Making The Passive Stereo Audio Mixer Model "PAM-4"

For about $20 (if you have the tools), you can build the above passive mixer for four-or-less CD changers
that will work quite well to combine the CD players signals. A passive mixer just uses resistors to isolate
the players from each other so they don't cause an impedance mismatch being all connected together.

An impedance mismatch distorts the sound quality, changes the frequency response, and usually
affects your volume level. Passive mixers, like the one described above and below, will drop the audio
gain just a tad (about 3db), but do not usually distort the audio. You can make up for the audio drop by
turning your amplifier or receiver up about 1 notch.

IMPORTANT: You should NOT use a "Y-cable" because of the player's solid state output. Players not
playing will clamp their output to ground and cause an impedance mismatch and drop in volume level as
described above. This effects players that ARE playing.

We have several links to audio music equipment supply companies on our web site that sell four, six,
and eight-channel stereo mixers that take up one-or-two rack spaces (19"W x 1.75"H -or- 19"W x 3.5"H).
These are brands such as Ashly, Rane, Symetrix, Fostex, and Samson, and run from $210 to $500,
depending on the features and quality. This is the ideal route, but if you're on a budget, read on.

The following text-based schematic is for a four-channel passive mixer. We at Wintek SC, and many of
our other customers have built and tested the passive mixer below. It works quite well, with no bad side effects.

You can build it with more or less channels if you wish, but if you need to combine more than four
players, we strongly suggest an active mixer, due to the greater signal loss.

The parts you need for this four-channel passive mixer are available from Radio Shack, Fry’s Electronics,
Digi-Key, Jameco, Mouser, or other electronic parts stores.

TOOLS:
1 - 25-watt soldering pencil (Radio Shack Cat# 64-2070, $6.99)
1 - Roll rosin core solder .5 oz (Radio Shack Cat# 64-001, $1.19)
1 - Nibbling tool (Radio Shack Cat# 64-823, $10.99)

OR optionally (if you feel uneasy with a soldering iron) you can use:
1 - Solder tape strips (Radio Shack Cat#64-010, $1.59). You wrap the
tape around the connection and heat with a match to solder it.

PARTS:
8 - 4.7k ohm 1/4 watt resistors (Radio Shack Cat# 271-1330, 2@$0.98)
(The resistors come 5 to a pack, so you only need two packs)
1 - 4" x 2" x 1" project box (Radio Shack Cat# 270-1802, $2.29)
1 - Octal Phono Jack Board (Radio Shack Cat# 274-370, $2.19)
1 - 6 foot Stereo Patch cable (Radio Shack Cat# 42-2351, $3.99)

OR optionally you can use:
1 package - Panel Mount RCA phono jacks (Radio Shack Cat # 274-346, 4 in a pack @ $2.59)

Circuit Description

Input 1 Left o-----resistor----+
Input 2 Left o-----resistor----|
Input 3 Left o-----resistor----+-------------o Left Output to Amp CD In
Input 4 Left o-----resistor----+ (Left Stereo Patch cable)

Inputs 1-4 Grounds o-------------------------o Left/Right Grounds

Input 1 Right o----resistor----+
Input 2 Right o----resistor----|
Input 3 Right o----resistor----+-------------o Right Output to Amp CD In
Input 4 Right o----resistor----+ (Right Stereo Patch cable)

Each input for each channel must go through a 4.7k resistor to the output cable to be combined with all
the other inputs from other players.

Mount the octal jack board onto a project box. This is done by cutting out a square big enough for RCA
connector strip to protrude through. This can be done with a drimmel or notch tool.

Cut the plugs off one end of a standard audio patch cable, strip back the end, and feed it through a hole
in the project box.

Solder a resistor to the center tab of the four input jacks as shown above. Then connect the other side of
all four resistors to left output cable. Do the same for the other four jacks for the right output.

Buss all the grounds of all those jacks to the shield on the output cable.
It should end up looking like this complete with case. (Pardon the stick drawing).

+-----------------------------+
| CD AUDIO INPUTS |
| 1 2 3 4 |
| +----------------------+ |
| L | O O O O | |-----> Left output to amp CD input
| R | O O O O | |-----> Right output to amp CD input
| +----------------------+ |
+-----------------------------+

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