Apologise for this not being on the main thread, but:
Can anyone tell me how to get rid of mains hum on soundcards?
I have basically similar SB16 cards in three computers and have the same
problem on all -
Unconnected there is no hum, but when I connect the soundcard "Line IN" to
the rx "audio out", with audio gain set to zero, on any of my rxs, mains
frequency lines appear on Spectran/Argo or whatever like this :
50 Hz = -70 dB
150 Hz = -67 dB
250 Hz = -80 dB
350 Hz = -95
450 Hz = just above noise at -110 dB.
Figures relative. There is no trace at all of any even harmonics.
Of course I use shielded cable, and have tried connecting shield to earth
only at one end - no change.
I've tried all combinations of grounding - separate ground direct cptr to
rx, separately to ground,
and so on. No change.
I get a slight (3-6 dB) reduction by running a half-inch copper strap from
the metal shell carrying the soundcard sockets direct to a proper ground
(NOT mains ground) but that's all.
There is no trace of any hum on the direct audio output so I can
"eliminate" these harmonics by turning up the rx audio gain high enogh to
swamp them (they don't come up as gain is increased) but I don't think they
ought to be there at all.
All my rxs/txs are powered from a single 12v 60 amp stabilised PU using a
bridge rectifier supplied by a transformer. It is well grounded through a
half-inch strap and there is no ripple on it even at 50A. Cptrs have the
usual switch-mode supplies.
Any ideas apart from running the lot off car batteries?
Walter G3JKV.
|
