Getting rid of low level hum in Aragon 4004MKII

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I just completely went through this amp trying to get rid of the slight hum it had. New caps everywhere, thermal grease through cleaning and biasing and dc offset. I still have some slight hum and can't track down where its coming from. If I short the inputs I can't hear it. Also if only one channel is hooked up I can't hear it. This makes me think it is solveable. I'll attach my folder full 'o schematics for ya.

https://dl.dropboxusercontent.com/u/3011761/4004 Service Info/004mkIIUpgrade.pdf
https://dl.dropboxusercontent.com/u/3011761/4004 Service Info/4004 board layout.pdf
https://dl.dropboxusercontent.com/u/3011761/4004 Service Info/4004 power supply.pdf
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
I don't know your previous experience with amplifiers, but an open ended input lead is usually an invitation to environmental electrical noise of all descriptions. Often there is a muting switch to avert this noise when it affects the normal use of straight power amplifiers such as this one. Integrated amps have volume controls etc. which attenuate the input and noise will be less noticeable.

There is an unspecified earthing recommendation amongst the mods, which shows that power supply ground is set above chassis ground by a resistor but without figures, i can only assume this gives lowest noise when the amplifier is connected to a signal source. It would seem to be the way it is intended.

As you've realised, when there is a signal lead that provides a ground connection to the preamp ground, there is less noise. I can't imagine anyone wanting to use the amplifier without an input so unless there are issues with having input leasds always fitted in the normal way, I think you have your answer already.
 
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