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OTL, Output Transformerless Amplifier User Group.

If I understand this drawing correctly,..,

You are bi-amplifying the speaker. You are using a dbx223 as a hi-pass filter in front of a Wolcott amplifier that drives the treble transformer. The treble transformer seems to go right into the mixer box. That's fine for the "ESL panel".

But what the heck are the speakers you have labeled "back ribbons" and "front ribbons"? Are those ESLs or ribbon speakers? If ribbon speakers, I can indeed imagine that the impedance can go down to one ohm, all of which is in parallel with the Wolcott driving the ESLs. So you are using that 8-ohm L pad in SERIES with the "front ribbon" speaker. That will raise the impedance of the front ribbons as seen by the amplifier, but it has no effect on the impedance seen by the amplifier due to the "back ribbons", and it dissipates lots of amplifier power as heat in the Lpad. (Parenthetically, if that is the Sound Lab Lpad, it is very bad, sonically, in my opinion.) Thus the impedance of those back ribbons is dominating the load seen by the Wolcott. (The net impedance seen by the amp will be the sum of the inverted values of the impedances. Therefore, the impedance can never be higher than the lowest value of all those different speakers.) If indeed they are ribbons, I would guess that the impedance can go quite low, down to one ohm or below, even. That is not good for the Wolcott, if you want the best sound quality. In fact, it's not good in any way.

With respect, this set-up could be a lot better. Best of all would be to ditch those extra speakers or to run them completely independently using another amplifier. At the very least, you have no need for that "12db" passive crossover coming after the Wolcott, if the dbx is itself an active crossover.

I am editing this post to say that I did not take into account the fact that the passive crossover should, if it really does afford a very high impedance at frequencies below 12kHz, ameliorate some of the potential problems I cited here. (I think I must have been thinking "12db", as in slope, instead of "12kHz", as in the -3db point of the passive filter.) In my next post, I did take the effect of the passive crossover into account. Here the active dbx crossover and the passive one were not in fact redundant, because they affect very different parts of the spectrum.



Edits: 05/12/14

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