Which high power SET

Many have expressed that harbeths are incompatible with SET.
Is this a problem of topology (SET vs push-pull) or stated power?
I'm using a el34 push giving out 28 watts and I hardly go have to go past 9 o'clock on the volume knob. To my ears, the 28 watts seem more than enough for me in terms of power with good frequency extension.

If it's topology, it means harbeths with its low sensitivity are generally incompatible with any SET amp, irregardless of it's power output. Given a hypothetical world where both push pull and SET generate the same power output, does push pull drive harbeths better simply because of topology?

As Almarg pointed out, it is not necessarily efficiency, but the impedance curve of the Harbeths that MAY make them incompatible with most SET amps. Most SET amps have a fairly high output impendance compared to other topologies. Push pull amps can also have a highish output impedance, but, typical SET amps tend to be even higher. The high output impedance of most SET amps will interact with the low impedance of the speaker in ways that will cause frequency response to be uneven. The same can be said of OTL amps which also tend to have higher output impedances. The only way to really find out if such interaction renders the speaker/amp combination unacceptable is to try the amp with the speaker (it is even possible, though less likely, that such interaction will be felicitous).

I am somewhat on the fence as to whether the 28 watts will be adequate or not. Clearly, that amount of power with the Harbeths will not allow for playback at extremely high volume in a large room. But, if you play at reasonable levels, this amount of power may be adequate. As Atmasphere pointed out, long before a SET amp has reached its rated power, it will already be straining. I have personally found that SET amps do distort somewhat gracefully--they become "thick" and slightly muddy and stop getting louder, while pushpull amps will distort more harshly (albeit at higher volume levels for the same rated power). It may well be the case that the higher powered SET amp will be pushed into obvious distortion only for brief moments and you might be willing to live with that. Hence, it is really hard to say one way or another if a particular power level is adequate, which is why some people advocate huge amounts of reserve power (I don't because I like the sound of lower powered amps of all kinds).

Hi Charles and Jeff, I have not heard the Absolare. My comment was more general. Just like a 300B, an 845 also has its own signature. In fact companies like Line Magnetic audio have amps where they use 845 output tubes driven by 300B as driver tubes just to compensate for the that certain lack of delicacy. But even with those kind of designs I could not accept the 845 way of presenting music. If I get to hear the Absolare, I will definitely post my thoughts in some or the other thread, thats a promise :-)

Edoit, I second all of Larry's comments, including his emphasis on the word "may," except that I would add "widely varying speaker impedance" to his mention of low speaker impedance.

The fact that the speaker's impedance varies widely, from about 6 ohms at mid-bass and upper treble frequencies to as high as 34 ohms just below 2 kHz, will cause the frequency response of the amp/speaker combination to be highly sensitive to the output impedance of the amplifier that is used. Since SETs tend to have particularly high output impedances relative to most other topologies, and also because output impedances tend to differ significantly among different SET designs, the acceptability of the resulting sonics pretty much comes down to a matter of chance.

An ideal speaker candidate for use with SETs would have high efficiency, high impedance, and relatively little variation of impedance as a function of frequency. Which is not to say that results will always be unacceptable if those criteria are not met, but you would be taking a big chance if you were to buy (without return privileges) before trying the amp in your system.

If you do try out a SET in your system, given the impedance curve of your speakers what you should particularly look out for with respect to potential frequency response issues would be an over-emphasis of frequencies in the 1 to 3 kHz area. And, correspondingly, an under-emphasis of the mid-bass and upper treble.

Regards,
-- Al

In my experience, push pull has dramatically more shove than SET. 28 p/p EL34 watts isn't equivalent to 25 845 SET watts. In fact, I had 15w Quad p/p amps that were tremendously more powerful than the 25w Audions SETs. As others suggest, impedance much different between topologies.