The Zero (Speltz) Autoformers are not a free lunch. As with any transformer, they trade off current and voltage. So if you mate an amplifier with a high-ish output Z to a speaker with a low input Z, via the Zero, you lose voltage in direct proportion to the gain in impedance seen by the amplifier. This affects apparent amplifier power. It does work, but it’s not a perfect world. I happily used the Zeros for many years as an interface between my Atma-sphere amplifiers (OTL amplifiers with a high output Z) and my Sound Lab M1 speakers, which had a midrange impedance dip down to 2 ohms and was under 5 ohms impedance across a considerable portion of the midrange. The Zeros made it possible to drive them. Then I traded my M1s for a pair of Sound Lab 845PX speakers. The match was just barely acceptable, because the 845s + Zeros were sucking more amplifier power than the M1s, but with the intellectual input of a guy in Australia and by changing one of the two audio step-up transformers in the SLs and ripping out the crossover components, I was able to completely eliminate the impedance dip typical of SL ESLs. (Ralph knows this story very well.) In fact, my 845PXs now present an impedance that is never lower than 20 ohms from around 200Hz to 5kHz, and they are also much more efficient, for technical reasons too arcane to go into here. The load presented by the Sound Labs is just heaven for my Atma OTLs, and I don’t need autoformers. So, use ’em if you need ’em, but it’s still best to select either a speaker suitable for your amp or vice-versa, in terms of impedance.
To the OP. I advise you to set those controls the way you like them, then measure R across each rheostat, then substitute discrete resistors of that value, one for each rheostat (if I am correct that each speaker has 2). This will get you a big upgrade in clarity. When you do that, select very high quality resistors of very high wattage rating; that also pays off.
To the OP. I advise you to set those controls the way you like them, then measure R across each rheostat, then substitute discrete resistors of that value, one for each rheostat (if I am correct that each speaker has 2). This will get you a big upgrade in clarity. When you do that, select very high quality resistors of very high wattage rating; that also pays off.