High voltage audio amp? - Page 1
Forum section: electronics / projects

author: ssashton
author_rank: Frequent Contributor
timestamp: December 22, 2018, 05:12:20 pm
content: Hi guys and gals, I designed the little speakers in Dave's latest video.I've always got ideas floating around and for a long time I have wanted to make my own co-axial electrostatic driver. Maybe slightly bigger than an A5 sheet of paper with two concentric areas that work down to about 500Hz. A digital delay could be used to produce a virtual point source effect.The problem is driving voltage, since good quality step-up transformers are expensive and rare as hens teeth.Ideally it would be great to design an amplifier that can directly output audio up to 2000V. Discrete amp design is beyond my experience though, so I wanted to see if anyone had thoughts on the best way that could be achieved? Could it be as easy as using an existing discrete push-pull design but choosing some special high voltage output transistors? The load would be capacitive of course.Just throwing it out there!Simon

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 22, 2018, 05:26:26 pm
content: Define 2000V- rms, peak to peak? It's a problem because the needed slew rate at high frequencies is huge. Getting stability with a capacitive load is also an issue. Good HV outputs are a problem. Maybe an OTL tube amp? IMO, transformers look better and better.

author: ssashton
author_rank: Frequent Contributor
timestamp: December 22, 2018, 05:32:14 pm
content: Thanks for your answer.2000V RMS I guess. I wouldn't know the required voltage until I build a panel but 200V is pretty conservative for ESLs (ElectroStatic Loudspeakers).Tubes would be out of the question IMO. Is 20KHz high enough frequency to be a problem for slew rate?

author: Benta
author_rank: Super Contributor
timestamp: December 22, 2018, 05:32:46 pm
content: Huh?Normally you'd supply the stator plates with a high DC voltage and simply drive the diaphragm with standard audio voltages. What's different in your design that makes high voltage drive necessary?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2018, 05:34:09 pm
content: High voltage linear amplifiers are kind of tricky. There are very few transistors that go much beyond 1000 V. In addition the power handling capability at high voltage is often relatively poor (e.g. more like 10-20 W power handling capability for a TO3 / TO247 case BJT at 500 V).High voltage MOSFETS can be a little better.There are a few such designs out there. The usual trick is to have several high voltage transistors or MOSFETs in series.There are few plans from higher voltage SMUs around that at least show the principle for some 500 or 1000 V.I also remember some circuits for bootstrapped OPs to get a high output voltage.So this would be really about a discrete design and not that similar to a normal class AB amplifier. A capacitive load does not make things easier.Using vacuum tubes would be another alternative - not that exotic at high voltage.

author: ssashton
author_rank: Frequent Contributor
timestamp: December 22, 2018, 05:40:41 pm
content: The best way to do ESL is have the centre diaphragm with high resistance statically charged, then supply high voltage audio to the conductive 'stater' panels front and back.This might be interesting:

author: duak
author_rank: Super Contributor
timestamp: December 23, 2018, 06:21:01 pm
content: A blue sky idea is to design a PWM driver with a pulse type step up transformer to generate the HV AC output. It would need a few fast HV diodes artfully arranged to provide demodulation and some filtering to reconstruct the audio. If a carrier frequency of, say 100 KHz, is used, the filter wouldn't have to be all that sharp for a 20 - 20 KHz bandwidth.I was looking for prior art (HV PWM amplifier) and found this paper on an ESL driver:http://orbit.dtu.dk/files/110626083/Thesis_deni.pdfCheers,

author: nfmax
author_rank: Super Contributor
timestamp: December 23, 2018, 06:39:39 pm
content: Quote from: ssashton on December 22, 2018, 05:40:41 pmHeh! Got a pair of those (slightly older version) sat right next to me here as I type! One thing you should be aware of is that the secondary leakage inductance of the two audio transformers actually forms the input inductance of the first stage OS the delay line (which uses T-sections)You might try combining an audio amplifier with a higher than normal, but still manageable, output - say a few hundred volts - with a transformer with a smaller step-up ratio. That will make for a simpler transformer design. You might even be able to use an autotransformer, if you can build the amplifier into the speaker, and use optical isolation (e.g. Toslink) on the input.Quad considered this approach back in the day, but PJ thought the market was not ready for a loudspeaker with integrated amplifier.