low power tube amp

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...

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

Generally speaking tube amps are designed to provide approximately the same amount of power when a 4 ohm load is connected to the 4 ohm tap as when an 8 ohm load is connected to the 8 ohm tap. If an 8 ohm load is connected to the 4 ohm tap, though, maximum power capability will usually be reduced somewhat. However in the case of the Sopra 2 the speaker is essentially a 4 ohm load in those parts of the spectrum that typically require the most power.

I’ve found an impedance curve for that speaker here, near the bottom of the page. As you can see, the impedance is in the vicinity of 3 ohms between about 80 and 150 Hz, and is around 5 ohms between 150 and 500 Hz, while rising to much higher values in the mid-range and treble regions. Obviously the sub won’t be handling much if any of those frequency ranges, so depending on the output impedance of your amp (and Charles makes good points about that) the combo could very possibly still have the tonality issue I mentioned earlier.

Also, it’s interesting to note in the listening impression part of that review that the reviewer did some of his listening with an ARC GS150 tube amp, which is a 155 watt amplifier that incorporates a substantial amount of feedback (14 db). As with many ARC designs that presumably results in lower output impedance than in the case of most other tube amps, on its 8 ohm as well as its 4 ohm tap. Yet the reviewer commented that:

I also noticed that the GS150 sounded somewhat more forceful in the bass through its 4-ohm than its 8-ohm taps.

Regarding the sub, it will certainly mitigate the power limitations of your 20 watt amp, by preventing it from having to deal with deep bass frequencies. But given the Sopra 2’s impedance characteristics I don’t think it will help with respect to the tonality concern, which involves higher frequencies than the sub would be handling.

Good luck. Regards,
-- Al

P.S: I also note that the measured sensitivity of the speaker was lower than specified, at 87.5 db/2.83 volts/1 meter. Which means that in the bass region, where the speaker’s impedance is around 4 ohms, its efficiency is only 84.5 db/1 watt/1 meter.

Klee123, 
AL has provided you with with excellent technical information and explanation. 
Several questions.
What speakers are you presently using with your 20 watt amplifier?
Does this amplifier utilize NFB?
Have you any knowledge of the power supply design?
Most relevant,  can you hear the amp and speakers paired prior to purchase?
Charles 

Doesn't the tonality issue apply to any amp given the not so flat impedance curve?
if my amp is able to drive the speakers without clipping at my listening level, then the tonality problem is the same as with a 200w amp correct?

from what I read so far, what's the advantage then to use a low powered amp? There are so many Class A SET out there I can't imagine everyone using speakers with 95+ efficiency