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In Reply to: Re: question on driving Sound Labs with Atmas posted by Ralph on July 22, 2002 at 11:00:14:
Ralph: The SL spkrs have bass and mid-range level controls. One can select among -6, -3, 0, and +3db contours. In the +3db position, there is no roll-off of the bass transformer, as you surmise. But in the 0 db position, I am told there is a 5mH inductor in series with the primary. For -3db and -6db contours there are series inductors with increasing inductance. When he optimized his A3s for impedance, Kevin Covi used the +3db position, but he inserted a 1 or 2 mH inductor in series, as per your idea on the desireability of having at least some roll-off.
Two day’s ago I took one of my A3 speakers outside onto my patio well away from the house. I re-tested the frequency response. All the tests were done with a 1uH inductor in series with the base transformer. The non-linear roll off on the base transformer still remains. If a 5, 10 or 20uH inductor is selected to be in series with the base transformer the non-linear roll off is still there, the contour just changes slightly. I don’t know if this is particular to my A3’s of if it is a general trait of Soundlab speakers. I am still waiting for Roger West to reply to my email.
This is basically the way we have set up my friends A3s with the exception of the modifications you have made. So stock toroidals, +3 mid, brilliance open, 0 bass sounds the best. Otherwise that 100 hz bump is too audible. YMMV.
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