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Old 2nd October 2001, 07:16 PM   #1
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Some friends of mine and I are in the midst of an electrostatic speaker project. They work great, and now we're in the post-build process of setting up; getting them prepared for everyday duty.

After some experimentation, we ran them on an impedance analyser, and got this god-aweful plot that peaks around 170 ohms at 700hz, and drops to less than .4 ohms around 20khz. The phase shift swings more than 70 deg around 20 hz, and -80 deg around 5khz. This is dangerous to our amps, so we're looking for methods of cleaning up the complex impedance that the amp (or crossover) sees.

For those unfamiliar, the electrostatic speaker consists of a stepup transformer, and two parallel plates with a charged membrane suspended between them. The amp plugs into the the transformer, the transformer powers the plates, and the membrane gets pulled back and forth. So the impedance the amp sees is some combination of capacitive and inductive impedance.

SPICE seems to have trouble realistically representing a transformer, so my efforts of computer modeling the system have hit a roadblock. If anyone can provide some assistance in this aspect, that'd be great. . .

I'd heard mention of a Zobel network for smoothing inductive impedance for regular magnetic coil speakers. I was wondering if a similar configuration would work for the ESLs. RIght now, we've just got a big, honker 2ohm power resistor in series with the transformer on the amplifier side, so that helps boost the resistance a bit (though of course we're dissipating a lot of wasted power as heat). But that doesn't help with the wild phase shift swings, which could induce all kinds of crazy destructive oscillations in our amps. If anyone could give me some help, or point me in the right direction, it'd be most appreciated. Thanks.

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Old 3rd October 2001, 05:33 PM   #2
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G-Daddy

The resistor in series with the transformer will help tame the impedence somewhat and help the bass, but it can sometimes shut the highs down. I have found that bypassing this resistor with a good quality film cap can help.
You will have to experiment with values, try starting with 1uf.

Jam

P.S. What is the turns raito of yuor transformer?

[Edited by jam on 10-03-2001 at 11:38 AM]
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Old 3rd October 2001, 08:06 PM   #3
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With the current setup, the turns ratio of our step-up transformer is around 50:1. They're the transformers that are for sale at the Electrosatic Speaker Exchange. There's little to no documentation on these things, so I'm a bit uneasy about them. I DID pump them with white noise, and took the 100-second average of the output power vs. frequency plot. It wasnt very linear . . . but then I dont really have anything to compare it to. If anyone can tell me what I should expect, I'd be happy to try that again.

I don't quite understand how a simple resistor could affect frequency response, other than shifting the whole plot up two ohms, but I'll take your word for it. One interesting thing is that what you're describing with the capacitor is starting to resemble what I've seen of a zobel network, for instance here http://sound.westhost.com/lr-passive.htm . It'd be nice to flatten out the resistance component of the impedance, but what im really worried about is the phase shift. We've blown 3 amps so far, and while my compatriot Dan doesn't agree, I think it was from excessive phase shift at the low and high audible ends. So right now we're using an equalizer to heavily deaden the lows and highs - needless to say it sounds less than full. Thanks for the help, we'll try the shunted capacitor and see what happens. Thanks again.
- Jonathan
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Old 3rd October 2001, 09:36 PM   #4
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Jonathan,

What amplifiers are you having trouble with (and their power ratings)? I may have a solution for you.

Jam
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Old 3rd October 2001, 10:20 PM   #5
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Jam,
The first blown amp was my friend Dan's Aiwa shelf system, i dont rememeber the model or power rating. The second amp was a Yamaha receiver, rated at 100 wpc, I dont remember the model. The third blown amp was a Yamaha R-9, I think rated around 150 wpc. The R-9 has since been repaired, and we dont run it abouve -34db, along with the heavily damped equalizer I described before.

Dan believes all three amps were blown because we turned them up too far. Opening them revealed blackened output stage transistors in all three cases. Oddly enough, the Yamaha website recommends not turning up any of their amps beyond the halfway mark on the volume knob. But surely with a normal commercial set of speakers, its not this easy to blow an amplifier.

