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Old 3rd March 2002, 11:05 PM   #1
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Default Tube Amp for ESL

I have Stax F-83 at home and I listen to vinyl recordings. I like electrostatics, Lowther loudspeakers and of course Single Ended Tube Amplifiers. 3 years ago I developed a dedicated amplifier for Lowther. Impressed by this amp, few of my friends with ESL57 have requested me to consider developing a dedicated Single Ended amplifier for their speakers. However, I would like to develop one dedicated amplifier (DIY kit) for the great majority of ESLs if such is possible. Wee bit ambitious, I think!

During the past three to four months, I have designed 3 Single Ended tube amplifiers on paper. I am not happy with any of these designs even though the Pspice simulations appear to be OK.

Looking at discussions at ESL forums, it appears that ESLs are voltage driven devices. Electron Tubes and Audio Output Transformers are efficient at swinging voltages. Everyone knows this. Why then do ESL manufacturers, knowing fully well that ESLs are voltage driven devices, incorporate a low impedance primary winding and high step up ratios in their speaker transformers? This interface appears to me as the stumbling block to a dedicated amplifier design.

Am I correct or missing by a mile.

Mohan
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Old 4th March 2002, 06:06 AM   #2
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Default Re: Tube Amp for ESL

You would to need to generate large voltage swings -- say between 3 and 6,000 volts -- and perhaps have a way of dialing this in for different ESLs. That means some pretty exotic tubes and lethal supply voltages

I have been talking with a few guys about building our own ESLs and our approach was to use a single voltage step-up OPT/ESL input trafo. The tube amp could be built with less lethal voltages and commonly available tubes. A 1:10 (or so) step-up trafo can be wound by any number of manufacturers (Menno van der Veen confirmed he could wind one).

dave
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Old 4th March 2002, 09:01 AM   #3
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Default Why SE?

every es speaker I have ever seen consists of the charged diaphram stetched between two stators that are driven in opposite phase by the (usually) input transformer. I am fairly confident the STAX es speaks work the same way, so how are you going to do it with direct drive SE?

It seem to me that this is perfect for a PP amp.

I recall that Acoustat did direct drive tube amps incorporated into their speakers in the 70's. Don't remember the model, but I don't think they were very reliable beasties.

I agree it seem pointless to have a tube amp step down thriugh the transformer, and then step up again into the speaker. Perhaps you could find someone who understands output transformer design (maybe Lucas at <a href="http://axis.jeack.com.au/~lucas/index.html">Black Art</a>, though I don't know if he'd be interested) to design and build one, perhaps with a low ratio transformer that is optimised in your topology. That would also possibly make the amp design easier and would allow for the PP drive needed by the speak to be generated by a SE amp. Direct drive would be a design challenge for the high voltages needed for a speak as against an es headphone like my Lambda Pros. Remember an es speak is a big capacitor, about 1 nF depending on panel size and stator spacing, and that is the load you will have to drive.

Even if you get it running and optimised, please be very cautious about releasing the design as a kit. The voltages involved in a direct drive amp could possibly be higher than even things like 211 SE amps, and so are potentially lethal. My es headphone driver needs to swing 350vpp to get max (loud!) output, and it has small, closely spaced elements. A speakers' requirements will be MUCH higher, as the drive voltage required is a square law function, determined by stator spacing. Most DIYers IME would not be up to doing it safely. High voltage dancing, whilst stimulating, is something to be avoided wherever possible, if longevity is part of the game plan.

Good luck, you have a challenging project ahead of you.
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Old 4th March 2002, 09:11 AM   #4
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Default Re: Why SE?

Quote:
Originally posted by Brett
It seem to me that this is perfect for a PP amp.
For our application we were thinking PP.

dave
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Old 4th March 2002, 09:23 AM   #5
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Hi Dave,

My post was mainly directed to Mohan. I like your approach of using a step-up o/p trannie. Seems to me to be the most logical solution. Be inetersted to hear how it turns out.

Cheers
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Old 5th March 2002, 11:28 AM   #6
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Default Mohan

If you want some info on how to build ESL's. Try to get a hand on this book "The Electrostatic loudspeaker desing cookbook, by Roger r. Sanders, ISBN 1-882580-00-1". If got it my self and it's really pratical.
About your Tube Amp. You surelly need a step up transformer. In some cases you need a polarizing power supply above 10 [kV].There are some guidelines to be recon' with.
Like prophet10 said. The transformers are provided by Amplimo but designed by Menno van der Veen. These are the best money can buy.
If you're looking for some good Tube Amp designs. Try to get this book "Modern High -End TubeAmplifiers, by menno van de Veen".
I only know the ISBN of the Dutch version (ISBN 90-5381-089-7.

Good luck with you project.
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Old 5th March 2002, 01:19 PM   #7
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Thanks Dave, Brett and MazzPotential. Looks like I am not alone in trying to make a dedicated amp for ESLs.

