Using tube amp with electrostatic speakers.

P.S. to my previous post: The Audio Research V70 that was referred to in the post by Lostbears had a MUCH lower effective output impedance than most tube amps, due in part to very liberal use of negative feedback. Specifically, according to arcdb.ws it was spec’d as utilizing 20 db of overall feedback, resulting in a damping factor of 30 (the highest damping factor I can recall ever seeing for a tube amp), which theoretically corresponds to an effective output impedance on the 4 ohm tap of 4/30 = 0.13 ohms, essentially in solid state territory.

Generally speaking, Audio Research and also McIntosh tube amps tend to have lower effective output impedances/higher damping factors than most other high quality tube amps.

Regards,
-- Al

Most all tube amps use output transformers with impedance taps that work just fine into 4 or 6 or even 16 ohms!

The Zero is of NO VALUE to most tube amps!

Don, the impedance of many electrostatic speakers, including most Martin-Logan models, decreases to very low values at high frequencies. The SL3 model the OP referred to, while having a nominal impedance of 4 ohms, is spec’d at 1.5 ohms at 20 kHz. Some other M-L models have impedances at 20 kHz in the vicinity of 0.7 ohms.

Most tube amps have effective output impedances on their 4 ohm taps that are comparable to or even greater than 1.5 ohms. The interaction of that kind of output impedance with that kind of load impedance variation will result in a greatly under-emphasized upper treble region relative to the response that would occur with a solid state amp. A Zero would either eliminate or considerably reduce that problem (note Ralph’s reference to avoiding high end rolloff), and I suspect it would also reduce amplifier-generated distortion.

Regards,
-- Al

Don_c55 10-12-2016
Do not buy into this "theoretical nit picking" impedance BS.

The impedance interactions I referred to can easily make a difference of several db or more in the uppermost octave, with electrostatic speakers such as the M-Ls.  That is certainly not nit-picking or BS. Although within the range of possible interactions preferences may of course vary widely depending on the particular listener, system, and room. And yes, many people use M-L speakers with tube amps, without using Zeros, with results that are satisfactory even if not theoretically optimal. But Ralph’s suggestion was legitimate and well worth considering, IMO, and did not warrant the cynical response it received.

Regards,
-- Al

Tomcy6, what Ralph is saying that relates to your first question is that the very low impedance at 20 kHz is a byproduct of M-L’s decision to avoid extremely high impedances at low frequencies. The maximum power capability of nearly all solid state amplifiers decreases dramatically as the load impedance increases to high values, and therefore what M-L was trying to avoid was forcing the customer to use much more powerful solid state amplifiers than their designs require. Especially given that most music contains most of its energy in the lower part of the spectrum.

In contrast, depending on the specific design a tube amp will often be able to supply more power into high impedances than into 8 ohms, up to a point, and in any event whatever falloff may occur in its maximum power capability into impedances that are above a certain level (but still within reasonable limits) will almost always be much less than in the case of a solid state amp.

So M-L’s choice of impedance characteristics works in the direction of making their speakers more practical choices for users of solid state amps, since less powerful amps can be used than would otherwise be the case, while making them less friendly to tube amps than would otherwise be the case. While also, to some degree, adversely affecting amplifier sonics in both cases, as Ralph indicated.

Regards,
-- Al

Great post, Lew.  Thanks!

The only thing I would add is in connection with this statement near the end:

... it is my opinion that so long as the impedance remains reasonable at least up to 10kHz, the low Z above that frequency is nothing to worry about; there is very little energy demand on the amplifier for frequencies above 10kHz...

The low energy demands in the top octave will of course work to the amp's advantage, as you indicated.  I would add, though, that there can still be effects in that region on tonal balance and in how accurately high speed transients are reproduced that will vary as a function of the relation between amplifier output impedance and the speaker's impedance characteristics in that octave.  Those effects will become increasingly pronounced as speaker impedance in that octave decreases relative to amplifier output impedance, everything else being equal.  Although as you indicated the specific relation between those characteristics that will be sonically optimal very much depends on the particular speaker and the particular amplifier.

Best regards,
-- Al