it short...

The idea behind the transformer in a feedback loop developed when I had switched from building DD amps (fun but pointless) to winding my own transformers. What I learned from that is that an ESL transformer is always a terrible compromise. You want high stepup to get decent output but increasing stepup decreases bandwidth and results in lower impedances at the high side of the audio range. With a stepup of 1:100 or higher impedances of 1 ohm or less at 20khz are not uncommon. That has a rather negative impact on sound quality, amplifier distortion increases steeply with increasing output current (beta droop in the output devices). Especially at higher frequencies where openloop gain is already lower so there is less feedback to compensate. This is imho the main reason why esl’s with low stepup ratios often sound so much cleaner.

What you ideally want is :
1. reduce to load on the amplifier, that should ideally not drop below 6 ohm or so.
2. do something about the non-linearities in the transformer (caused by core physics and other boring stuff)
3. increase stepup if possible

The solution I found is to place a LR network before the transformer. This radically transforms the frequency response and impedance of the transformer, see picture. Top curve is a normal transformer + esl, bottom is the same transformer with the same esl but now with 10 ohm//2.5mh in series with the primary. The frequency response now starts to drop at approx 2-3 khz with a nice 6db/oct slope. Transition to 12 dB does not occur before well above 200 khz. Even better, the impedance seen by the amp does not drop anymore at higher frequencies.
The price is the amp has to deliver more voltage at high frequencies to compensate for the dropping response. Basically we are trading current demands for voltage demands but that is a good deal. The energy content of music drops steeply at high frequencies, so there is plenty of headroom in the voltage department.

Now we place a feedback loop around the whole thing. If we do that correctly and we add enough loop gain we get a nice straight frequency response to above 20 khz, straightening out any transformer non-linearities along the way. Those are most pronounced at low frequencies and wow, there we have plenty of feedback. Two beers for the price of one

The challenge is to get the feedback loop stable. You have to use nested feedback loops, using the frequency response of the transformer/RC network to set the dominant pole in the outer feedback loop. It requires a power amp with a bandwidth of at least 1 MHz, to keep enough phase margin within the loop. All of this is no big issue with modern components, in fact with a good design there is even room to increase stepup to 150-200.

I built a good sounding prototype with a self-made 1:125 transformer that reaches 40khz bandwidth driving 500 pf and I believe that is quite impressive.
The whole project got a bit dusty lately as life puts other demands on a man but I plan to pick it up again using 6/230V toroids, got tired of winding. If anything good comes out I'll post it here.

So much for my attempts of keeping it short
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Old 11th October 2012, 06:13 PM   #29
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This indeed looks like best alternative to dd drive. The high voltage dividers will need bypass caps. A circuit diagramm of the two long tailed pairs and compensated HV dividers would be great...
One problem is haunting me for a long time : the two primary - secondary capacities of the step up will leed to considerable current into the main earth..further deatails can be extracted in the very worthwhile reading white paper AN004 (<hum & buzz...> JENSEN TRANSFORMERS, INC. - APPLICATION PAPERS AND SCHEMATICS) by Bill Whitlock. This should be valid for all ESLs with step up connected to main earth...
great idea maudio!
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Old 11th October 2012, 07:02 PM   #30
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A suitable high voltage N channel FET for dd drive : STW3N150 (STP3N150 - STMicroelectronics) by STMicroelectronics..
A comment to my last post : The problem with the current into the main earth from the step up center tap is, that it will lead to a voltage drop across the cable shield resistance of preamp - power amp connection (also cd - preamp etc.) - this is not 50Hz or so, but phase shifted fullrange music signal spectrum, which is seen simply as input signal by the receiving amp..signal gets covered by "fog"..distortion..
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