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Old 9th October 2002, 07:52 AM   #1
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Default Driving ESLs, thoughts wanted

I'm a college student, and I'm all for making sacrifices for cost effectiveness. That said, I'm still (passively, now) looking for a cheaper method to drive ESLs than spending $300 per channel on audio transformers. I don't yet have an amp for the ESLs, so I am in favor of direct-driving them if possible.

Here are some links I've gathered (I have a couple more, but they're dead now):

http://gilmore.chem.northwestern.edu/koss.gif (lousy sound)

I have a bunch of nice MJL1302A/MJL3281A bipolars, so I'd be in favor of doing some sort of SS/tube hybrid, or even bipolar/MOSFET hybrid. What about making a tube voltage amp that takes inputs from the outputs of an ESP 3A? Any advantages, or would the 3A be wasted? The tube amp shouldn't have to deal with gain, because current waveforms are converted to voltage waveforms, right? I'm also in favor of buying the 3A boards from Elliott, to simplify the process...

I know little about SS amp design, and even less about tubes, so the less I have to figure out for myself the better. Any help is appreciated
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Old 9th October 2002, 10:58 AM   #2
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Default Driving ESL by hybrid SS/tube amp

You can get excellent results with simple complementary Darlington emitter follower made from these MJL3281/1302 transistors driven by a triode voltage amp. The internal resistance of two halves of 6922 connected in parallel is low enough. Use two pairs of the mentioned BJTs with a pair of MJE15032/15033 as their drivers at40Vcc (single rail supply) and you'll avoid the use of any protection circuit except Vcc rail 5A fast-blow fuse. The power dissipation ability of these transistors will allow you to put a lot of idle current (up to 2 a) into them making pure class A amplifier. No loop NFB will be needed. The re is no reson to use more high Vcc voltage to drive ESL63s.

Best regards

Denis N. Afanassyev
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Old 9th October 2002, 03:37 PM   #3
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Default schematics?

Do you know of any schematics of amps such as the setup you described? Also, wouldn't the bipolars drive the triode, or rather the triode be the actual output device? My terminology's probably screwed up.

Even better, would it be possible to make a simple modifiaction/addition to something like the 3A? Like, slap the triode on after the usual output stage? I haven't had the opportunity to sleep since yesterday morning, so if I'm not doing so well picking up on ideas, please forgive me.
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Old 9th October 2002, 03:49 PM   #4
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You don't need to spend $300 on step-up transformers.

You can use a tube step-down transformer in reverse. You want one with at least 10K impedance on the primaries (you will use as secondaries), and it must be center tapped for the high voltage bias.

Try www.tubesandmore.com under the push pull audio transformers. P-T1609 worked well for me. Only $37.

Also www.handwoundtransformers.com/pp has a similar model for only $29! I haven't tried it, but I can't see a reason why it won't work.

These are some cheap and easy solutions to the transformer problem. My suggestion to you is to try them and listen. If you want better quality later you can always upgrade (maybe in time to return the old ones).

This way you can listen to your beautiful ESLs while you build your direct-drive amp. Good luck.

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Old 9th October 2002, 04:06 PM   #5
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And another thing...

If you don't have an amp to power the ESLs with, yet, you can buy a mid-fi amp to power them now. After you finish the direct-drive you can bi-amp the system and use the mid-fi amp to power the woofers. (Unless your ESLs are huge you will find that the bass will be a little less than desired)

Just my thoughts, from one poor college student to another.

Keep us posted.

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Old 9th October 2002, 04:34 PM   #6
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Thanks for the info Dan. I had been wondering about using step-down transformers in reverse, but it seems like the power specs are rather odd. 10W output @ 10000Ohms, with an 8Ohm "primary." How does that work out in terms of voltage and current? 60W into 8Ohms with a rail-to-rail voltage of 70V -> 10W @ 7kV? I could even use the 4Ohm one as a primary. I'm not fresh on the formula, though.

BTW, my ESLs are 18"x36" and are costing me ~$50 each without the transformer. I haven't planned for an aditional sub per channel or anything. Also, do you think it would be prudent to not make one dimension divisible by the other? It seems like I might get peaking, beaming, resonating, etc.
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Old 9th October 2002, 06:41 PM   #7
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You're asking the right questions Altaic.

I'll try and answer them to the best of my knowledge.

When you use the step-down transformers in reverse the 8 Ohm side will not be exactly 8 Ohm. Mine was nominally 4 Ohms and dipped down to 1 Ohm at 40khz! So, to make it a little safer for my amp (a 200 Watt Yamaha) I used the 16 Ohm configuration instead. I personally have watched multiple amps give up the ghost while trying to play the low impedances (before I knew what I was doing). Do not make the same mistake! In regards to this the bigger and more robust the amp is, the better. You'll probably need an amp larger than 60W. Electrostatic Loudspeaker Design Cookbook by Roger Sanders spouts a minimum of 100W. I would do at least 150W to be sure. I don't know what else to tell you about that.

