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Posted by Lew on April 18, 2012 at 12:42:41:
The 90:1 step-up transformers are made by Rob MacKinlay in Perth, AU. You can find directions to his website over on SLOG. Ironically, he had dropped these transformers from production in favor of using a pair of transformers dedicated for bass and treble, respectively. I bought his last pair, and they were used. As a result of the new interest, he geared up to make six more pairs. These are now already spoken for before they are even built. If enough more SL/OTL owners are interested, I am certain he will make more of them. Cost about $600/pair.

I messed around with Rob's new treble transformers as well. They have a 100:1 step-up ratio and can go about an octave lower than the SL toroid (to about 300Hz). This is a useful improvement, since it takes the crossover out of the most critical midrange. These EI types also do not need that power-sucking resistor in parallel with the primaries as does the SL toroid, a huge advantage over the toroid. I installed them in my friends pair of U2PXs with a low value capacitor in series to effect a 300Hz hi-pass filter (based on my Z vs f measurements, he only needed 8uF). He is quite happy with that. If cost is an object, Rob's treble transformers are less expensive than the full-range ones, and there would be no wait to get a pair.