Using tube amp with electrostatic speakers.

I realize this is an old thread but it has a lot of information related to the issues of using a tube amp with electrostatic speakers.  I have a question related to the information provided.  It seems the issue is the impedance variation from high frequency hf (low impedance) to low frequency lf (high impedance) of an electrostatic speaker.  SS amps output a voltage for a given input so they will emphasize the hf vs the lf at a given power output because of the impedance variation of the electrostat over the audio spectrum.  From what I gather tube amps deliver constant power over the spectrum but you can be limited at low impedance (if the amp can't drive the load) or at high impedance if the output voltage does not swing high enough.  Please let me know if my understanding is incorrect.
Should you always use the lowest tap on your amplifier (e.g. 2, 4, 8 ohms) corresponding to the lowest impedance of the electrostat?  What is the trade off at low frequency high impedance using the 2 ohm tap.  If you went with the autoformer and it gives you the ability to drive the 2 ohm load what is the tradeoff at low frequency high impedance (e.g. driving 30 or 60 ohms).  Any insight would be appreciated. 

Best Regards,
Gary

It may seem that I am trying to avoid your actual question, but that is not the case, really.  First, most transformer-coupled tube amplifiers provide a choice of 4, 8, or 16 ohm taps, not 2, 4 and 8 ohms.  Second, the very low impedance at very high frequencies exhibited by an ESL is not so much of a problem for the amplifier, because very little energy is required at those high frequencies.  (For example, I made impedance vs frequency curves for my Sound Lab ESLs after eliminating the crossover components; impedance does not drop off significantly until 5kHz and goes to 2 ohms only above 10kHz. But every ESL will be different from every other one, in this respect.) I suggest that you try each pair of output taps for yourself and see what you prefer.  There is no right or wrong answer to your question, except to say that most amplifier energy will be required in the bass and midrange frequencies.

Nordicnorm, M-L candidly admits that they design their speakers to favor SS amplifiers.  (See also my post of 11-02-16.) This means that there is circuitry at the input that dramatically lowers the impedance seen by the amplifier, well below the native impedance of the speaker itself.  Thus your finding is not surprising. This is why I sold my last pair of M-L ESLs about 20 years ago and never looked back.  No way I would give up my OTL tube monoblocks just to suit one particular speaker.

lewm,

Thanks for the response.  I have an old set of Soundlab speakers (A1 and B1S) that I have been working on and upgrading.  I am currently driving the system with SS amps (Innersound ESL 800) but would like to try a couple of tube amps I have laying around (both are 60W).  One of the tube amps does have a 2 ohm tap (it is a Margules U280C).  I would use one amp to drive the A1s and the other to drive the B1S (with a crossover).  I have the luxury of optimizing the frequency band for HF and LF by using the B1S as my bass speaker.  Thus my question about using the 2 ohm tap for the A1s if bandwidth limited.  Would there be any benefit driving the B1S with a tube amp at the high impedance or would this work equally well with a SS amp.  Once again, thanks I really appreciate your input.

Also, since you seem to know Soundlabs, would there be an optimal frequency band for using the A1s from an impedance standpoint (e.g. minimize the wild impedance swings that the amp sees).

Regards,
Gary

I am fairly sure that the A1s would look like my 845PX with respect to impedance, which is "very high" up to 100Hz (50 to 100 ohms), then around 4-5 ohms up to around 1kHz to 2kHz where there is a sharp notch dip to 2 ohms or so, then back up to more reasonable values, then back down at 5kHz and above, to 2 ohms or less at 20kHz.  This is from memory.  (I could REALLY hear an improvement in vocals when I got rid of that 2-ohm dip; see below.)

You can probably assume, if you don't want to or cannot measure, that the B1S will present a fairly high impedance to the amplifier, and in that respect it should be an easy load for any tube amp, but the question would remain how much amplifier power you need to make it sing.  You might ask the folks at Sound Lab.  

One option you didn't mention but which works quite well is to bi-amplify your A1s.  I've never done it, but you can experiment with that idea.  See if you can find the old "Sound Lab Owners Group" archives for some hints.

And finally, you might do what I did with my 845PXs: (1) Acquire a full-range ESL step-up transformer with a turns ratio of 1:75 up to 1:90.  Plitron makes a nice one, toroidal at 1:75, but there are several others. I used a EI type transformer made in Australia that is unfortunately no longer available.  (2) Remove the existing treble transformer in your A1s and also remove the RC network that effects a high pass filter feeding the treble transformer. That RC network is the problem; it creates the low impedance dip right around the crossover point, because of the value of the R that SL chose to use. Also, the R (typically 5 to 8 ohms in value depending on how old your speakers are) soaks up amplifier power, because it is in parallel with the panel at all frequencies. The intrinsic impedance of the panel is mostly way higher than 5 to 8 ohms, so most amplifier energy flows through that R.
(3) Install the full-range transformer in place of the existing treble transformer AND the RC network, so there is no longer an RC at all.  You can retain the inductor that feeds the bass transformer.  Make sure the two transformers are "in phase".  And then your A1s can be driven by your 60W tube amplifier all day long. The resulting impedance is going to be 20-30 ohms from about 100Hz to 5kHz, going down from there but fairly constant between 100 Hz and 2kHz. And the efficiency is ridiculously improved.  I doubt my speakers draw much more than 20W, steady state.

Or finally, talk to Roger West about upgrading your treble transformer to his latest device, which is more nearly full range. That allows for a lower crossover point and also gets rid of that midrange dip in impedance, according to them. With that option you keep the crossover (but change the values according to his instructions). Plus, this mod is factory sanctioned, unlike what I did.