When you make a dedicated amp for a speaker, the compromises have to be minimal and the combination should give a reproduction that approaches a live experience. This is my goal.
Single Ended design is my first choice because no PP amp can achieve the musicality of a good (I mean good) SE amp. The only amps that come close are Class-A PP topologies. Stax DA-300, DA-50, Sugden P50 and the old Electrocompaniat (25W) & ElectroResearch…. to name a few. My memory is rather rusty with names and model numbers.

I have listened to DIYers in our club graduate from 300B to 2A3 to 45 amps. These guys use Altec Voice of the Theatre, Lowthers, Tannoys … and so on. In my opinion they were graduating from a 4W amp to 3W amp to 1.5W amp. This is another way of looking at their claims, which I do not dispute because I know them and I have a high regard for their tastes. What I am saying is that it is rather difficult to build a good amp at high power ratings.

So far, in my design efforts, I have identified the input transformer as a compromised component. ESL manufacturers provide a standard interface (8 ohms or 16 ohms) so that the consumer has the flexibility to change either the speaker or the amp. In the DIY domain this need not be so. However, I feel (being ignorant and trying to re-invent a lost wheel) that this is not the appropriate interface for tube amplifiers. Perhaps, we should be looking at a 1:50 or lesser ratios for the transformer instead of the standard 1:287 in the ESL57. This is my present line of thought. It is good to know that Dave is already on his way.

I am burning the midnight oil (some maths) to convince myself that this is indeed a valid approach. From the limited calculations that I have been able to complete, it would appear that using the ESL transformer primary to load an output tube could compromise bandwidth available from the system. I am continuing my calculations.

I know both Lucas (Black Art) and Hugh (aksa). Great blokes and I respect them. We meet at the good old Melbourne Audio Club.

Mohan
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Old 5th March 2002, 04:11 PM   #8
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Default OTL and ESL

Output Transformerless tube amps are well known to be one of the best type of tube designs for ESL speakers. These can be made with triodes such as 6C33C running in pure class A. This tube appears to be one of the best OTL type tubes. It also seems that OTLs are exceeding the performance of push pull and single ended type designs. I have heard OTLs and see no reason to go with push pull or single ended transformer coupled. OTLs have the power of push pull and the musicality of single ended. Why have a huge lump of a transformer in the signal path with other tube designs?
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Old 5th March 2002, 10:14 PM   #9
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I used to build OTLs in the early to mid eighties mainly because no one would wind an audio output transformer for me. When companies do not want to do something, they ask simple questions like “Why can’t you use transistors….Why do you want that for…” and stare at you as if you are Rip Van Winkle incarnated.

I had Magnepan MG-IIB in those days. The load was ruler flat and easy to drive. OTLs excel at
driving simple loads. My 20W OTL sounded as if (in dynamics…transient capabilities) it was far more powerful, open and more revealing than my Audio Reseach D76, which I continue to proudly own. However, the OTL was not comfortable with Quads and Ohm (the inverted cone) speakers. Stax-F83s were brought home in 1988? and the OTL was quite inadequate to drive these. I tried to build high power OTLs but faced oscillation problems. Hence my reluctance to go the OTL way. I agree that with simple loads and very small feedback, OTLs are the king of amplifiers.

High transconductance tubes such as 6C33C need a large cathode area to maintain their profuse emission characteristic. Along with this comes other evils like high input capacitance, grid current etc.. I have listened to Transcendent amps and some SE amps employing this tube. I am not impressed. Yes, in terms of rp, the tube is great.

Take a good Single Ended amp and apply a little local feedback. Looks great on the scope with symmetrical clipping, wider bandwidth etc..But the magical qualities of sound reproduction disappear. For this reason, I would not use even cathode followers in my preamp.

My present inclination is to change the input transformer in ESLs to suit valve amps. Interpolating 30V peak input into the ESL57, it would appear that 9000V is about the maximum recommended plate voltage. What I do not know is as to how much current is required to maintain this voltage across the capacitor. This will determine the choice of appropriate tubes.
Is this a relevant question?

For the record, amplifiers that I have so far designed (for this project) on paper use 572-10 and 811-10 tubes. I can use 3CX-300A if required. But the magic of the glowing bottle will be missing. Few pulse triodes are also under consideration. Looking at my calculations, it seems that a low step-up ratio is mandatory to achieve wide bandwidth. Is this correct?

Mohan
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Old 6th March 2002, 01:11 AM   #10
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Default OTL for ESL

Low power and unstable OTLs are a thing of the past.

www.tenoraudio.com and www.joule-electra.com and www.atma-sphere.com make high power into any load OTLs.

www.TranscendentSound.com does not use 6C33C tubes in their OTLs.

Here is a simple OTL that I am considering.

http://www.geocities.com/researchtri...otl/index.html
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