Threshold 400a hum/buzz

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Joined 2011
Ok, is this a low pitch hum, or more of a buzzing, similar to when the turntable ground wire is disconnected?
If your uncle is good, then the soldering should also be good.

How long ago was this work done, and how much was the amp used after that?
 
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Most commercial amps has a bit of hum if you have sensitive speakers. If you build your own amp(s) + PSU(s) you can build a PSU(s) that has most lower ripple than any commercial amp. For me DIY is the ability to build amps to a much higher standard that you can buy :)
If you go for max. 1 mV ripple on PSU.......you are almost there....... :) .......filter chokes does a nice job.......
 
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Joined 2014
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try measuring the VAC coming off of the power supply caps. I don't think the PS is very sophisticated on these amps. As in, just a plain cap bank. Not even CRC. As mentioned, a little hum coming from the speakers is pretty normal. As in having to put your ear up to the speaker to hear it.