We're both planning to use different amps in the final setup; Dan recently bought a Yamaha MX-830 power amp rated at 190 wpc. We have no desire to blow more amplifiers, but we also dont want to have to limit our sound as severely as we have in order to be safe. The goal as I see it is to make these speakers as safe to drive as a commercial set of speakers (or at least as close as possible). Any help you can provide is of course, greatly appreciated. Thanks.
- Jonathan
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Old 3rd October 2001, 11:36 PM   #6
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Jonathan,

Yamaha amplifiers are not particularly great on driving reactive loads, I should know because I used to repair them.
I would suggest a Hafler, Adcom or Aragon as a better choice.
The main problem that you are facing is the low dc resistance of the transformer. The solution would be to use two large back to back elactrolytics, about 1000uf 50v, bypassed with a film capacitor in series with the transformer. Remember that capacitors in series are half the value of one capacitor. This should prevent your amplifier from blowing up, though I do not recomend using anything built by Yamaha to drive electrostatics.

Jam
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Old 3rd October 2001, 11:43 PM   #7
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Alright, cool that gives me something to work with. I'll give the electrolytic cap method a try. With that setup, I'll do the impedance analysis bit again and see how it come through. Thanks for the info.
- Jonathan
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Old 4th October 2001, 12:39 AM   #8
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Before my compadre Jon gives any false ideas, let me describe what happened to these amps, as they belong to me. The Aiwa system was rated at 30 wpc, a meager amount when trying to drive electrostatics. To its credit the music was barely audible at the maximum volume of the unit very shortly before its smoking demise. The second amp blown was a Yamaha A-27. It was playing a softly recorded tape when it blew. The soft recording demanded that the volume be turned past its dreaded halfway point. However, it had put in almost a week of demonstrations prior without any problems. The third amp blown was a 150 wpc yamaha that was purchased off of ebay. In an attempt to hook the speakers up to the the B+C setting (3 Ohms), I, through ignorance, shorted the terminals. Amps do not like this. Obviously the electrostatic load did not have any impact. The 100wpc amplifier was repaired and sounds as solid as ever on the electrostatics. Of course, I'm playing CD's which don't carry as much sound covering static as the softly recorded tape. I find that the volume knob at around -34dB (one fourth) is loud enough as I need it. At this level the wattage indicators only peak in the "red zone" once in a while.
It seems to me that the problem with the electrostatics is human error rather than a mechanical problem. Of course with a larger more robust amplifier I feel more comfortable. Hence, the MX-830 with a 190wpc safety net and impedances as low as 2 Ohms.
Hope that this will shed some light on a possible problem and/or solution.
-Dan

PS If any one is in the market for a less than perfect R-9 let me know!
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Old 4th October 2001, 09:36 AM   #9
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Obviously, as stated before, we have differing opinions. But of course, thats what makes for healthy debate. Viva la differance.
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Old 4th October 2001, 04:32 PM   #10
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Putting a 2 ohms resistor in series with the transformer, while preventing the amp from seeing 0.4 ohms at 20kHz (it now sees 2+ 0.4 = 2.4 ohms) will also attenuate the sound level by over 15dB at 20khz, 7dB at 10kHz, 3.5dB at 5kHz etc, so no wonder you now don't hear any highs from the speaker. Kind of defeats the purpose of having a low mass diaphragm, doesn't it !! Basically, you have to much diaphragm area /too small plate speacing / too high transformer turns ratio, take your pick. The resultant capacitance is too high so the reactive impedance is too low at high frequencies.
As for the low frequencies, the impedance is partly governed by the value of bias voltage you are using, as this cuases a negative compliance in series with the positive compliance due to diaphragm tension. Electrostatics are very complex beasts, despite the apparent simplicity of their construction. They do have wild impedance swings and demand very capable amplifiers to drive them. If you are interested in reading the most comprehensive and thorough treatment ever published on electrostaic speakers, look at he book "Loudspeaker and Headphone Handbook" edited by John Borwick, Focal Press, ISBN 0 240 51371 1. It is excellent though very complex.
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