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Old 18th February 2015, 12:03 AM   #1
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Default Noisy amplifier?power supply

Can anyone walk me thru, or point me to a tutorial for finding and diagnosing Noise in amplifiers/power supplies? Using an Oscilloscope?
I have a very noisy valve guitar amplifier. That is, when its turned on and the volume is past 10 o'clock, there is a lot of unpleasant noise. Its volume dependant. i have poked a wiggled everything, confident of layout and grounding, regulated every thing (B+ heaters etc). I am hoping that i can see the noise on an oscilliscope and identify it.
So, where can i learn to do this?
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Old 18th February 2015, 12:05 AM   #2
rayma is offline rayma  United States
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Originally Posted by bullpeters View Post
I have a very noisy valve guitar amplifier.
Can you post the schematic and some good photos?
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Old 18th February 2015, 03:22 AM   #3
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i would rather someone point me to or provide a method to investigate the display on a scope to give me a hint as to whats causing it. that way not just being relative to THIS amplifier. I have tried the obvious.
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Old 18th February 2015, 09:15 AM   #4
DF96 is offline DF96  England
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Get the circuit diagram, or reverse engineer it from the item itself. Note the major stages, and roughly calculate the gain of each stage. Use the scope to see where there is noise, and where there is not noise - taking into account the gains you just calculated. The noise probably originates at or just before the earliest point in the circuit where you can see it.

If appropriate, disconnect any feedback loop. If the noise is only present part of the time, note when it is present and when it is not present. From that work out what might be causing it.

Almost any component can cause noise. In old equipment suspect carbon composition resistors.

PS I assumed that when you say "noise" you actually mean noise, and not hum or oscillation. However, fault tracing (or is this debugging of a new build - you haven't told us) is much the same whatever the problem. It involves comparing what the circuit does, with what it is supposed to do. The big difference is that in debugging the circuit itself might be wrong, whereas in fault-tracing it is a component (or joint).

Last edited by DF96; 18th February 2015 at 09:18 AM. Reason: add PS
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Old 18th February 2015, 09:18 AM   #5
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Location: West Australia, near to the beach, natural ambient sounds only.
If the noise is volume dependent you should see it by connecting your CRO probe to the 'top' of the volume pot.
What is the nature of the noise ?.
You could make a short recording of the sound, encode it to MP3 (128k mono would be good enough), compress to ZIP file and attach to your post...max file size is 975 kB.

Never ignore the noise, it carries information - The author low-frequency noise in MOSFETs and metal films
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