Tracing hum

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Graeme
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Joined: Tue Oct 28, 2008 9:25 pm

#1 Tracing hum

Post by Graeme »

My kel84 hums audibly through the speakers.

Its slightly annoying so figured i'd see if i can do anything about it.

The wiring isnt that neat inside as its had a few mods done which were quickly implemented and never tidied up so i guess thats the first place to look at.
Its running as a power amp only, i seem to remember the conversion introduced some hum, but for some reason its either louder now, or it just bothers me more.


What im wanting to ask though, is how do you go about looking for hum?

I have a scope, but is it possible to explain to a clueless beginner where to start looking, and what im looking for?
Andrew
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#2

Post by Andrew »

Short the inputs so that no noise is picked up from a floating input.

The work through from the inputs with the scope probe looking for where the hum starts to appear.

Stick to one channel at once, the hum might be predominantly 50Hz or 100Hzm, or 150Hz etc or above.

Poor grounding, pick-up between AC wires and ones carrying DC, AC heaters on gain stages and noisy HT are all possible suspects.

If you have a DC bench supply using that instead of AC heating is good way to eliminate possible causes and reduce the number of variables.

Andrew
Analogue, the lost world that lies between 0 and 1.
Graeme
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Joined: Tue Oct 28, 2008 9:25 pm

#3

Post by Graeme »

Thanks.
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