When building PSU's (especially for audio/radio use) I always place 4 x =
0.1uf caps across the each of the rectifier diodes in the bridge.
Open up a good quality audio amp and have a look - there usually there =
especially if it has a radio tuner input.

The switching action of the diodes modulates any RF present in the mains =
supply and can produce a 100Hz hum in certain conditions.

Joe



-----Original Message-----
From: Tal Bejerano - AMC [mailto:kooter@ZAHAV.NET.IL]
Sent: 23 June 2002 01:51
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]: Hum problem


Hi 2 All

Attach a quick schematic that I draw to explain my problem.
The problem is a hum I can't rid of.
I try to increase the 1000uf capacitors with larger ones (4700uf) but no
change, only the hum sound changed to lower. I try to increase the =
output
caps' from 100uf to 470uf but no change at all.
what I can do more? maybe the bulky transformer that on the pcb can =
cause
this hum problem?

if you think the component placement is important I can send the pcb
picture/drawing (even I put it in the way that unregulated / unfiltered
traces will not be near the audio circuit)

I really need your help!

Tal

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