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Subject: Passive mixer design.

Date: 01/16/01 at 1:41 AM
Posted by: Phill Jones
E-mail: naffsmeggingnaff@yahoo.com
Message Posted:

Hi!

I've recently designed a little circuit for a passive mixer.
The concept behind the thought of this project was, I wanted
to be able to plug in a lot of multimedia microphones into my
computer, but being able to control the gain of the mics at
at the same time. This led me to think that all I'd need was a
basic passive mixer. So I thought, ok, can't be too hard. 6.5mm
mono jacks (I know computer mics are 3.5, but I've got my reasons.)
straight through to a pot, and here's the first question. I've got
a few pots to try out, such as a 1k a 5k and a 10k. I guess I'll
just have to try, but I was wondering which one woud be the best
bet. There's not going to much current coming through from a tiny
dynamic mic, so you'd hardly need much resistance, was my theory.
I'm not a electronical engineer though, so I'm most probably wrong!
So, once we're through the pot, I figured that each channel is going
to need to be individually isolated. It was suggested that putting a
resistor in place between the pot and the output bus on each channel
should do the trick of impedence matching the channels. Could someone
verify this? It was my thought that a simple .01 cap would do the trick
by cutting out the dc current going through. Well, not exactly my thought,
because someone told me this one too, but hey! I'm wondering what's the
go with these two options. I'm trying to achieve an outcome in which
if channel 1 is shorted to earth, then it's not going to short everything
else on the bus. So that's question 2! After that, all the channles collect
on an output bus. Half the channels go to a left on a 3.5mm stereo output jack,
and half go to the right. So this in theory should give me a basic mixer,
working off the controlling of the gain in each channel. I don't
want to be able to boost individual gains, so I don't need seperate pre-amps
for each channel! I don't need a pre-amp at all, and if so, then I'll
put one in situ. after the entire mixer. So, with this in mind, I got thinking again.
(Dangerous thing to do!!)I play guitar, and because I'm not a moneybags, I plug my
guitar direct into the soundcard. This works fine, except that you get the left channel,
and nothing else! The mixer I decided should be able to be turned into a bridged stereo one.
So I figured that with a SPST switch in place after the cap/resistor in channels 1 to 4,
I should be able to patch the right channels into their respective left partners.
By doing this, I hope to be able to have a 'stereo' input to the computer for my poor
guitar! Or indeed, any single mono source that I want bridged! Question here... is the
way to go about it? I can't see why not, but if I'm wrong, then let's have it!
Anyway... enough from me. I've got the thing planned out, and I'm pretty sure it'll
work, but I'm just basically running past a few sets of eyes to see if there's any pro's
who think what I'm doing is wrong. Thanks heaps!

Phill.


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