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Old 21st March 2015, 11:59 PM   #1
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Default Concern about humming amp

Hey all,

I'm not much of a do-it-yourselfer, but I have a Sansui AU-999 integrated amp from the early '70s that has a slight mechanical hum. I don't think it did this before I moved into my current house earlier this year. I have done a little reading and discovered that it is likely due to DC on the mains. Is this right? More importantly, do I need to worry about damage to the amp? I can't imagine running that much DC through the power supply is good; should I do something to clean the power? Alternatively, if I'm not risking damage to the amp, it'd be easy enough just to ignore.

Thanks,

Jordan
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Old 22nd March 2015, 07:39 AM   #2
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Location: Coffs Harbour
Quote:
Originally Posted by JordanHunn View Post
...I have a Sansui AU-999 integrated amp from the early '70s that has a slight mechanical hum. I don't think it did this before I moved into my current house earlier this year. I have done a little reading and discovered that it is likely due to DC on the mains. Is this right? More importantly, do I need to worry about damage to the amp? .......Alternatively, if I'm not risking damage to the amp, it'd be easy enough just to ignore.....
Mains DC problems are real enough in many locations, particularly where heavy industry power demands also cause power factor and phase sharing problems. The effect on audio power amplifiers though, is not as often as might be suggested by the number of threads raising the issue.

Unless you compare with another, say a borrowed amplifier or hear the problem with other audio equipment too or neighbours also have a similar persistent problem, the fault is more likely in your aging amplifier. DC presence on the mains usually varies over a daily cycle so you wouldn't anticipate a constant effect over a period of a day or a week. However, the rest and relocation by moving house may just have been enough to bring an existing problem to your notice.

It's also possible that the transformer windings or iron laminations have loosened enough over time to become noisy. A check over any transfomer assembly bolts and a careful prod with a finger or makeshift stethoscope to identify where the buzz is coming from, should help narrow down the source of the hum. Large and older transformers often use a bolted lamination assembly and carefully re-tightening the long, exposed through-bolts can help there. However, I doubt that just a slight mechanical hum from the transformer will cause much harm. Some amplifiers, notably larger pro. amps, hummed noticeably, right from new and continued to for decades - DC or not!

Though this is more about audio hum, have you replaced the amplifier's electrolytic caps in the last 20 years? I have come across amplifiers where dying power supply caps cause mechanical noise too. Maybe time for an overhaul?
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