low power tube amp

You need to be more specific naming the amp and speakers if you want an answer to your question that is meaningful.

i am looking at a few speakers
one is sopra 2 (rec power handling 40-300w)

Amp is built locally, class A SET, KT88, 20wpc or so

thanks

Hi klee123,
The potential problem would be driving the lower power amplifier into the range of clipping which could damage the tweeter. So it depends on how loud you listen and how hard you’re pushing amplifier.

Jperry raises pertinent questions, as there’s such variation among audio components. Two amplifiers rated at the same 20 watts can in reality be virtually night and day different as to their performance. The quality of their design, implementation, transformers and power supply could place them at polar ends of a spectrum. This is not exaggeration to simply make a point. Other than the same power rating (watts)there may be no other common ground.

If possible I’d listen to the pairing and judge based on what you hear. Depending on your listening volume and music genre you may be using just a few watts the vast majority of the time. Power is an important consideration but so is the overall  quality of the amplifier.
Charles

It's a serious problem, don't do it. A good chance you will damage the speakers when you clip the amp. If you absolutely love the speakers, then get a more powerful amp, but I'd suggest finding speakers that are made to mate with your lower powered amp. Cheers,
Spencer

A tube amp will clip "softly" by rounding off the wave, as opposed to the much more dangerous hard clipping produced by a transistor amp that's pushed too hard.  My speakers are recommended for 150-800W, and I've driven them for years with a 38-watt tube amp.  If I can remember not to be greedy by chasing slam and impact by turning up the volume knob, I'm rewarded with stellar imaging, tone, and texture.  Going too loud flattens the imaging and transient incisiveness.

You won't get pants-flapping bass and room pressurization out of your SET, but if that's not the highest priority you may still be pleased with it.  Hook 'em up and listen.

Soft clipping tube amp is less likely to damage speakers when clipping. 

Cymbop,
Your 38 watt amplifier is able to do what it does no doubt based on high quality power supply design and transformers. Another (lesser quality) 38 watt amplifier could fail miserably with these same speakers. As you said, hook them up and just listen.
Charles

Klee123,
A minor correction the KT 88 is a tetrode/pentode not a DHT (directly heated triode) so it would be a (SEP) single ended pentode )rather than a SET (single ended triode. Regardless,  if it is of good design, built and implementation it will sound very good. Good luck to you.
Charles

Charles,

You’d be hard-pressed to find a lesser-quality tube amp than mine!* It retailed new for a thousand bucks, with a pair of monitor speakers bundled for free.

That said, it is an over-achieving little beast. :-)

*Perhaps this sentence has never been written before on Audiogon. ;-)

Hi Cymbop.
Quality can be had for reasonable cost as your amplifier demonstrates. It couldn’t handle your speakers if it lacked good solid design and parts. High quality parts don’t always have to imply expensive. For me the true test of quality is actual performance as opposed to price tag. 
Charles

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...

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 

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?

When a speaker’s impedance varies widely over the frequency range, generally speaking the tonality issue that can arise as a result of the interaction between that variation and amplifier output impedance will arise to the extent that the amp’s output impedance differs from the output impedance the speaker’s designer anticipated the speaker would usually be driven from.

Nearly all solid state amps have effective output impedances that are near zero ohms. While tube amp output impedances vary widely among different amps, from a fraction of an ohm to several ohms or more. So if the speaker was designed with the expectation that it would generally be used with solid state amps, as may very well be the case with the Sopra 2, a tonality issue is likely to arise with tube amps to a degree that is greatest with those tube amps that have particularly high output impedances. Such as many single-ended tube amps, and tube amps which use minimal or no feedback. The power rating of the amp is not directly related to that.

The 155 watt ARC amp referred to in the review I linked to undoubtedly has lower output impedance than the majority of other tube amps, single-ended or not. So I suspect it (and many other ARC amps) would be a better match for this speaker in terms of tonality than most other tube amps.

Regards,
-- Al

Klee123, This is why I asked earlier in this thread if your amplifier utilizes NFB as this will lower the amplifier output impedance and likely making it less susceptible to widely varying speaker impedance loads. Problem is that NFB can have adverse effects as well (depending on the amount and how it’s used). Ralph (Atmasphere) has covered this in excellent detail numerous times on this forum.

For example my SET amplifier has no NFB and has an output impedance of 1.2-1.4 range. This is pretty low for a no NFB amplifier but would be considered very high for the typical solid state amplifier. This works out fine for me as my speakers are 14 ohm nominal and have a relatively flat impedance curve.

So as Al has clearly explained the amplifier and speaker must have reasonably compatible impedance characteristics. You need more information about your amplifier.
Charles

found the website

I will be using this with the sopra 2.  The kt88 doesn't sound as nice as this with my last speaker (quad esl 63)

http://www.finaleaudio.com/829b-special/

talked to the amp designer, it does have negative feedback

Klee123,
I’m unfamiliar with this double tetrode tube. I’m aware of the company Finale Audio and they have an established and good reputation. This particular amplifier seems to be an all out effort toward high quality. I get the feeling that this is a very good amplifier design. I suspected it’d have "some" NFB as tetrode/pentodes aren’t quite as linear as pure triode tubes. I hope you can hear  the pairing, it could be really good! I like their attention placed on high quality transformers and chokes, always a good sign.😊
Charles

Klee123,
If the Focal  speakers don't work out you should really consider looking at more efficient and easy to drive speakers.  I think that this 829 SEP  amplifier you have is special.  The right speaker match would allow the amplifier to shine to its full capability. 
Charles 

thanks
you guys are a wealth of knowledge

yes, i also think the 829 amp is something special.
It had a great review by wall of sound in canada.

I compared it side by side with simaudio moon's flagship integrated amp, and the difference was not even subtle. The 829 had texture and detail way beyond simaudio's

I would ask your amplifier builder once you narrow down your speaker selection as you have a unicorn for an amplifier.   I would also ask him how he built at kt88 SET amplifier. In my 35 years of owning various tube gear have never seen a KT88 SET.   Would definitely audition amplifier and speakers hooked up together.   Power needs are gonna be dependent on material, room size, volume levels etc.   Most likely you have a SEP (pentode not triode) amplifier.

Hi timber77,
Yes,  the distinction between SET  and SEP  was explained earlier in this thread. The actual tube in this amplifier is an 829b. 
Charles 

The low wattage won't damage your speakers.  The only question is whether or not you're getting enough level and do they sound good.  If not and you love tubes, check out:

http://www.tubes4hifi.com/bob.htm#ST120

If the speaker choice is still open, consider some speakers that are more efficient (that is if you are planning to keep the amplifier).

I would be on the lookout for something in the 93-96db efficiency range; generally speaking its not a good idea to push a single-ended amplifier very hard (and FWIW, this amp poses no threat to the proposed speakers).

thanks guys.....

any thoughts on adding an autoformer?