Spectra 1100, Tube drive ESL

Read more on the Acoustat Spectra 1100 in our home audio section

Martin2008-08-22 21:41

Dear Andy, Thankfully to your help I have acquired proper transformer for my acoustat 1100 and make panel operational, and I bought a pair of acoustat 11, which I have repaired. The speakers sounds really good with one problem they need a lots of power, my tube amps restored HK-A700 with 7591 PP tubes puts about 35w per channel, and it is enough for quiet listening.. furthermore as a tube amps gets more distortion with higher volume now I can hear distorted sound when I drive it to the max.. (on solid state mosfets both sounds better, but amp is over 200w) So counting on your kindness and professional expertises I am wondering if you know which tubes will be good to build an push pull AB output stage, for electrostatic panels I mean OTL, with bias voltage only, do you have any links or ideas about the power strage schematics made with modern tubes ? the other question is about bias supply voltgae, I have bought a 110 ac to 7.5 kv dc module used in ion generators (like in air purifiers) there are also same modules with 15 and 18 kv.. do you think it would make any sonic difference to supply higher bias to electrostatic panels, I know the humidity makes a difference in performance so my idea was that higher B will make more dynamic sound (of course I remember about safety and 500M 5W resistor like those in acoustat interfaces..) many thanks for all help ! MArtin

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Andy Szabo2008-11-16 14:47

I agree that 35-wpc is not enough for Acoustat speakers. All ESL\'s are inefficient, and require lots of power. With care, you can safely use 250-wpc with these speakers. \'Care\' meaning avoiding signs of distortion from the speaker, and not allowing your teenage son to play his music when you\'re not home! The only OTL schematic I can recommend is Acoustat\'s original servocharge amplifier, available from this site. It uses tubes designed for the horiz. output stage of B&W televisions. Personally, I think the idea of building your own OTL amp to drive ESL\'s is a waste of time, and it could also kill you (the HV power supply is nothing like the wimpy panel bias supply). In all fairness, there are plenty of folks who would disagree with that statement, but no one has yet to show me a working example. I would not recommend deviating from Acoustat\'s original 5K-VDC bias supply. That value, along with the stator spacings of the panel, was determined after significant research to find the best compromise between efficiency and humidity-resistance and longevity for the Acoustat panel. 7.5K-VDC would increase efficiency, but at the risk of arcing the panel, depending on your climate. Voltages as high as 15K or 18K-VDC would cause the panel to arc instantly and fail. At those spacings and voltages, the air itself would break down and conduct electricity (think lightening).

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