From: John Woodgate
Subject: Re: Output Impedence
Date: Mon, 28 Oct 2002 07:00:48 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Mon, 28 Oct 2002 09:05:13 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Tom Bruhns wrote
(in <firstname.lastname@example.org>) about 'Output
Impedence', on Sun, 27 Oct 2002:
>"news.cable.ntlworld.com" wrote in message news:email@example.com>...
>> Cheers Guys,
>> 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 ??
>Well, it does matter some. What's the input impedance of your mixer?
>Use a transformer matched to it; that's more important than matching
>the output impedance of the soundcard, which likely is pretty low. If
>you load the transformer with a load equal to the transformer's rated
>secondary impedance, then the primary will present (nominally) its
>rating to the sound card output, and that shouldn't be a problem.
No, we don't do that. The mixer input impedance is likely to be 10
kohms, and if you want a transformer that 'matches' that, it implies a
primary inductance well above 10 kohms at 20 Hz, which is a LOT of
Audio interfaces these days are all low-impedance sources driving high
impedance loads, which means that the transformer primary inductance
needs only to be high compared with the source impedance.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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