Content-Transfer-Encoding: 8bit
Message-ID: <5.1.0.14.2.20011208143147.009fc590@mail.pncl.co.uk>
Date: Sat, 08 Dec 2001 15:05:42 +0000
To: rsgb_lf_group@blacksheep.org
From: "Walter Blanchard" <blanch@pncl.co.uk>
Subject: LF: Mains hum
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii; format=flowed
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

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.