Help in Troubleshooting noise in audio Amplifier

[HLong]

Hi all, I just recently built my SE Tube amp with parafeed output transformer. Everything looked fine and as expected when I tested it with a 8 ohm resistor dummy load. However, as soon as I hook up to a speaker and crank up my signal generator, I heard a loud high pitch noise and the noise vary in pitch as I vary the input frequency. The outputs apprear normal on scope. I did not see any oscillation. I tested it on a different speaker and I have the same result.

I ran out of thought what would be wrong. Do you have any suggestion?

 

[felixc]

If the noise only happens with the function generators, then perhaps you are working on a false problem. The noise may very well be within the specs of the function generator.

If you use voltage divider resistors at the input to reduce the output from the signal generator, but you drive the signal generator at a higher voltage, is the noise the same?

If say you add an RC filter at the input to filter out whatever exceeds say 8KHz, is the noise still there?

You have to find if the noise comes from your generators, from a ground loop somewhere, or from within your amp.

There is virtually nothing that you can do about magnetostriction noise. It is caused by the current modulation in the magnetics. Some manufacturers pot the parts in epoxy to reduce the noise. All you can do is try a similar part from another manufacturer.

Is there a high-end audio store in your neighborhood? They may have a closed-loop analyser that can tell about the performance of your amp.