low power tube amp

The Focal Sopra 2 is specified at 91 db/2.83 volts/1 meter, with a nominal impedance of 8 ohms and a minimum impedance of 3.1 ohms. Based in part on my experience some years ago with a high quality 18 watt tube amp and 90 db speakers having fairly benign impedance characteristics, listening to classical music having wide dynamic range in many cases, I would expect that for many (but certainly not all) users 20 high quality watts would be adequate. However a greater concern IMO would be the sonic synergy between the amp and the speaker.

Specifically, impedance curves I’ve seen at Stereophile for several Focal speakers, which in most cases are specified similarly to the Sopra 2 in terms of sensitivity and impedance characteristics, show impedances of 3 to 4 ohms in much of the bass region, and much higher impedances at higher frequencies.

The interaction of that kind of impedance variation with the relatively high output impedance of most single-ended tube amps will result in weaker bass and over-emphasized mids and highs, compared to many other tube amps and nearly all solid state amps. Although if your amp provides a 4 ohm tap the degree of that impedance interaction will be reduced somewhat, compared to an 8 ohm or higher impedance tap.

Good luck. Regards,
-- Al

Heartily agreed, Charles and Al.

Hi Al,
Agree with your assessment. Given that this is a KT 88 amp there may be some judicious use of NFB (lowering the output impedance) which "may" be beneficial along with a 4 ohm tap. An audition if possible will settle his case. If unable to audition then the safer choice would be a higher power push pull tube amplifier or solid state.
Charles

really appreciate all the info here....

I am planning to add a JL E-sub, which has a build-in active crosscover.
(ie preamp output to sub, then to power amp)

So the difficult low frequency load will be handled by the sub instead.

Any advice about this setup?

Also, my amp has a 4-ohm output, but doesn't that decrease the power rating to half? ie 10wpc?

thanks again

If the distortion from overdriven tube amps harmed speakers, angry electric guitar players would be everywhere instead of just here and there. My tube hifi amp is somewhere around 65 watts per side and seems to have plenty of headroom (the speakers are around 90 db efficiency). I grew up with tube guitar amps going back to the early 60s when I first fooled around with my cousin’s new ’61 blonde Fender Bandmaster. I eventually bought that amp from him to use for live stuff off and on for many years, and learned how damned loud 40 to 60 watts can be. What? Speak up…I can’t friggin’ hear you...