Don't worry about the 10W spec of the trannie unless you want your music loud. I found that I could listen to quite loud volumes with no problem.

As far as I know there is no problem to building your ESL with a ratio in the dimensions. You may want to build some supports in the middle of the 18" to stabilize the diaphram. It depends on your spacing, but 18" gives the membrane alot of room to travel and it may touch/arc. Think about gluing a strip of spacer in the middle, so that essentially you will have 2 9" x 36" or 3 6" x 36" ESLs working in tandem.
Since the bulk of the music is well above the resonant frequency of the membrane (mine was ~115hz) you won't have to worry about peaking. Beaminess is a characteristic of ESLs. While making the sweet spot very small, it does make the "soundstage" at that point very wide and open. There is no easy solution to that problem.

I am not an expert in all of these problems, so I encourage anyone else to point out the errors of my advice.

Good luck,
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Old 9th October 2002, 07:41 PM   #8
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It seems like Altaic is looking for means to directly drive the electrostats, which means a minimum of 2kV peak in push-pull (meaning with respect to the diaphragm). The 40v emitter followers may be fine to energise a step-up but that's all they can do.


I am not a transformer guru but some of the specs of the step-up are clear. You need a step-up ratio in the region of 1:100-150 and insulation between primary and secondary capable of coping with a few kV. Dunno if all valve outputs can cope with this. Frequency response requirements otoh seem to be more easily achieved than with normal loudspeaker loads as the load is almost entirely capacitive and there is very little resistive component so very little real power transfer. I've found that even low quality non-interleaved transformers can sound quite good if they're not expected to drive directly huge areas of the panels. What this means is that you can have separate transformers for the bass and treble panels, thus allowing you to use a larger core for the bass. To get you started you can use even simple mains transformers, two per panel, primaries in parallel and secondaries in series. This requires each transformer to provide just half the step-up ratio and needs only to cope with half the voltage requirements.
As in most electrostats only thin strips are expected to work at high frequencies, this will reduce the capacitance the high frequency step-up transformer is seeing. It's easy to come to a compromise between the filter, hf radiating area and transformer quality.
I once ended up with a 3-transformer configuration, each driving a single frequency band and in turn being driven by a separate power amp. Very dynamic sound with excellent bass.

Unfortunately your real question about the direct drive i can't really answer. Obviously you'll need some pretty high voltage valves like 845, 211 etc. With all the complications and obvious dangers. Even if you decide to follow that route, i feel you should first get a transfomer based setup up and running and then improve on it. It will be so much cheaper and easier.

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Old 10th October 2002, 06:49 AM   #9
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Default Re: schematics?

[QUOTE]Originally posted by Altaic
[B]Do you know of any schematics of amps such as the setup you described? Also, wouldn't the bipolars drive the triode, or rather the triode be the actual output device? My terminology's probably screwed up.

In this circut the vacuum triode with resistive late load drives a bipolar (Darlington emitter follower) output stage via a capacitor (10 - 15 uF polypropylene). The loudspeaker is capacitor(>4700 uF electrolytic) connected to the output too.

I've not seen yet a schematic exactly what I've described.

I was experimenting with a Darlington emitter follower consisting of MJ15024/15025 output pair driven by MJE15032/15033. The output transistors were biased to 1.5A. I drove this output stage with also a BJT resistor-loaded common-emitter stage (MJE340) operating with much higher B+ voltage (160V) rather than a tube. The THD appeared quite low too. This prototype makes quite remarkable sound and I'm continuing refining this circuit.

Of course, this amplifier needs a step-up transformer to drive the ESLs.
Moreover, the directly driven ESLs is a bad idea, because the properly designed transformer greatly improves the performance. In such masterpieces as QUAD ESL63 the step-up transformers are actually the parts of equalizing circuit, which is essential to make a dipole radiator producing a flat response.

It seems indeed feasible to design the step-up output transformer for a tube amp, which will directly matched to ESL panels.

Some example of vacuum triode / bipolar hybrid without global NFB can be found at www.audiodesign.tin.it, but this is not class A design with some complications.


Denis N. Afanassyev.
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Old 10th October 2002, 07:19 AM   #10
vbd is offline vbd
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Hi all, I will not enter the debate about transformer vs direct drive, I just give a link to a DIY amplifier for ESL:
This design has MOSFETS outputs, feedback, and opamps... I found it quite interesting...
If you are ready to cope with high voltages...
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