16 ohm speakers: any amp sounds better with more resolution. speaker cables less critical.

Dear Raul, Of course you are correct that no speaker has a flat impedance.  Yet it's common practice to refer to speaker impedance with a single value.  I am guilty of that as is almost everyone else in the audio world.  If it were of some importance to me personally in the course of evaluating a speaker for purchase, I would demand to see the full impedance curve over the entire frequency range.  As you know, an impedance dip in the bass or midrange can be much more consequential than a falling impedance at very high frequencies, for the interaction of any speaker with any amplifier.  I don't argue with any of the points you made about speaker impedance.  Also, all remarks about speaker impedance are subject to the choice of amplifier to drive that speaker, unless the speaker is just crazily variable in impedance across the audio frequency range, which will be challenging for any amplifier.

Ralph, The older Sound Labs sounded anything but "transparent" in the midrange, using your OTLs to drive the unmodified version of my 845PX. I remember listening to Frank Sinatra during the first week or two after I bought the 845PXs and thinking to myself, "Is that Frank Sinatra?" Yes, Dr West did respond to our findings and our fix by revising his circuit and substituting the old treble audio step-up transformer for a new one that works down to lower frequencies, at least an octave lower.  This allowed him to make changes in the crossover such that it sucks less power and provides for a higher impedance at midrange frequencies.  I am told it's a big improvement.  I continue to drive my own 845PXs using a treble transformer capable of full-range response that does not require any crossover components at all to differentiate the input to the bass vs treble transformers.  Now, getting back to the amplifier factor, it is quite likely that the old version of the 845PX could have been driven more satisfactorily at midrange frequencies using a typical SS amplifier, because of the lower output impedance exhibited by most SS designs, but then I wouldn't have the OTL-ness to which I am addicted. I was told that SL use SS amplifiers at their factory, which is probably why the problem went unnoticed for a while.

Elliott do bypass those L pads!!!!!!

Now, getting back to the amplifier factor, it is quite likely that the old version of the 845PX could have been driven more satisfactorily at midrange frequencies using a typical SS amplifier, because of the lower output impedance exhibited by most SS designs, but then I wouldn't have the OTL-ness to which I am addicted. I was told that SL use SS amplifiers at their factory, which is probably why the problem went unnoticed for a while.

@lewm  I have customers with solid state amps and they report the same thing that our customers with tube amps do- that when the error in the backplate was corrected, the speakers instantly were easier to drive and sounding better at the same time.

Sound Lab was using a Boulder amplifier, which is why they may not have noticed a problem. But remember that resistor that got pulled out? It was a composite of eight resistors, totaling 200 watts! Obviously a lot of amplifier power was being used to heat those parts up. So its no wonder when that problem was corrected that the speaker got instantly better.

Dear @atmasphere  @lewm  and friends:  In reality " things " are not so " simple " as both of you posted to me.

The whole speaker impedance subject is a lot more than only the deviations through the frequency response of the speakers because we have to take in count to phase and angle that could " helps " to the amplifiers to develops even higher distortions and if you look to the amplifiers measurements I linked you can see that how good or not an amplifier can drive an specific speaker model.

If can be true that some SS amplifier can have some " problem " with some specific speakers ( I never found out one SS amplifier that had problems about or at least I just coul not detect it and I'm extremely demanding listener, not an easy one. ) where exist a higher trouble is with tubes even that the system owners are satisfied with and likes what they listen with.

Problem is that speaker manufacturers never gives the whole impedance facts/measurements/charts. Tha's why generalizations to say 8 ohm speaker spec means almost nothing. I choosed at random ( between hundred of ST measurement. ) and you can read there that your words are not exactly rigth with some of those speakers, same for maplifiers.

I love Soundlab speakers and for me is the only today electrostatic that I really like it.
My friend Guillermo ( a wealthy one gentleman. ) owned big Wilson that performed very good. Suddenly he took a fligth tp SLC in Utha and he bougth and brougth with him a pair of top of the line Soundlabs and the JC-1 amplifiers recomended by the designer him self.
For me was a surprise because never told me he wanted to change the Wilson, so and before I listen his new system and when both been at my place testing one of the tonearm prototype he told me about and I asked why and he told me that were the only speakers with the kind of sound quality performance levels that the very old ones in my room/system. For me was a true unexpected nice comment by him but when I listened at his place I really fall in love with Soundlabs/JC1 combination and if not exactly it sounds similar, obviously I like it.

This Soundlab measurements are not the today ones but more or less gaves an idea about and confirm that in the bass range what you said of very high impedance that for a SS amplifier is not " welcomed " however I heared no single trouble in those Soundlabs driving by the truly good JC-1 monoblocks:

https://www.stereophile.com/content/sound-lab-1-electrostatic-loudspeaker-measurements


R.

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.