Using tube amp with electrostatic speakers.

Ralph-  I've been using a MacIntosh MC275 with a set of custom Quad 2805's-  the chinese panels are not well made and I had Kent McCollum rebuild using UK-produced 63 panels, then add all his upgrades. Measurements on the Quad 63 & 2805 show 4 ohm as the low impedance. Measurements on the MC275 show good linearity and very low distortion (for a tube amp) when using a tap where the load equals or is higher than the tap utilized. While I originally ran the Quads at 8 ohms, I switched to the 4 ohm tap and it is very nice. I crossover at the appropriate point for the Quads and supplement the bottom with a Velodyne SW. The PA is a one-off SS design which is very clean and has almost nothing in the signal path- made by Ron Sutherland for me. Does the 4 ohm tap in this case make sense to you?

BTW, I have an unusual collection of ELS's including the original ML CLS pair. I don't mean one of the production models, I mean the original prototypes. Gayle Sanders voiced these to match a set of contemporary STAX ELS headphones as closely as possible. They really are spectacular but a difficult load to say the least. The production models were tamed down and never sounded as good at the original prototypes. Back in the day, I drove them with a Yamaha B-1, a rare beast that could handle very difficult loads and was quite an amp in the late 70's-  all vertical Static Induction Transistors in the output stages.  Speaking of Stax's, I also have a pair of 8x's- another superb and very rare electrostatic. 

Gary Osoba

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