In Reply to: RE: Ping Ralph? ESL speakers and OTL amps? posted by David M. on July 4, 2015 at 19:12:45:
The impedance of an ESL is really a function of the turns ratio of its audio step-up transformer; the higher the turns ratio, the lower will be the impedance seen by the amplifier. Plus the fact that an ESL is like unto a giant capacitor; native impedance is very high at low frequencies and vice-versa at high frequencies. In the "transistor era", some ESL manufacturers have geared their product to work well with solid state amplifiers, unfortunately. But here and there there are good matches to be found. With Sound Lab ESLs the problem was mostly related to their use of two audio step-ups in parallel, one for bass and one for "treble". The bass transformer is fronted by an inductor that rolls it off at midrange frequencies. This causes no problems and leaves a nice high Z at low frequencies. The treble transformer is fronted by an RC network that serves as a high pass filter. Problems arise, because the value of R in the RC network has ranged from as low as 4-5 ohms to 8 ohms, over time. That R sets an upper limit for impedance independent of the treble transformer turns ratio, at the worst possible place, in the midrange. Some of the more vintage SL speakers have an impedance dip as low as 2 to 4 ohms at 500 to 1kHz. In recent years, SL have addressed the problem in several ways. As many know, I cured my problem with my 845PXs by ditching the RC network altogether and using an aftermarket treble transformer, but one need not go to such an extreme to enjoy the OTL/ESL match that is indeed made in heaven.
Quad 57s and to a lesser extent 63s are a good match. Later Quads may be excellent also. KLH 9s are perfect for an ESL. Martin-Logan CLS model I speakers are great too. Later iterations of the CLS suck.
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Follow Ups
- Ditto - Lew 08:55:33 07/05/15 (0)