For those who don't know, Sound Lab drive their full range ESLs using two audio step-up transformers wired in parallel, one for bass and one for treble. Audio frequencies are divided by a first order crossover network (inductor in series with the bass transformer, RC network before the treble transformer) before being fed to the bass and treble transformers, respectively. I have done impedance vs frequency curves for my 845PXs, measuring Z at 50, 100, 200, 500, 1K, 2K, 5K, and 10K Hz, with the original bass and treble transformers in place, with only the OEM SL bass transformer in place, with only the original treble transformer in place, and with the aftermarket full-range 1:90 ESL transformer alone and in combo with the OEM SL bass transformer. I have the data in a notebook. So I think I have a pretty good understanding of the impedance of an 845PX. I use the speaker with the OEM bass transformer preceded by a small inductance, in parallel with my aftermarket audio step up transformer preceded by no filter at all. Efficiency is hugely improved; I would wager it could easily be driven by less than a 50W amplifier. My Atma-sphere amplifiers probably make 100W into a 16-ohm load, and they are coasting at high SPLs while driving the 845PXs, which appear in my system as about a 20-ohm load, using the generalization that Raul dislikes, but I can guarantee that Z never goes below 20 ohms below 5kHz. All ESLs are in effect giant capacitors, so it is inevitable that Z goes down as frequency goes up above 5kHz. As Ralph said, the big problem was the resistor in the RC network that was there to create a high pass filter for the (old) treble transformer. That resistor, depending upon the vintage of the speaker, could be as low as 5 ohms, or in later speakers as high as 8 or 10 ohms. The resistor creates an absolute upper limit of impedance at frequencies around the crossover point, because it is in parallel with the output of the amplifier. Thus, if you were unlucky enough to have one of the older speakers with a 5 ohm resistor, you were dealing with 5 ohms or less impedance at around 500Hz to 2kHz. That's also why you benefited from a very high wattage resistor in that RC network. Raul has a point about the very high impedance in the bass region using the OEM bass transformer; it's way up around 100 ohms and maybe higher below 100Hz. Yet, my OTLs drive it fine, as does Raul's friend's SS amplifier.
Elliot, For Sound Lab owners, the first thing I recommend is to bypass the "Brilliance" control, which is an L-pad in series with the treble transformer, much like the controls on your speaker. Alternatively, if you feel you need some treble taming, you could use a discrete resistor of an effective value in place of the L-pad. L-pad bad. No L-pad good.
Elliot, For Sound Lab owners, the first thing I recommend is to bypass the "Brilliance" control, which is an L-pad in series with the treble transformer, much like the controls on your speaker. Alternatively, if you feel you need some treble taming, you could use a discrete resistor of an effective value in place of the L-pad. L-pad bad. No L-pad good.
