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Old 23rd April 2005, 07:36 AM   #1
tade is offline tade  United States
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Default esl transformer.

I emplore you of the internet!
i am a pooor college student interested in ESLs. i need someone more competent than i to recommend a high voltage transformer to me! i would like for it to be cheap and readily available.
I expect for my panels to be four feet square and with 1/4 inch spacing.
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Old 23rd April 2005, 06:43 PM   #2
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Default Trannies from old tube amp

There are two types of high voltage transformers associated with ESLs, the high voltage bias supply for the diaphragm, and the step-up transformer that interfaces your amp and the ESL's stators. The cheap trick for both of these is to use transformers salvaged from an old tube amp (push-pull type). For the bias supply, use the power transformer’s B+ winding. You would make a voltage multiplier (stacked diodes and caps) to multiply the high voltage to even high voltages for the ESL bias, which can be anywhere from about 1000 to over 5000 volts.

For the step-up transformer, you use the tube amp’s output transformer “backwards”. You drive the 4 or 8 ohm secondary (now a primary) and connect the former plate taps to the stators. The bias supply goes between the center tap and the diaphragm. Usually you want a step up ratio of better than 50:1, so try to find a high primary Z tube amp. A smaller amp using, say, EL84s might have 8K primaries. If you drive the 4 ohm tap, you’d get a step up of about 45, barely enough, but maybe good enough for getting started.

There are a thousand factors to consider to really do all this right: leakage inductance, winding capacitances, voltage ratings, ratios versus panel designs, etc. I’d suggest you buy books on ESLs by Roger Sanders and Ronald Wagner. You’ll see references in this forum to those two books, which are especially good for beginners.

IT HAS TO BE SAID: THE VOLTAGES USED IN ESLS ARE DEADLY. IF YOU AREN’T EXPERIENCED WITH WORKING WITH HIGH VOLTAGES, CONSIDER FINDING AN EXPERIENCED FRIEND TO HELP YOU. Hate to be preachy, but we’d hate to hear of you getting hurt. We need all the ESL DIYers we can get!
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Old 23rd April 2005, 09:20 PM   #3
SY is offline SY  United States
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Since you're in the US, you should keep your eyes open for old Dynaco SCA35 or ST35 amps. The output transformers work very well in this application used backwards as Brian described. The bias supply needs almost zero current, so a tube amp transformer is overkill (though it will work fine). A high-voltage supply from an old copying machine can easily be pressed into service.
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Old 24th April 2005, 01:18 AM   #4
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Default Re: esl transformer.

Quote:
Originally posted by tade
I emplore you of the internet!
i am a pooor college student interested in ESLs. i need someone more competent than i to recommend a high voltage transformer to me! i would like for it to be cheap and readily available.


There are no cheap, readily available audio transformers. You must either pay a fairly hefty price, or be very good at scrounging them from an old tube amp (this is getting harder and harder to do these days).

You can buy new audio transformers from any of several sites that advertise on the web. Get output transformers with about a 10k Ohm primary winding and 4 or 8 Ohm secondary winding. The primary must be center tapped or you need to get four transformers. 4 transformers will usually make for more sensitive speakers, so if you can get four, do so. 15-20W transformers will be just fine, even if you drive the speakers with a 200W amp.

The high voltage bias supply should be made using the smallest possible transformer if it is to be line powered. It needs to supply very nearly zero current, so to stay safe, you want a really small transformer. Better yet would be a dc-dc converter that converts a low voltage DC input from a wall-wart up to the required bias voltage (3-5 kVDC). There is a plan for such a supply here:
http://www.rehorst.com/esl

Quote:
I expect for my panels to be four feet square and with 1/4 inch spacing.
If you mean stator to stator spacing, that should be OK. If you are talking about stator to diaphragm spacing, it is too large and you will not get much sound from your speakers, especially if you use only one audio transformer per speaker.

If your panels will be flat, you will need to break them into smaller pieces. If you try to build a driver that is 4' x 4', the weight of the stators, suspended only from their edges, will behave like a drum with a very low resonance frequency. If you bump the speaker frame with your hand the stators will wobble back and forth for quite a while. This may never be excited by playing music, but it does not give a feeling of confidence about the whole assembly to have parts of it wobbling like that. Also, as it wobbles, it flexes and the diaphragm may wrinkle and make undesirable noises in the process.

I would break a 4' x 4' driver into 4 drivers, 1' x 4', and put some sturdy mechanical support (part of the frame to which they are mounted) between them.

The alternative to breaking the drivers into smaller pieces is to curve the stators. A curved stator is much stiffer than a flat one. This is the reason why Martin Logan curves their stators. Curving the stators creates other problems with tensioning the diaphragm that may be quite difficult to overcome.

I_F
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Old 24th April 2005, 01:23 AM   #5
wrl is offline wrl  United States
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They aren't really "cheap" but they are definitely less expensive than some other suppliers. They are also very helpful if you have questions about wiring or ESL's in general. I'd recommend calling them.

Also you might want to consider using 1/8 inch spacing rather than 1/4 inch. That way you could use a smaller step up ratio (cheaper transformers) and not sacrifice as much output.

No joke though be careful when working with these.

-Wes
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