Really weird problem with volume control

I am doing this hobby for some 40 years, but I never encountered a problem like this before. The subject is a Kenwood KA-80 "high speed DC integrated amplifier".
Everything works fine, DC offset 10-15 mV on both channels, bias set to factory values, then.... when I turn the volume pot there is a scratchy sound on the left channel at around 9-10 o'clock position -like a dirty pot with contact problem-, but ONLY when playing music on it, Without signal, there is nothing, no noise all over the volume range. Happens on all inputs, from any source I am using.
Any ideas, what might be causing this?
 
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I knew it.....:D This darn thing works up to 400 kHz, too much for my old ears :cool:
 

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An audio amplifier which works up to 400kHz could be regarded as faulty!

Humbly may I ask why? Having heard a few (hundred) amps in my life, the HF speed and resolution of this amp is quite exceptional. And I am not talking about over emphasized ear-piercing "hiss" generator type treble, but smooth ultra clean highs resolving ambient noises, nature sounds (water, rain, street noises) and instrument overtones with an eerie reality never heard before. Not only my opinion, a few fellow veteran listeners all agreed on this particular feature...
 
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Ohh come on Welcome, you are worse than Sreten....
What HF "speed" means is what you do NOT hear with (too) many other amplifiers. Subtle detail, specially on live and field recordings. Low level minuscule noises, overtones which give you the feeling of being there .If you ever heard -for example- a street interwiev with someone on a busy day listened on the old Quad electrostats, then you know what I am talking about, otherwise have no idea.
Chipmunks... you listen to those often? :spin:
 
Ohh come on Welcome, you are worse than Sreten....
What HF "speed" means is what you do NOT hear with (too) many other amplifiers. Subtle detail, specially on live and field recordings. Low level minuscule noises, overtones which give you the feeling of being there .If you ever heard -for example- a street interwiev with someone on a busy day listened on the old Quad electrostats, then you know what I am talking about, otherwise have no idea.
Chipmunks... you listen to those often? :spin:
You cannot be serious!
The human ear cannot even feel sound pressure above 40kHZ let alone hear it. I expect the noise you are hearing is a faulty HF unit.
 
When I joined this community a few years ago, I was hoping for something better than it turned out to be in reality. (with all my respect to those who are really helpful, and serious about this great hobby).
Really hard for me to understand why it is so cool to post stupid and cynical comments for some people.
 
dragonweed said:
Humbly may I ask why?
It will amplify any lowish frequency RF which it sees. Not a good idea, unless you live on a desert island far from radio sources.

It is unnecessary, and in engineering doing unnecessary things almost always means compromising on necessary things. Even a person with quite wild ideas about their own ears and speakers could not need anything beyond 50kHz.

Your original problem may be bursts of parasitic oscillation at certain signal voltage levels. These can be difficult to see with a scope as they could be at very high frequencies.
 
One last thing. I studied psychoacoustics for some time, made lots of experiments with wideband amplifiers and HF speakers (real ribbons tweeters, B&K 1/4"mikes with 50 kHz bandwidth), and let me tell you, there is much more the human hearing mechanism is capable of, than you could ever dream of....:worship: (and hearing bandwidth is only a small part of it)
 
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Your original problem may be bursts of parasitic oscillation at certain signal voltage levels. These can be difficult to see with a scope as they could be at very high frequencies.[/QUOTE]

Well, with this one I agree 100%, and now I think this is what is causing the problem mentioned in the starter post.
 
I am doing this hobby for some 40 years, but I never encountered a problem like this before. The subject is a Kenwood KA-80 "high speed DC integrated amplifier".
Everything works fine, DC offset 10-15 mV on both channels, bias set to factory values, then.... when I turn the volume pot there is a scratchy sound on the left channel at around 9-10 o'clock position -like a dirty pot with contact problem-, but ONLY when playing music on it, Without signal, there is nothing, no noise all over the volume range. Happens on all inputs, from any source I am using.
Any ideas, what might be causing this?

Did you check for DC offset while you adjust the volume? Might be worth a look.

Mike
 
Your original problem may be bursts of parasitic oscillation at certain signal voltage levels. These can be difficult to see with a scope as they could be at very high frequencies.[/QUOTE]

Then, if I apply a slow sweep from a few Hz to say100 kHz, I should see the oscillation at the critical frequency on the carrier wave on my scope (40 megHz)?
Or is it possible that it happens only with complex signals?