Bolserst's always helpful post 18 has an interesting circuit: the ESL is a direct-drive amp (albeit through a capacitor). The 6V6 tubes are connected directly to the ESLs (through a capacitor) and have a 9400 Ohm load for their basic push-pull operation (as well as a transformer for the subs). The bias is the B+ (plate voltage) of 340v.
Although a few years since I sold my direct drive amp (no capacitors for me!), I remain convinced it is the best quality sound. Of course, years ago, 5% distortion on your radio wouldn't seem so bad.
But I'm still confused about single-stator ESLs, grounded and otherwise. With a high bias voltage, they perform as well as double-sided???
Ben
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No, single-stator ESLs will not perform as well as push-pull ESLs when used as a loudspeaker.
In the top octaves where there is very little diaphragm motion and if the bias voltage is 10x or more the signal voltage you can get the distortion down to similar levels. But, with that ratio of bias to signal voltage the SPL output capability is limited because the single-stator setup is limited on how high you can set the bias voltage before the diaphragm collapses to the stator. Many(most?) of the single ended ESL tweeters from the 50s-60s actually operate with the diaphragm mostly collapsed to the stator except for the areas adjacent to the spacers. Others used a thin sheet of resilient insulation between the stator and diaphragm to improve the situation somewhat.
The real issue with a single stator (just like magnapan speakers), is that the field strength is no longer linear with diaphragm displacement. There's a linear field between parallel pates, but with only one plate, the field lines diverge. No amount of playing with bias voltage levels will fix this.
That is absolutely correct but not relevant for these supertweeters because they are crossed over that high and have a very limited max spl. That means they hardly make any excursion (~0,05mm) and the changes in the field are very low. Yes, that's not ideal at all but the distortion comes mostly from other sources (amp, 'suspension' and uneven tension of the foil etc.) so that's not the first concern in this case. Other ESL which have to play in the midrange or even fullrange need some kind of push-pull drive though.
You are correct that there is a distortion mechanism related to diaphragm motion. But, even if the excursion is vanishingly small as it is with these tweeters there is still considerable distortion. The reason is because the force on the diaphragm is not proportional to the voltage difference between diaphragm and stator, rather the square of the voltage. Push-pull arrangements cancel out this distortion as shown in Attachment #2 here:
http://www.diyaudio.com/forums/plan...ted-ask-why-bias-voltage-esl.html#post3589946
@ chinacave,
I would be interested to see pics and schematics of your Philco tweeters.
You could post here or over in the Philco thread.
Thanks!
Thanks! look forward to it.
Philco thread is here: http://www.diyaudio.com/forums/plan...stereo-phone-electrostatic-2.html#post4195822
Philco thread is here: http://www.diyaudio.com/forums/plan...stereo-phone-electrostatic-2.html#post4195822
When I was in high school in the early 1970s, a college friend who was a med student helped me sputter aluminum on some Saran Wrap with their electron microscope sputtering machine. I connected the aluminized sheet to one 6BQ5 plate of an Eico stereo amp, and laid it on the chassis (ground). Loaded the output transformer with a 10 ohm load for safety. It made music!
Anybody seen these on eBay? (Kidding, must be a vast number of dollars)
B.
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