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How much ripple is too much ripple?

CrazeeD

CrazeeD

New Member
Working on an amp that is noisy (110hz hum) and I’m measuring 250-300mV of ac ripple when cold dropping back to about 200mV when warm, off the power supply. I thinking this is too much and likely the source of my noise. Am I on the right track here or totally wrong?
 
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CrazeeD said:
Working on an amp that is noisy (110hz hum) and I’m measuring 250-300mV of ac ripple when cold dropping back to about 200mV when warm, off the power supply. I thinking this is too much and likely the source of my noise. Am I on the right track here or totally wrong?
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sounds pretty high.
 
mcgjohn said:
sounds pretty high.
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That’s what I was thinking. I’ve decided to bite the bullet and swap out the remaining 3 big filter caps (have already changed 1 because it leaked). They seem to test well uF wise in circuit with a cheapo meter but curiously show 0 esr. Parts are ordered and we’ll see if it helps with the hum.
 
never measured it but with an unregulated single filter supply I'd expect a fair bit of it. The thing is though, the ripple should largely cancel in a split rail amplifier. Thats one of the major reasons for using that topology. The parts of the amp that run single ended generally have a couple extra filters, and often they are regulated. The regulator does a fantastic job of dumping power supply noise.

might be worth isolating the preamp from the power amp section to see if the noise remains or not.
 
gadget73 said:
never measured it but with an unregulated single filter supply I'd expect a fair bit of it. The thing is though, the ripple should largely cancel in a split rail amplifier. Thats one of the major reasons for using that topology. The parts of the amp that run single ended generally have a couple extra filters, and often they are regulated. The regulator does a fantastic job of dumping power supply noise.

might be worth isolating the preamp from the power amp section to see if the noise remains or not.
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You are clearly on a level higher than me on this sort of thing. I do understand what you are saying for the most part. This being a power amp there isn’t really a preamp section per se but there still must be a way to probe different areas to see where the noise comes in. I received my caps today so they are going in regardless. Once that’s done I will reevaluate and if the problem persists. Will update! Thank you very much for your insights.
 
the thing with it though, it only cancels if the ripple is equal and opposite. If you have one funky cap, the ripple won't match whats on the other side so its not exactly perfect

normally you can see this on a 2 channel scope if you hook one probe to each channel and add them. If all is right, it should cancel and give you a basically flat line. If it doesn't cancel, thats what you hear
 
gadget73 said:
the thing with it though, it only cancels if the ripple is equal and opposite. If you have one funky cap, the ripple won't match whats on the other side so its not exactly perfect

normally you can see this on a 2 channel scope if you hook one probe to each channel and add them. If all is right, it should cancel and give you a basically flat line. If it doesn't cancel, thats what you hear
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I see (I think), I am definitely learning things today. Will continue to work and report back. Thank you!
 
should have said to each rail, not each channel. one on the + supply and one on the - supply.

anyway, hope it goes well. Sometimes it really is just a junk cap. I'm guilty of over-analyzing things sometimes.
 
gadget73 said:
should have said to each rail, not each channel. one on the + supply and one on the - supply.

anyway, hope it goes well. Sometimes it really is just a junk cap. I'm guilty of over-analyzing things sometimes.
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I figured you were talking about the rails but to be honest didn’t want to sound stupid if I was misunderstanding you! Thanks again!
 
gadget73 said:
sometimes my fingers go faster than my brain, so occasional bits of "huh?" tend to come out. Its even worse in person, I don't have a verbal backspace key.
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Well, changed out the filter caps and nada, same damn problem, still have the 120hz noise. I'm done for tonight but tomorrow will put it all back together and scope the rails. I assume if the rails are equal (but opposite) then I can assume my problem is not power supply related?
 
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