CGS stereo mixer

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CGS81 the CGS stereo mixer is DC coupled, it will also work with DC signals and control voltages. Any DC offset that is present at the inputs will affect the outputs, so keep this in mind when interfacing to external equipment.

It will work on either +/- 12 volts or +/-15 volts without modification.

A little on how it works

The schematic for the basic stereo mixer. Note that many parts left of the squares marked "IN" are mounted off-board.

The circuit is a standard two-inverting-stage op-amp DC Mixer. For each channel, a panner circuit is wired before the inputs to the op-amp stages, which operate as mixers and gain recovery circuits to compensate for the losses in the panning stages.


The component overlay for the VER1.0 PCB. Note that as this board is being repurposed for the mixer, you need to follow the parts overlay given above, not the on the PCB. Click through for an enlarged, printable version. Print at 300dpi.

Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges.

When you are happy with the printed circuit board, construction can proceed as normal, starting with the resistors first, followed by the IC sockets if used, then moving onto the taller components. Note that as this board is being repurposed for the mixer, you need to follow the parts overlay given above, not the on the PCB.

Take particular care with the orientation of the polarized components, such as electrolytics, diodes, transistors and ICs.

When inserting the ICs in their sockets, take care not to accidentally bend any of the pins under the chip. Also, make sure the notch on the chip is aligned with the notch marked on the PCB overlay.

If you are not going to separate the board into two units, you do not need to install the beads and power connector at one end of the PCB. The 10uF electrolytics can be installed, as they will help with power rail decoupling.

There is provision on the rear of the board to install 10n to 100n, 1206 or 805 SMT capacitors across the power rails near the 10uF Electrolytic capacitors. These are optional.

The remainder of the circuit needs to be wired, point to point, between the jacks and pots.

Pad identification
Pad ID Function
0V 0V connection for jacks and pots
IN Input
IOUT Inverting output, if required.
OUT Output.


No setup is required.

Parts list

This is a guide only. Parts needed will vary with individual constructor's needs.

Part Quantity
47pF 4
100n (0.1) decoupling caps 4
100n 1206 SMT (optional) 4
10uF 25V 2
330R (330 Ohms) 6
47k 1% 8
100k 1% 8
150k 1% 8
330k 1% 6
50k lin pot 4
50k log pot 4
100k lin pot see text
TL072 2
Jacks as needed
Ferrite bead 2
0.156 4 pin connector 1
CGS81 VER1.0 PCB 1


  • 330R refers to 330 Ohms. 100 nF = 0.1 uF.
  • PCB 6" x 1" with 3mm mounting holes 0.15" in from the edges.


Readers are permitted to construct these circuits for their own personal use only. Ken Stone retains all rights to his work.

See also


External links