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Old 4th October 2012, 05:53 AM   #11
jitter is offline jitter  Netherlands
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Originally Posted by Mooly View Post
I too think layout and grounding is probably the most likely cause and from what I can see from your picture it all looks really neatly built.

If the "hum" has a harsh sounding edge to it (harmonics present) then grounding is a prime suspect.
I agree, see also this thread. On symmetrical PSUs with single bridge and centre tap as GND, where that centre tap connects with respect to signal reference (GND) is all important.

50 hz hum. If it were induced hum then I would think the output would be more sinusoidal and tbh at that level probably not audible. 50 hz as a sinewave is a very deep pure tone (if the level is high enough).
Worth an experiment.
I have a DIY class A headphone amp that is powered by a toroid. Because of limited space, it was near the amplifier board and with no signal on the input I could hear a very faint hum. After hanging the toroid on the outside of the case, the hum disappeared.

Move all transformers away from the wiring through which the signals flow as far as possible. If hum goes away, it was induced.
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Old 14th October 2012, 08:36 PM   #12
Bensen is offline Bensen  Belgium
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Default Problem solved

1: after the right frequency measurements of 50Hz and 100Hz, I stopped thinking that the problem was the power supply

2: as mentioned by Nigelwright7557, I put in a 33R before the cap that buffers the LTP's and VAS ==> humm was still there, but amp seems to be having more detail

3: I splitted the audio ground and power ground by a 10R resistor.
==> this solved the humming problem.

With the input shorted and my ear against the tweeter, I can't hear a thing!

Thanks guys!
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Old 14th October 2012, 08:46 PM   #13
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Originally Posted by Bensen View Post

3: I splitted the audio ground and power ground by a 10R resistor.
==> this solved the humming problem.

With the input shorted and my ear against the tweeter, I can't hear a thing!

Thanks guys!
Classic ground loop problem.

I built a mixer on a pcb with no thought to grounds and it hummed really badly. When I split off the power supply and had one star ground the humm went completely.
Murton-Pike Systems PCBCAD51 pcb design software.
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