"H. Dziardziel" wrote:

On Sun, 04 Jan 2004 23:42:21 GMT, "author" wrote:

The power supply in my radio has audible hum in the background
and bypassing the diode rectifiers with capacitors does not eliminate the
hum. There is no hum when using battery power. The solution is to
build my own power supply. Does anyone have a design for a compact 6V
supply that I could build for this purpose?

Do you mean the dc output was bypassed i.e. capacitors were
paralleled with the dc output? Bypassing rectifiers will
increase or create hum. I would suggest replace all the
rectifiers (could be leaky) and capacitors (could be leaky too)
first. One can add too much capacitance too by the way. They
become the load instead of the actual load. What is the voltage
and current requirement and the actual under load? That too can
be the source of hum.

The purpose of bypassing (in parallel) the power supply diodes
(rectifiers) with capacitors (.010-mfd) is to remove RF switching
transients (caused by the diodes) from the DC output. This can often
eliminate the kind of hum which is heard when the radio is tuned to a
strong station but the hum goes away when the volume is turned down all
the way. Regular AC hum caused by a problem with the power supply filter
capacitor(s) can still be heard when the volume is down.


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On Thu, 08 Jan 2004 00:02:45 -0500, starman
wrote:



The purpose of bypassing (in parallel) the power supply diodes
(rectifiers) with capacitors (.010-mfd) is to remove RF switching
transients (caused by the diodes) from the DC output. This can often
eliminate the kind of hum which is heard when the radio is tuned to a
strong station but the hum goes away when the volume is turned down all
the way. Regular AC hum caused by a problem with the power supply filter
capacitor(s) can still be heard when the volume is down.



Hmmnnn...no pun intended of course....
http://home.computer.net/~pritch/shortwav.htm
This occurred to me too but if in fact the diodes were bypassed
and there was no effect I dismissed that source -- too quickly on
second thought, thanks.

In article , "starman"
wrote:

The purpose of bypassing (in parallel) the power supply diodes
(rectifiers) with capacitors (.010-mfd) is to remove RF switching
transients (caused by the diodes) from the DC output. This can often
eliminate the kind of hum which is heard when the radio is tuned to a
strong station but the hum goes away when the volume is turned down all
the way. Regular AC hum caused by a problem with the power supply filter
capacitor(s) can still be heard when the volume is down.

There's a third pssibility: the hum may be coming from the AC line itself
(as 60 or 120 Hz modulated RF in common mode). The design of the power
supply has little influence on this. Common mode chokes are sometimes
helpful, but not terribly effective. Measures to reduce common mode
coupling to the receiver's antenna input (balanced antenna, grounded coax
feed) are more effective in this case.

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