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In Reply to: RE: Roger Modjeski and direct-drive ESL amplifier posted by Lew on December 31, 2013 at 07:58:08
to get it to the equivalent of 200 watts or so. Is that realistic with current tubes? The good news is that one only needs 28 mA into 28Kohms :)
dee
;-D
True terror is to wake up one morning and discover that your high school class is running the country.
quote by Kurt Vonnegut
Follow Ups:
It may be true that you need more than a 4000V swing to drive the SL's adequately, but not because of Watts, I don't think. Once you remove all the power sucking components in the back plate and now also eliminate inefficiency associated with a step-up transformer, I think the speakers will be very happy with much lower Watts, more like 50W or less. I think (emphasis on "think") the extra volts are needed to make the diaphragm respond, because I am told that the SLs have an unusually wide gap between stator and mylar, so as to permit deep bass response. That's why you need that huge bass step-up transformer that SL uses. If 4000V is fine for a Quad, I doubt it is sufficient for a full range Sound Lab speaker. But I would yield to RM of course, in terms of knowledge on this subject, assuming he knows the ins and outs of Sound Lab speakers.
Edits: 01/01/14
I am just going off of good old ohms law. if you put 200 watts across 4 ohms you get a voltage swing of about 28.28V multiply that by 200 to 250 and you get the 5700 to 7000 volts :)
dee
;-D
True terror is to wake up one morning and discover that your high school class is running the country.
quote by Kurt Vonnegut
With no audio step-up, the impedance of the panels is much, much higher than 4 ohms. In fact, I measure about 25 ohms at 1kHz using a 1:90 step-up with no RC network, which means that the intrinsic Z of the panel is way up there in the thousands. (I am thinking it would be 25R times the square of the turns ratio, or 8100 X 25.) With the Sound Lab crossover network, yes, Z is as low as 4 ohms, at certain frequencies. But even with their massive bass transformer, Z does rise at low frequencies way above 4 ohms; I think I got around 50R at 50Hz via the SL bass transformer.However, your basic premise that increasing V necessitates an increase in power, all other things being equal, is of course correct. My only point was that I do not believe Power has to be anywhere near 200W. Because, for a big enough V swing, the current demand is nominal when you're driving the panels direct.
I edited this to change "your" into "you're" (you are). An error I commonly make in a first draft.
Edits: 01/01/14
I used it as a nominal number with the crossover...i know it is not really 4 ohms and not a real impedance but rather capacitive reactance....and and and but just to estimate what would the voltage swing be on the amp and on the panel...gets you somewhere in the ballpark.
dee
;-D
True terror is to wake up one morning and discover that your high school class is running the country.
quote by Kurt Vonnegut
So my point was that 4 ohms is not the operative impedance of the panel with nothing between the stators and the amplifier outputs.
Anyway we both agree that more than a 4000V swing afforded by the RM amplifier shown in the advertisement is probably necessary to drive a Sound Lab with authority. The question is whether RM can get it up to 5 or 6kV, or whether he agrees with us that more V is needed and why.
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