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From: fish@ihlpg.UUCP (Bob Fishell)
Newsgroups: net.audio
Subject: Re: question
Message-ID: <336@ihlpg.UUCP>
Date: Mon, 1-Apr-85 17:54:40 EST
Article-I.D.: ihlpg.336
Posted: Mon Apr  1 17:54:40 1985
Date-Received: Tue, 2-Apr-85 07:25:09 EST
References: <458@bonnie.UUCP>
Organization: AT&T Bell Laboratories
Lines: 41

> *** REPLACE THIS LINE WITH YOUR MESSAGE ***
> Here's the situation:
> I have two headphone signals (walkman + stereo keyboard) that I
> want to mix together to hear on one headphone.
> Can you 'Y' this stuff togther directly or do you need a mixer.


> Is is possible to mix speaker level signals?


***     AC  T              YOUR     AGE ***

It depends on the output impedance and levels of the devices.  Generally,
it's not a good idea to "y" outputs together, since you'll frequently
wind up with two C/S collector circuits glaring at each other -- bad
for both of them, especially if they share a ground connection.

For higher impedance headphones, e.g. Sennheisers, all you need to
do is put a couple of resistors in series with each output device,
i.e.
               R1
device 1 o-/\/\/\/\-:----headphone
               R2   |
device 2 o-/\/\/\/\-'

R1=R2=150 ohm.

This will provide sufficient isolation, in most cases, to eliminate
any problems that might be caused by the amps loading one another
down.  If you have a floating ground output, you'll have to use
two sets of resistors, one on each lead.  This scheme will work
(I've done it) for headphone jacks as well as speaker drive
terminals, as long as you're driving a relatively high impedance.

If you have speakers or other low-impedance loads, you'll have to use
a mixer.

/_\_

				Bob Fishell
				ihnp4!ihlpg!fish