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Old 19th April 2015, 12:13 AM   #1
mazur is offline mazur  United States
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Default Driving ESL from tube plates

Hi.

I want to drive some homemade ESLs with my Fisher KX-200 which has about 475V on the plates. The ESLs only play from 250Hz up (I use an active crossover). Questions:

- If I connect a dummy load on the output can I just connect the stators to the plates?

- Should I connect the bias supply negative to the amp ground?

Thanks.
Elias
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Old 19th April 2015, 05:16 AM   #2
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Well, if you never worked with HV staff, do not do it, period.

As a safety measure you can not connect stator plates directly to the tubes' plates. Use good quality film capacitors of the value 10..20 times of the panel's one, preferably snubber film foil (Wima, Epcos) in series between plates and plates 1000..2000 VDC rated.
In worst case you will be shocked by capacitor discharge but nor killed by dc power supply of the amplifier of yours. Install capacitors inside the safety enclosure i.e. do not fan out wires directly from the plates. Some interlock measures, like blocking power if HV connectors are not engaged would help.
Do not use ceramic HV caps: they are prone to fail short.
You need to organize dc path from plates to the ground 1..2 meagaohms.
Be aware of high AC voltage across dc path resistors. In your case two 2..3 watt rated resistors connected in series will suffice. Bias supply has to be connected to the same ground.
And yes you have to load amplifier's output for stability reasons.
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Old 19th April 2015, 05:44 AM   #3
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Adding DC blocking will indeed make the circuit somewhat less dangerous, but you can always get killed by the AC voltage driving an ESL when it is playing loud, no matter whether it is driven from an amplifier with or without DC blocking or by a transformer. The only way to make it safe is to put everything into an enclosure that makes it impossible to get too close to the high-voltage stuff.

Anyway, are you sure the output voltage swing is sufficient?

About twelve years ago I built a lethally dangerous amplifier with 2660 V supply and 1100 V output common-mode voltage to directly drive the home-made ESL of a colleague of mine, see
Elektrostatic Loudspeakers
When we tried it, the sound was very good, but the maximum sound pressure level was just sufficient for soft background music. Of course his ESL was full-range.
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Old 19th April 2015, 07:49 PM   #4
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The Roger Sander's article might be of help. See page 23. Please be safe. http://sanderssoundsystems.com/downl...ic_speaker.pdf
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Old 19th April 2015, 11:35 PM   #5
tyu is offline tyu  United States
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oneway................good info............I had not seen this for some time...I still have
Dynaco mk3s...............I have done this setup with the mk3.........Acoustat.panels
but these panels... only had med SPL...............sound was good....
got have high SLP panels......not sure this will work less the panles are. crossover....higher like 250hz. up.....................good luck
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Old 20th April 2015, 12:40 AM   #6
mazur is offline mazur  United States
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Thank you for all the replies. Thanks oneway for the link. Great info.

alexberg, thanks for the caution working with HV. I've done several tube amp restorations and I'm always extra careful. Although Sander's article does not show a dc blocking cap, I will certainly use one.
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Old 20th April 2015, 10:21 AM   #7
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Isn't the bias DC? Bypass the caps with 20-megOhm?

I built a Sanders amp - parameters like Marcel's - used it for a few decades. Direct drive is the best sound I ever made. Drove Dayton-Wright cells 140-20kHz fairly loud but not Mahler-symphony loud. 250Hz and up far less demanding of power.

Ben
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Old 20th April 2015, 03:10 PM   #8
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Quote:
Originally Posted by bentoronto View Post
Isn't the bias DC? Bypass the caps with 20-megOhm
Bypassing the caps with high-value resistors would be slightly safer than a direct connection to the plates.
But, in either case, the effective bias voltage would only be the quiescent plate voltage which the OP stated was 475V.
For reasonable sensitivity, most ESLs need a bias voltage of 5x or more this value.
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Old 20th April 2015, 03:19 PM   #9
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Quote:
Originally Posted by bolserst View Post
Bypassing the caps with high-value resistors would be slightly safer than a direct connection to the plates.
But, in either case, the effective bias voltage would only be the quiescent plate voltage which the OP stated was 475V.
For reasonable sensitivity, most ESLs need a bias voltage of 5x or more this value.
Thank goodness we are not talking about ESL headphones.

In first post, OP mentions "Should I connect the bias supply negative to the amp ground?". I took that to mean he planned on an additional source of bias. And the answer is the opposite way (so that the two voltages would add together; I don't think bias polarity matters.)

20mOhm - even if I never actually did the math - should be sublethal. But can be tested on a neighbour's pet you don't like (kidding mostly). Unless panels are leaky or you live in Florida, more resistance using HV resistors might work too.

Ben
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Old 20th April 2015, 03:31 PM   #10
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Quote:
Originally Posted by bentoronto View Post
I took that to mean he planned on an additional source of bias. And the answer is the opposite way (so that the two voltages would add together; I don't think bias polarity matters.)
Oh, ok. I thought your comment meant that you didn't think an additional source of bias was needed at all.
Either bias polarity could be used, but using negative polarity relative to amp ground would be the most efficient use of supply voltages.
(as shown on page 23 of Sanders article linked above)
Don't forget to connect a dummy load resistor to the output of the step-up transformer.
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