From: John Woodgate
Subject: Re: Output Impedence
Date: Sun, 27 Oct 2002 22:33:07 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Mon, 28 Oct 2002 06:38:32 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that news.cable.ntlworld.com
wrote (in ) about 'Output Impedence', on Sat, 26 Oct 2002:
>The reason I want to know is because I have a soundcard that is conectted to
>a mixing desk via a line out. However, there is really bad hum on the line
>(about -36db) which is noticably audiable. I have checked the desk,
>computer, soundcard and cables and have come to the conclusion that is a
>earth loop problem.
>I am going to put an isolation TX 1:1 on the soundcard output but all the
>specs I have seen on different TX's state input/output impedences. Thats
>why I need to know.
>Thats unless it doesn't really matter ??
OK, now you have explained the problem more completely, it can be
answered. The output source impedance of the line output of your sound
card is likely to be 1 kohm or less. A small 600 ohm 1:1 transformer, as
used in modems etc., is unlikely to have enough primary inductance to
give you a decent low-frequency response. You need a full-bandwidth 1:1
line transformer, which will not be cheap.
There may be a cheaper solution. You should be running a
screened/shielded cable from the sound card to the mixer. Disconnect the
screen/shield in the free connector at the mixer end, leaving it
connected at the sound card end. If you had an earth loop, you haven't
got one any more. If the hum is still there, it wasn't an earth loop.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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