What's wrong with this power amp?
Discussion
The symptoms: Amp is just wired up to the speakers; no sources plugged in. Both speakers produce a low volume 100Hz buzz/hum that is constant and independent of the amp's volume setting. The amplifier itself, a Mk1 Cambridge Audio A1, is not making any detectable mechanical noise.
The buzz/hum is completely drowned out by any music, so it is not really a big deal, but it would be nice to fix it if possible.
Any ideas? E.g. are these symptoms consistent with excessive ripple from the amp's DC supply circuit? If so, should I be replacing big electrolytic caps? I'm thinking 100Hz implies its the output of a bridge rectifier, so either there isn't enough capacitance or the internal resistance of the caps is excessive.
I'm not entirely clear why the hum would be independent of the amp's gain control. I could speculate that the gain is set in a stage that is effectively a small signal current multiplier where the gain is independent of the supply voltage and therefore rather tolerant of a noisy supply, and the noise is leaking into later stages of amplification ... but that is just a guess.
Would be good to hear from someone who actually understands how these things work!
The buzz/hum is completely drowned out by any music, so it is not really a big deal, but it would be nice to fix it if possible.
Any ideas? E.g. are these symptoms consistent with excessive ripple from the amp's DC supply circuit? If so, should I be replacing big electrolytic caps? I'm thinking 100Hz implies its the output of a bridge rectifier, so either there isn't enough capacitance or the internal resistance of the caps is excessive.
I'm not entirely clear why the hum would be independent of the amp's gain control. I could speculate that the gain is set in a stage that is effectively a small signal current multiplier where the gain is independent of the supply voltage and therefore rather tolerant of a noisy supply, and the noise is leaking into later stages of amplification ... but that is just a guess.
Would be good to hear from someone who actually understands how these things work!
Well, the amp is old. Capacitors don't last forever. You're getting some DC leakage somewhere. This is probably somewhere after the pre-amp circuits.
I'm sure there are one or two contributors here who could answer in more detail, but they're not regular visitors since the typical level of questions here are much more basic. TBH, you're much better off asking this sort of question on somewhere such as DIYaudio.com. You've got a much higher concentration of hobbyists who build amps all the time, so there's a far better chance of getting answers faster and with greater knowledge.
I'm sure there are one or two contributors here who could answer in more detail, but they're not regular visitors since the typical level of questions here are much more basic. TBH, you're much better off asking this sort of question on somewhere such as DIYaudio.com. You've got a much higher concentration of hobbyists who build amps all the time, so there's a far better chance of getting answers faster and with greater knowledge.
Edited by Lucid_AV on Friday 10th September 13:01
Not that it's needed to locate the very obvious power smoothing capacitors, but I thought it would be interesting to see if I could get hold of a circuit dig or a schematic, and after an email to his current firm, I got a reply from Mike Creek, the amp's designer. As when meeting celebs like Jason Plato, I tried not to come across as a star-struck weirdo with alarmingly sweaty palms.
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