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Low Powered Tube Amp Distorting High Efficiency Speakers?

There may be a simple answer to this, but I can’t seem to find anything in my online searching. I have a pair of Zu Soul 6 99db efficient loudspeakers connected to a 15 watt Leben CS300xs integrated tube amplifier. When I crank up the volume to about 10 oclock on the volume dial, the speakers get pretty loud, but I hear a bit of distortion when certain high frequencies come into the recording.

My understanding is clipping / distortion occurs when the amp is driven too hard, but the volume being not even at halfway around the volume dial, I’m wondering why there would be distortion. Granted the Leben’s 15 watts gets pretty loud at 10 o’clock on the volume dial through the Zu’s. If anyone can explain why this is happening I’d love to know. Below are the Zu specs:

 

Impedance: 8 ohm nominal, 5.1 ohm minimum, balanced load.
Sensitivity: 100 dB-SPL @ 2.8V, 1m ground plane

Bandwidth, In-Room, Power Response: 38 - 28k Hz ±3 dB within 10 deg of axis at listening position greater than two meters.
Horizontal Listening Window: 45˚ @ -6 dB
Vertical Listening Window: 45˚ @ -6 dB

Group Delay: <5 ms

Max Power: 150 watts (full bandwidth)
Max Power LLF: 400 watts RMS (80 Hz high-pass, 12 dB/octave. Assumes power is unclipped, amp rated at 500 or more watts @ 8 ohms)
Avg Room/Moderate SPL: 2 - 4 watts
Avg Room/Loud SPL: 5 - 20 watts
Avg Room/Big Concert Levels: 20 - 150 watts

 

Thanks!

 

 

128x128avgonzalez77
perkadin's avatar
perkadin
286 posts
 

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.   

hoosierinohio
162 posts
 

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. 

avgonzalez77's avatar
avgonzalez77 OP
20 posts
 

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%.

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