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OTL, Output Transformerless Amplifier User Group.

It's always the current that matters, whether AC or DC, but..

AC has a higher propensity to interrupt conduction in the heart muscle, resulting in cardiac arrest. I am guessing that's the basis for the difference in lethality between AC and DC.

Are you saying that it is an intrinsic characteristic of ESLs to present a one-ohm load? I bow to your far superior knowledge, but it does not seem to apply to my Sound Lab 845PXs. I measure impedances in the range of 20-30 ohms, from 50Hz to 2000Hz, about 8 ohms at 5kHz, and reaching as low as 2 ohms only above 10kHz, where the energy demands are minimal. This is using a non-factory full-range audio step-up transformer with a 1:90 turns ratio. (Below about 500Hz, this transformer is in parallel with the Sound Lab bass transformer, which has a much higher turns ratio, probably 1:250, but there are no other passive parts between the amp and the paralleled primaries of the transformers except an inductor in series with the bass transformer.) Since the impedance seen on the primary side will vary as the square of the turns ratio, I concluded that the step-up is a major factor in determining what the amplifier sees and that the intrinsic Z at the panel is much higher. In my view, many modern ESLs are "ruined" for tube amplifiers by added circuitry in front of the step-up audio transformer, such as low value resistances in parallel to effect a crossover, as SL does, all of which tends to bring down Z to values that indeed are very difficult at times for tube amplifiers. (In stock form, Z was around 4-5 ohms at mid-range frequencies for my 845 PXs. There may have been a 2-ohm dip in there too, at 1-2kHz.) With a solid state amplifier, these added circuits do not present much of a problem, but I have yet to meet an SS amplifier that can come close to the musicality of an OTL driving an old school ESL, such as a KLH9 or Quad 57 or my modified Sound Labs.

But I sure do not disagree that direct-drive is Nirvana. I bought my Bev 2SWs just on a lark, to have something to listen to in my basement far from the madding crowd; now I am addicted. Did you sign every Bev DD amplifier? You signed the ones in my 2SWs and dated "1979". Thanks for your contribution to this great speaker.


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  • It's always the current that matters, whether AC or DC, but.. - Lew 09:40:11 05/07/16 (0)

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