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{subhead}DIY Project: Audio Mixer Part 1{def}{p}
Article by Paul Harvey
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I started this little project a while back when I got a SCSI CDROM
for my A1200 and got a bit  annoyed when I couldn't hear the CD
audio. I never got around to finishing it, as I ended up  buying an
A4000 anyway, which has an onboard audio mixer. This month I'll show
you what's involved in building a summing amp.
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If you have ever tried mixing audio signals by just hooking the two
inputs in parallel, you would  have heard a great deal of noise, and
decided it was a bad idea. A better way is to use an op- amp IC to
add the two signals together properly. The IC will cost you about $2
AUSD at the  most. Together with a few discrete components, you can
configure the IC as a 'summing non- inverting' operational amplifier
for less than $5 AUSD.
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What you need:
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- 4x 10k ohm resistors{p}
- 2x 10uf electrolytic capacitors{p}
- 2x 100uf electrolytic capacitors{p}
- 4x 47k ohm variable trim pots/potentiometers{p}
- 2x Operational Amplifiers - Get a 'dual' or 'quad' IC that has 2 or more in it, and is made 
specifically for Audio applications. Alternatively, use 2x 741 general purpose Op Amps (or it's 4
in one package equivalent, the LM324N), though you may not get as good quality output.
Alternative Op Amps and their pinouts are listed at the end.{p}
- A piece of 5x5cm prototype matrix PCB (predrilled with copper pads) or veroboard. Alternatively 
you could etch your own PCB.
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To use Op Amps, you need a few special power supply requirements: 0v
(ground), +9v and -9v.  There are some fancy tricks you can use to
get -9 and +9v from a 0v/18v source, but I think the easiest way is
to use the -12v and +12v lines that are running around inside your
computer. The 741 can actually handle a potential difference of up to
36v; -12v and +12v should work fine for most Op Amps. Check the
specifications of the chip you purchase, though.
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For Amiga 1200s in the keyboard case with external PSU:
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Square PSU Plug:{p}
Pin No. Description:{p}
 1       +5v DC{p}
 2       Shield Ground{p}
*3       +12v DC{p}
*4       0v/Ground{p}
*5       -12v DC
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For Amigas towered in a case with an AT PSU:
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AT mainboard power connector plug (ie. not the Molex/FDD power plugs):{p}
Pin No.  Description:    Usual colour:{p}
 1.      Power Good *    Orange{p}
 2.      +5Vdc           Red{p}
*3.      +12Vdc          Yellow{p}
*4.      -12Vdc          Blue{p}
*5.      Ground          Black{p}
*6.      Ground          Black
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*7.      Ground          Black{p}
*8.      Ground          Black{p}
 9.      -5Vdc           White{p}
10.      +5Vdc           Red{p}
11.      +5Vdc           Red{p}
12.      +5Vdc           Red{p}
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Items with asterixes next to them are the wires you will need.
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For wiring this thing up, follow the schematics below.
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{image gfx/mix16.iff}
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Well, that's all you need to know. As always, keep in mind that what
I've suggested here could be incorrect. If you are serious about
building this, *please* double check my PSU pinouts from another
source, and double check your own connections before you power
anything up! I don't accept any responsibility for any of the
information presented here; use it at your own risk. To the best of
my knowledge, and to the best of my ability, I've tried to make all
pinouts and information listed here as accurate as possible. I've put
the circuit through a software simulator, and nothing blew up... :)
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Next month I'll report how my own summing amp goes, ie. whether the
whole idea is flawed or not ;) If you make this yourself, tell us
about it!
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- Paul Harvey (paul@aio.co.uk)
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Pinout of:{p}
 the LM324 (General Purpose Quad Operational Amplifier IC) &{p}
 the LF347 (JFET Low Noise Quad Operational Amplifier IC)
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Pin No.  Description:{p}
 1       Out 1{p}
 2       In 1 -{p}
 3       In 1 +{p}
 4       +ve{p}
 5       In 2 +{p}
 6       In 2 -{p}
 7       Out 2{p}
 8       Out 3{p}
 9       In 3 -{p}
10       In 3 +{p}
11       -ve{p}
12       In 4 +{p}
13       In 4 -{p}
14       Out 1
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Pinout of:
  the LF353 (JFET Low Noise Dual Operational Amplifier IC){p}
  the LM833 (HiFi Dual Operational Amplifier IC)
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Pin No.  Description:{p}
 1       Out 1{p}
 2       In 1 -{p}
 3       In 1 +{p}
 4       -ve{p}
 5       In 2 +{p}
 6       In 2 -{p}
 7       Out 2{p}
 8       +ve
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