From: email@example.com (carltons)
Subject: Re: Output Impedence
User-Agent: NewsWatcher-X 2.2.3b2
NNTP-Posting-Date: Mon, 28 Oct 2002 09:12:44 GMT
Date: Mon, 28 Oct 2002 09:12:44 GMT
In article , John Woodgate
> I read in sci.electronics.design that news.cable.ntlworld.com
> wrote (in server.ntli.net>) about 'Output Impedence', on Sat, 26 Oct 2002:
> >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 ??
> 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.
Many times, these 600 ohm lines aren't even 600 ohms. They just say that
so that the voltage levels are defined. I would measure the mixer input
if I needed to know for sure. You can still drive the line with a low
impedance as long as you aren't sharing the input with some thing else and
you have sufficient voltage. It could be you have more than enough
voltage and require some attenuation. Check out all of this.
If your problem is hum, a transformer may or may not help. Is you hum
definitely coming from the source or is it due to different grounds
between the mixer and the source? If it's the source and a difference in
grounds, convert the source signal from single ended and go differential
into the mixer. If the mixer line is a real 600 ohm audio line then it
will be differential and balanced.