@charles1dad 

Great. I am just relating my experience. I run all tubes these days… left electrostatics and other planar in the past.

I had 2+2s for a couple years with the interfaces with Medallion transformers. Over time I acquired their servo amps, which sounded amazing, but in those days (1996) the amps were very unstable. There is a guy in Florida I think, who repairs/rebuilds them. Worth finding him

hth

@fortepiano50 , @charles1dad There are certain Acoustats that IME are very easy to drive.

First let's debunk the sensitivity thing. While rated at 81 dB, keep in mind this is at 1 meter. This means that the efficiency of the speaker is actually higher, since its a panel and a lot of its output will not be picked up by a microphone placed only a meter away. Typically add 6dB for a more realistic figure.  You will experience as much sound pressure at 10 feet away from the driver as you will at one foot.

The other issue is impedance. ESLs have an impedance curve that varies by about 9 or 10:1 from the peak impedance in the bass to the impedance at 20KHz. Therefore the frequency of the sensitivity measurement can have a big effect on the reading you get, since that can vary the result by + or - 3-6dB- the speaker is higher efficiency if the impedance was more than 8 Ohms at the frequency of measurement; less if the impedance was less than 8 Ohms.

In practice customers of ours used our M-60 amplifier to drive Acoustats with good results in fairly large listening rooms. So I suspect that they are really in the 91dB region, much like other ESLs once you winnow these things out.

With some of those Acoustats, to get the M-60s to play properly we had to use a set of ZEROs so they wouldn't sound rolled off in the extreme top end. But some of the Acoustats sounded fine without that, which suggest that their impedance was fairly high. This would account for why some solid state amps wouldn't drive them all that well, since the higher impedance would have reduced their power output. Most solid state amps will cut power in half with each doubling of the impedance.

So if our M-60 could drive the speaker that suggests an impedance curve similar to a Quad ESL57 or ESL63 - about 45 Ohms in the bass. A 100 Watt solid state amp would only make about 17-18 Watts on such a load where our OTL would make full power. 

Put another way, many tube amps could probably play this speaker.

@atmasphere 

Put another way, many tube amps could probably play this speaker.

I’m glad you took the time to reply and explain on this thread. I wasn’t aware of the detail and insight you provided.  However, I know what I heard on numerous occasions with the Acoustat speakers and VAC Renaissance amplifier combination. Again, truly wonderful sounding music presentation.

Thanks to your explanation I have a better understanding of what I clearly heard. The comments regarding sensitivity measurements vs real world performance, makes sense. The room was spacious yet this pairing had no difficulty filling it with music.

Charles

@stratmosphere I'm going to move a tube amp into my second system room where I have some mature Martin Logan CLSs and see how it goes.  I hadn't really tried it before.