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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
audiojan's avatar
audiojan
402 posts
 

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.

avgonzalez77's avatar
avgonzalez77 OP
20 posts
 

audiojan I’m not sure I understand.

hertzhead
22 posts
 

 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.

ghdprentice
6,965 posts
 

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.

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