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Darth Vader 23rd October 2009 03:54 AM

Driving ESL with valves/tubes
Hi all,

I am new to this forum and I have been doing some research on building DIY electrostatic speakers. I have a questions that may be a stupid one but please enlighten me.

Electrostatic panels work by having the normal amp output of a few tens of volts stepped up to a few kV at the stator. My question is, if I'm driving them with my tube amps, which already put out ~400V from the output tubes, can I just step that up with a 10X transformer directly driving th stators?

It just seems a little "silly" for the voltage to be steped down through the amp OPT and then back up again through the ESL transformer.



SY 23rd October 2009 08:53 AM

Certainly possible and ultimately better than the conventional step-down-then-step-up. But as a practical matter, it will require a custom transformer wound by an expert and may not be trivial to stabilize. For all that effort, intrepid experimenters will go all the way to direct drive, but that's no slam-dunk, either.

If you decide that this would be a fun project to keep you occupied all winter, one thing I might suggest is a tertiary winding that you can use in conjunction with an impedance network (perhaps just a resistor) to add a bit of damping.

Darth Vader 23rd October 2009 09:18 AM

Thanks for the clarification, SY.

Are there any articles or existing projects of this sort that you know of?

I am concerned about the frequency response of using a single transformer for the full freq range as well as the impedance on the input and output end of the tranny.

I'm not good at calculations so any help on this area will be much appreciated.


SY 23rd October 2009 09:24 AM

I don't know if someone has documented an actual project. It's certainly worth doing some searching in a library (it would probably be from the pre-'Net days).

I think there's a few papers floating around from the old Acoustat company on transformers and bandwidth- they ended up using two different step-ups for bass and treble to drive the same panel. There's probably a few tricks in there that you could use.

Good luck and if you try this, please post about it!

Darth Vader 23rd October 2009 10:07 AM


A lot of reading up to do before I get around to building. Besides, getting materials in my country is both difficult and costly.

I would definitely post once I start on something.

david yost 23rd October 2009 12:20 PM

You should also research the Beveridge OTL. If memory serves, the Beveridge amp not only drove the stators with the audio signal, but the diaphragm as well.

chinsettawong 23rd October 2009 02:05 PM

Hi Darth Vader,

This looks to be a very challenging and fun project. Looking forward to seeing your progress.

Wachara C.

arend-jan 23rd October 2009 02:18 PM


Originally Posted by SY (
If you decide that this would be a fun project to keep you occupied all winter, one thing I might suggest is a tertiary winding that you can use in conjunction with an impedance network (perhaps just a resistor) to add a bit of damping.

This is good advice. The Quad ESL step-up transformer has a few shorted turns on the core to serve as a 'loading choke' to reduce the input impedance peak.

I'm starting to develop an interest in OTL drive myself, this could be a nice winter project indeed!

Capaciti 23rd October 2009 02:35 PM

Hi Folks,

the direct coupled OTL tube amplifier is best matching to the high impedance of an ESL.

There were some systems avaiable in the past (Beveridge, acoustat, Pütz..), the sound has been amazing, but those systems had been not very relaiable.

In our days there is one interesting concept of an OTL-ESL-tube-amp. Currentyl i have two amps for testing. These amps are capable to deliver 4000 Vpp, which enables quite satisfying sound pressures. Former concepts were limited to room lisening levels.

Take a look: -innoxx audio- High voltage tube amplifier for esl (electrostatic loudspeakers)

I will present my reference ESL E2.5 on a show, driven by these amps. I intend to make a video presentation at the show. If this comes true you will be able to watch the amps "in action" at youtube.


ErikdeBest 23rd October 2009 02:45 PM

1 Attachment(s) built an amplifier employing 845's and a centertapped choke to drive Martin Logans. The site is down, but luckily we have Look here

Go to schematics (bottom of the page). Look at power amplifiers: 845 OTL

An interesting variation I saw once consisted of operating the whole output stage between 'ground' and a negative supply. Using a CT choke the plates are at about ground potential, and one doesn't need capacitors at the output. On the other hand, one still needs them at the input, or a transformer... See the attached schematic (I can't remember the author, maybe member forum revintage)

Still, I have no practical experience at all with these amplifiers.

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