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Grex Science Item 45: 60Hz hum
Entered by devnull on Fri Mar 5 08:05:53 UTC 1999:

What can be done to get rid of the 60 Hz hum in a boom box?

7 responses total.

#1 of 7 by n8nxf on Fri Mar 5 11:19:10 1999:

There is no cure-all for 60 Hz hum.  That should have been addressed
during the design phase of the project, not after it gets into a
customers hands. (Though if enough people complain about it, they
may decide to do something about it in the next model they design
Of course, yours could have a defect in it too...)

#2 of 7 by rcurl on Fri Mar 5 17:29:53 1999:

I once designed and built a tube amplifier that had fine DC filtering
but AC on the filaments that introduced a very faint but annoying 60 Hz
hum in very quiet passages (and I was much more of a hi fi fanatic
in those days). I added a circuit to inject an 180 degree out of phase
60 Hz signal with adjustable amplitude and phase to eliminate the
last vestiges of hum. By the way, is it 60 Hz or 120 Hz hum from
that boom box? Tubes aren't used anymore, and AC rectification introduces 120,
not 60 Hz, hum.

#3 of 7 by devnull on Fri Mar 5 19:57:34 1999:

I don't know of a way to determine whether it's 60 Hz or 120 Hz.  I assume
that it is caused by rectification.

The relavent device is about ten years old; I doubt complaining is likely
to buy much at this point.

#4 of 7 by tpryan on Sat Mar 6 00:18:46 1999:

        Use batteries.

#5 of 7 by keesan on Sat Mar 6 03:22:01 1999:

Could you replace the largest capacitor in the power supply, or replace the
power supply?  We have power adaptors to hard wire in, fixed someone's boombox
by wiring a power adaptor to it after he plugged in into 220AC.

We have a tube portable phono with a hum.  Bad tube?

#6 of 7 by russ on Sat Mar 6 06:09:23 1999:

A full-wave rectifier (cheap enough for everything these days) produces
hum at twice the input frequency, so it would be 120 Hz in the USA.

Adding additional filtering is easy.  You buy a filter capacitor of the
appropriate voltage and wire it across the power supply outputs.  The
bigger the capacitor, the more it will suppress any residual hum (but
you have to be careful that the turn-on surge does not overload the
power supply's rectifiers).

Re #5:  Could be a lot of things.  Probably not a bad tube, though.
A bad tube would amplify poorly, but the cathode (which is probably
heated indirectly and responds very slowly to changes in heat input)
seems unlikely to cause hum through aging.

#7 of 7 by n8nxf on Tue Mar 9 11:16:47 1999:

Even if you have perfect filtering there is a good chance that your boom
box will hum.  One time I built a power supply to run an old car radio
in my basement.  It had a lot of hum on it.  I had to physically move
the transformer to get rid of the hum.  The magnetic field around the
transformer was getting into the radio and being amplified.  Moving it
solved the problem.

Amplifiers can generally tolerate a fair amount of AC ripple on their
power supplies before they produce an audible hum.  This is because most
amplifiers employ feedback where the output signal is compared with the
input signal and adjusts the circuit accordingly. The feedback circuit
is fast enough to compensate for 60 / 120 Hz AC ripple.  Hence, if you
can hear hum it is usually part of the signal being amplified.

Keesan:  Check the B+ power supply with a scope or AC volt meter.  This
is an old phone and electrolytic capacitors tend to dry out with time.
(I have an old AM 2m radio with the same problem.)

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