Low Powered Tube Amp Distorting High Efficiency Speakers?

Is it truly that you are overloading the speakers or could it be the source is too high output for the Level input? You might very well need a certain volume out before this becomes clear.

audiojan I’m not sure I understand.

 I did a web search and found that Stereophile had done a review of the Zu Soul in 2016 (they also reviewed your amp in 2011). This was a review of the Soul and not the Soul 6, but the two appear to have similar woofers and may well have quite similar specs. The manufacturer states the Soul has a sensitivity of 97dB/2.83/m. The Soul 6 they rate at 100dB.

Some here on Audiogon are skeptical of John Atkinson's measurements, but I believe him to be unbiased and that over the years he has created a remarkable and consistent data base of speaker and amp measurements. As is often the case with speakers whose manufactures state a sensitivity over 92dB, his measurement came in at 91.5dB. Again, this is for the Soul and not the Soul 6. Assuming that the 6 is similar, I'd say there is a fair chance that your 12W amp is a bit under-powered for the speakers. Additionally, many of us notice some edginess in the upper mid-range when pushing our systems.

In general, it seems that very few speakers with front loaded woofers achieve sensitivity above 92dB. To honestly get to the 97 and above range the woofers must be horn loaded, such as the Klipsch La Scala.

My 50 year history in high end audio primarily comes from the massive power / current amplifiers in the beginning, working down in power to the 70wpc of triode tube amplification I have now. But what you describe is classic under powered.

@hertzhead +1. My JBL 4435’s are only 96 dB @ 1 watt 8Ohm’s, with 2-15’’ woofers and a horn.

Mike

Clip and burn.

1000 might mean nothing. What is your source? Some sources are higher level than others. that said, usually your amp won’t be the top of the range at 1000.

Every amp is different but I drive my 96 dB speakers as loud as you want with 2 wpc EL84 amp.

Do you have a spare set of tubes? Can you swap them out and see if it is any different?

Is it in both channels?

Finally, you mentioned "when certain high frequencies come into the recording". This is happening on more than one song isn’t it? it could be the distortion is recorded into the song and you’ve never noticed it before.

and one more thing, consider the possiblity of a bad driver in the speakers.

Jerry

Your comment on the distortion showing up in certain high frequencies makes me think it’s a bad driver.  Things like cymbal crashes will sound a bit fuzzy and not crisp. One of the negatives of some high sensitivity designs is that they tend to be less tolerant to clipped signals.  Being just 15 watts I’m guessing you may have had an aggressive listening session or two and pushed the amp, which clipped the signal creating excess friction and thus burned the coils. I hope I’m wrong because replacing drivers isn’t cheap. 

Before starting any troubleshooting process with tubed gear, it's always a good idea to substitute new tubes first (or a stash that you know works). It's an expensive undertaking but unfortunately even new tubes can be defective or out of spec right out of the box. The Russian factories have a very low QC baseline which is why certain retailers (Upscale, TubeDepot, Viva, etc) are worth the extra upcharge for their burn and matching efforts.

Also, EL84's in push-pull pentode mode have very high distortion, up to 10%, once you get above a certain power, usually 10 or 12 watts or so, since all is needed is about a 5 volt swing to produce full power. The 12AX7 have a high amplification factor which can easily swing twice that voltage. So it is possible that you are maxing out at that volume pot setting.

Another possibility is the recording itself. Often vocals are VERY closely miked which can magnify the sibilance. This is true of many rock, pop or new country recordings. Add to that the "loudness wars" recording style that still hasn't gone away in many recordings, or a variation thereof where some popular recordings are mixed and equalized in a manner that makes them stand out when played on so-so equipment like car stereos, cheap radios, ear buds and the like. These effects can really sound nasty when played on a full-range stereo at louder volumes. 

So, the question is: does this problem happen with all recordings or just certain ones? If just some recordings, I think the above comments might explain the issue. If all recordings, maybe it is the speakers, or the acoustics of the room, or a problem with the amp. However, don't forget to look at the source, whether phono, CD or streaming digital. Could be you have the wrong cartridge or phono preamp for your system, or a nasty sounding CD player or DAC. 

Actually that's a good point, hopefully its the tubes or even the source thats causing distortion. Try replicating the problem with a differert amplifier if you have something else available on hand.  If it still exists, it could be the speakers, but you certainly want to isolate the problem first to figure out where its coming from.   

I have a good old fashion Dynaco ST-70 that starts sounding distorted right before the tubes go.  Might try a whole new bank of tubes. 

Thanks for all the feedback everyone. My source is a PS Audio DirectStream Jr DAC with Tidal. The distortion is not necessarily in the recording, but I’m pretty sure its not a bad driver. Gonna get a bit specific here, but I can hear it in the louder passages of Caroline Shaw’s No. 2 Sarabande on her album Partita for 8 Voices (an incredible recording of her Pulitzer Prize winning album with Roomful of Teeth). Towards the beginning one of the sopranos on the left of the soundstage raises her voice a bit and the sound distorts. When I switched the speaker cables I heard the distortion on the right speaker, so its not the driver.

On reading your responses, I thought maybe I have too much gain coming from the DAC (I usually run it at 100 % volume and use the volume on my Leben to increase and decrease the sound), so I turned on the analog attenuator on the PS Audio (which drops the signal by 20db) and that definitely seems to have helped. I now have the volume around 10:00 for normal listening levels, and I found the sound overall got better - maybe its due to a few more watts pushing through my system when I’m listening at normal listening levels? I definitely heard a change - just a bit more depth of soundstage, fullness in the sound, clarity. But maybe just a slight bit of decrease in dynamics (though not as peaky which Zu’s can be from time to time). So I may have fixed the problem. However, when listening to classical tracks, it seems I may not be able to get the volume I want to really feel that immersive experience. Still playing around - gonna see what happens when I turn the attenuator off and just lower the DAC volume to around 70%.