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bigwill 5th May 2005 04:30 PM

Tube amp specifically for driving ESLs
Would it be difficult to design a tube amp that just outputs a high-RMS signal, with low distortion (no output transformer)? (I don't think so :))

If so, would it be nessecary charge the stators instead of the diaphragm, and drive the diaphragm instead to avoid having to produce two signals 180 degrees out of phase?

B Cullingford 5th May 2005 05:37 PM

What is difficult?
Probably harder than a chip amp for sure.
Certainly potentially dangerous - I have repaired such a beast - the only audio power amp I've ever worked on that had a skull and cross bones warning symbol :bigeyes:

No you would not want to drive the diaphragm, charge migration ... unstable ... diaphragm sticks to sators ... arcing ... not enough space here to explain. May I humbly suggest reading up on some theory, maybe someone else will have some good links, just remember not all you read is correct! Walker's (Quad) patent(s) is/are a good read.

I've wanted to do this for years, but some how something always came first. And of course I'm not sure I want to modify my 63s, so that means building some panels first.

bigwill 5th May 2005 05:58 PM

I might have to get a book :) All I know is that I want to avoid an output transformer!

SY 5th May 2005 07:12 PM

It's a hard problem. And mistakes are spectacular, potentially dangerous, and always expensive. I've built a d-d ESL amp that is a scaled-down (900V) version of the 5kV monster that I really need (and which just awaits more available money, hah hah). But even that scaled-down one was a real ****** to stabilize.

bigwill 5th May 2005 07:52 PM

I definately don't have the experience to do this. I think I'll stick with a transformer :)

djmiddelkoop 5th May 2005 08:04 PM


I definately don't have the experience to do this. I think I'll stick with a transformer

I've build one with a 4.8kV supply.
It's definately the most dangerous, read deadly, and tough tubeamp I ever built.

You must be very experienced with tubes and voltages above 1kV to make this happen, so like you said if you don't have the experience don't even think of trying.

What you can do is try to find an Acoustat X amp and replace the sand for tubes. Still very dangerous though.


BillH 5th May 2005 08:18 PM


Bigwill writes:Would it be difficult to design a tube amp that just outputs a high-RMS signal, with low distortion (no output transformer)?
I've been wondering the same thing lately. I've seen a diy direct drive esl amplifier on the net. Can't find the link at the moment, will check for it later.

I would like to try building a push pull tube amp to drive a pair of diy esl's. I'm thinking it would need transformers, though. I'm not feeling comfortable working on an amp with a 4-5KV power supply.

Just a rough idea would be a pp amp with a +- 200v power supply with a 1:5 step up transformer. The transformer probably isn't an off-the-shelf part, so I've been searching the net for transformer winding info.

I'm quite rusty at tube circuit design, haven't worked with them in about 30 years, so the whole thing is just an idea at the moment. Sy-feel free to correct me if I'm going down the wrong path.

SY 5th May 2005 08:20 PM

The step-up may work, but again, the hard part will be stabilizing it- reactive loads are difficult. You will probably need more step-up than that, depending on your panels and bias voltage.

BillH 5th May 2005 09:56 PM


The step-up may work, but again, the hard part will be stabilizing it- reactive loads are difficult.
Would it need more stabilization than a traditional esl setup of a tube amp driving an 8 ohm step down transformer, in turn driving a voltage step up transformer to the esl's?

Brian Beck 5th May 2005 11:43 PM

My direct-drive amp for the Quad ESL-63 (long)
(I've copied a posting I made to Audio Asylum on this subject, especially about driving the Quads, here:)
Itís not easy and it is incredibly dangerous. But of youíre determined and experienced with the hazards of very high voltage electronics, it can be done. I built a direct drive amp for Quad ESL-63s (and with other ESL projects in mind) and in the process gained some insight into the unique drive requirements of the Quads. To drive them to anywhere near full output levels requires a lot of voltage, even more than for many other ESLs, because the Quad uses a fairly high impedance panel design for an ESL. This is evidenced by the step-up ratio of the pair of input transformers which is about 1:170 according to my measurements (at 1 KHz with the transformers in circuit). Most other ESL step-ups are in the 1:50 to 1:100 range. The bias voltage of 5.25KV is also on the high side. The 63 is specified for ďprogramme peak for undistorted outputĒ of 40V (corresponding to 28.3 Vrms or 100W into 8 ohms, at which level the speaker will output 106 dB SPL in the midrange, IIRC). If you do the math, that means that maximum drive at the secondary equates to 4800Vrms or 6800 V peak!

The next thought is that you donít really need to drive the Quad to full specified power and that you can get by with less voltage. Thatís the first reasonable compromise that you ought to make. Even so I wanted to go for the full Monty, if only as a challenge.

To pull this off, youíll need very high voltage rated tubes, which probably means transmitter tubes and a very high voltage supply. You can choose among many types of transmitting tubes, although I prefer triodes for sound quality and the lower plate resistance which provides better coupling as the ESLís impedance drops at high frequencies. Youíll disconnect the Quadís trannies, and connect your new amp to drive the inputs of the Quadís delay line circuit which is on the secondary side of the trannies. The protection circuitry is therefore disabled unless you wish to incorporate it somehow into the new amp design (I havenít done so yet but probably will). Generally youíll use a push-pull arrangement with large HV plate load power resistors to B+, the plates then connected to the Quads via HV caps. Or you can use choke loading, or use a push-pull transformer primary as a load without a speaker attached (but it will be hard to find a conventional transformer with enough voltage rating). All of these require compromises and Iím leaving out tons of details, of course. Morganís Jonesí book has a nicely conceived 845-based ESL amp design, although it might not make enough voltage for ESL-63s.

I havenít mentioned my design yet because itís so gargantuan, impractical and crazy that nobody in his right mind will want to duplicate it. I use four 833 transmitting triodes per channel (not a typo) which are run well below rated dissipation in a push-pull mu-follower bridge configuration to achieve enough current drive (ie: slew rate) for high voltage/high frequency testing of other ESL designs. The thing runs off of +/- 4000 volt supplies and uses a pentode servo amp to correct output DC levels. I run the upper cathodes directly into the Quads without coupling caps (kids, donít try this at home). It will deliver more than 4800 Vrms to the Quads at full tilt. How does it sound? Well, pretty damned good to my ears. The Quads now sound leaner, tighter and airier, all of which were needed improvements IMO. Detail is astonishing in a very natural way, as is punch and ďsockĒ. Was it worth the effort? For me, yes, but then I enjoyed the journey. Now I still have final packaging ahead of me.

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