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RE: "Will" of SLOG

The idea of Will's (and then my) quest was (1) to increase the input impedance of the SL speakers, so they can be driven better by our OTLs, and (2) in the process, to increase efficiency, which was dreadfully compromised by the RC network used by SL, where the parallel value of R ranged from 4-5 ohms to 8 ohms, over the years as they changed it quite often. There was also scuttlebutt that the toroidal treble transformer "needed" the parallel R, in order to function properly. (I can no longer parrot their rationale for that.) Substitution of the treble transformer with a full-range transformer enabled Will (and I) to do away with the RC network entirely. Thus in one fell swoop both goals were achieved. In the standard Sound Lab set-up, there is also a low value inductor in front of their bass transformer. Will found that one cannot just do away with the SL bass transformer; both it and the AU90 transformer in parallel are necessary to derive a full bass response from the big speakers. Will did however eliminate the inductor, whereas I have not; I use a 12-ga 1.5mH Alphacore air core inductor in series with the bass transformer. Either way, the results were and are transformative. I measured the impedance v frequency on my speakers with the modified set-up, and across the frequencies from 50Hz to 2000Hz, it does not dip below 20-25 ohms. At 5kHz it's about 8 ohms and gradually rolls off at 10kHz and 20kHz. Roger West pointed out to me that there is a sharp impedance (not frequency response) peak between 200Hz and 300Hz. Big deal! If it's there, it causes zero problems for my OTL amplifiers.

The problem with Martin-Logan speakers, and why I have stayed away from them ever since the CLS II, is similar; they are designed for solid state amplification. Further, the audio step-up transformers they used (in the CLS, don't know about now) were puny. What's inside the CLX?


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  • RE: "Will" of SLOG - Lew 06:45:11 05/09/14 (1)

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