Date: Sat, 08 Dec 2001 10:07:35 -0800 (PST)
From: "John Sexton" <computernetworks@go.com>
Subject: Re: LF: Mains hum
To: rsgb_lf_group@blacksheep.org
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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"<blanch@pncl.co.uk>
To: rsgb_lf_group@blacksheep.org
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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