Hi Walter,
I think the problem is with the computer.
Here I use little audio transformers (I have several made up in boxes) to 
isolate the rx from the computer. It does make a big difference, but I still 
see 50 or 100 Hz lines especially at VLF, practically not at all on LF.
Hoping that someone comes up with the definitive solution.
73 John, G4CNN

-----Original Message-----
From: "Walter Blanchard"<[email protected]>
To: [email protected]
Date: Sat Dec 08 07:05:42 PST 2001
Subject: LF: Mains hum

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.

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