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From: smithj@hpsad.HP.COM (Jim Smith)
Newsgroups: sci.electronics
Subject: Re: Audio Mixer Question
Message-ID: <1840038@hpsad.HP.COM>
Date: 20 Feb 91 01:10:22 GMT
References: <1991Feb17.072807.25379@Neon.Stanford.EDU>
Organization: HP Signal Analysis Division - Rohnert Park, CA
Lines: 36

Andrew Zimmerman writes:
====
was curious about the stereo systems.  Do they use one potentiometer to
control both channels, or do the slide pots control two resistors?
Same question applies to a balance control.  Actually, I am more
====
Actually, most mixers use two pots with two sliders.  But for your application,typically one control would control two resistors.  Stereo volume control
pots are pretty common, last time I checked Radio Shack sold a 100k version.
Although a balance control can be implemented with a single pot element,
most of them that I've seen use dual pot elements, with opposite tapers.
Actually, there is a balance pot taper, where the resistance of one side
decreases to zero from one end to the center of rotation, then the resistance
of the other side begins to increase.  Again, last time I checked, Radio
Shack sold such a beast.

For your application, a simple summing opamp circuit in each channel should
do fine.  Any linear applications handbook (like National's or Signetic's)
should have a decent schematic.  Bells and whistles you could add include
buffer amps on each input and an input trim on each input to match the levels
of all the inputs at any particular fader setting.  One thing to be aware of
is that if you want to feed a phonograph to it, you will need to put a phono
preamp circuit ahead of the mixer.

Last time I built a mixer, I had trouble finding appropriate slide pots
(with more than a 1.25 inch throw and a decent feel and the right taper)
and ended up using rotary pots (ala Radio Shack).  It does the job...

Good luck!
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| Jim Smith                         | e-mail: smithj@hpsad.HP.COM             |
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