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RE: SoundLab tranfourmer MOD

I cannot take the credit for this mod, although I was yearning for some way to get where I am now prior to learning about it. I was contacted by a guy named Will, whose posts on this subject can also be seen now on SLOG. Will and I used to contribute on the old SLOG site, but I had not been there for years, nor had I been in touch with Will. Anyway, Will announced to me that he had discovered a full-range ESL step-up transformer made in Australia (Will is Australian)that he was using to substitute for the Sound Lab toroidal treble transformer. The beauty of it is that because this is designed to be a full-range step-up (with a 90:1 turns ratio), it can be used in parallel with the SL bass transformer in the total absence of any actual crossover parts. (Before you ask, let me say that Will tried using the AU transformer alone to drive the SL panels; the bass was grossly deficient that way; you need the two transformers.) I did not believe the parallel set-up would work so well at first, because I guessed (wrongly) that operating the two audio transformers in parallel would result in a low input impedance that would be less than ideal for OTL amplifiers. But, as the story is told now on SLOG, I made some impedance vs frequency measurements using either the SL toroid or the AU full-range EI type step-up, and I proved to myself that there is no actual problem; the impedance is quite favorable for an OTL amplifier. At this point, it is as simple as (1)buy that step-up from Australia, (2) remove the SL toroid, the capacitors in series with it and the resistors in parallel with it, and (3) install the AU transformer directly hooked up to the pos input of the speaker, in parallel with the OEM SL bass transformer. You must take care that the AU transformer and the SL bass transformer are in phase; the leads are color-coded to help you figure that out. And you are good to go.

As Will is fond of reminding me, I still have the 1.5mH inductor from my old crossover, in series with the bass transformer. Based on his and one other person's results, that probably is not needed.

There is one more tweak that has not been mentioned. As you know, the mixer box places small value capacitors in series with each arm of the secondary of the treble transformer. It took a lot of work, but I have upgraded the quality of those capacitors; I found 8kV .01uF Russian milspec teflon film caps for sale on eBay. They are huge and require a lot of insulation, but they sound a whole lot better than whatever SL used in the mixer box, the "pie'ce de la resistance".

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