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Old 2nd May 2012, 05:36 PM   #11
DF96 is offline DF96  England
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The circuit diagram you posted does not have a volume control, so where is the control?
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Old 2nd May 2012, 05:47 PM   #12
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It is actually on there...about A4-A5 on the circuit diagram (numbers and letters around perimeter)
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Old 2nd May 2012, 06:30 PM   #13
BZed is offline BZed  United States
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Is there any DC voltage on the output of the signal sources? I don't see any DC blocking here.
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Old 2nd May 2012, 06:43 PM   #14
DF96 is offline DF96  England
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Why can't the Chinese draw a circuit in a way which shows how it works, instead of hiding how it works?

Assuming the volume pot is normal, apart from motor drive, then DC on the input may be the problem as BZed says. Omitting input coupling caps in an amp is OK for DIY if you know that the source will have them, but poor engineering in a commercial item.
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Old 3rd May 2012, 02:33 AM   #15
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I wasn't sure about whether or not Yaqin had included blocking caps at the input...the schematic has a few errors. I did pull the bottom off the amp and there are no blocking caps, however I can measure no dc offset in the output of either source.

Curiously, the input tube's bias voltage is 1.9v though that's become a red herring I guess.
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Old 3rd May 2012, 10:57 AM   #16
DF96 is offline DF96  England
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Just to make sure we are all thinking about the same problem: you have a source which you know works fine, but always distorts with this power amp whatever the volume setting? Other sources are fine, and this source is OK with a different amp. Is that the story?
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