Audio Mixer MK1

 This is a an active audio mixer circuit. It has two audio inputs and mixes the two channels together.

What do I mean by active?
There is an amplifier in the design and the volume level for both input channels is variable.

The amplifier is rated at 250 mW RMS or 10W PMPO!*

Current Status:    Complete
Date:                  4th May 2004
License:               Creative Commons Attribution 3.0 Unported License

June 2012
Added the Eagle CAD files, which include the schematics and PCB.
An updated design, that is intended to interface with an amplifier and provides cross channel fading has been developed. Full details are here:

4th May 2004 update
The design is fully tested and working fine.

The schematic is shown below, click the thumbnail to enlarge.


PCB design
The Eagle CAD design files, including the PCB, can be downloaded here (152K)
Click the thumbnail below for a larger view of the PCB.


Parts list

The parts came from the following suppliers:
ESR Electronic components,
HCP Cables

Part number  Description      QTY    Supplier   Cost inc VAT    Total Cost
LM386N       Audio Op-Amp       2     ESR      0.53 ea           1.06
954-310      Dual 10K log pot   3     ESR      1.81 ea           5.43
110-080      8 way DIL socket   2     ESR      0.07 ea           0.14
710-530      5mm Red LED        1     ESR      0.08 ea           0.08
?            Mini toggle switch 1     ESR      0.63 ea           0.63
NC56L        Soft tch knob, blu 1     Maplin   0.29 ea           0.29
NC57M        Soft tch knob, yel 1     Maplin   0.29 ea           0.29
NC59P        Soft tch knob, grn 1     Maplin   0.29 ea           0.29
AQ89W        2.1 mm power plug  1     Maplin   0.49 ea           0.49
JK09K        2.1 mm power socket1     Maplin   0.99 ea           0.99
AQ81C        Power adaptor lead 1     Maplin   0.99 ea           0.99
DC plug 2.1  2.1 mm power plug   1    ESR      0.30 ea           0.30
DC skt 2.1   2.1 mm power socket 1    ESR      0.47 ea           0.47
XS47B        fig 8 power cable   2    Maplin   0.29 metre        0.58
JW64U        PC Power connector  1    Maplin   0.79 ea           0.79
JW59P        0.1" pin strip      1    Maplin   0.79 ea           0.79
JH95D        6 way phono socket  1    Maplin   0.99 ea           0.99
?            Phono skt, chassis  2    ESR      0.24 ea           0.48 order 1 red, 1 black
Many         Misc resistors      20   ESR      0.03 ea           0.60
Many         Misc capacitors     10   ESR      0.06 ea           0.60
76.190       CD audio lead       1    HCP      2.50 ea           2.50!
Total  (using ESR components)       17.33
By shopping around you can reduce the cost of the connectors/cables.

The final design has been built and here is a picture of the prototype, it is currently (as of July 2012) fitted inside my workshop PC, used to drive a pair of small speakers.


*Peak Music Power Output (PMPO) is a bull shit marketing term to make a product look as though it has large power output, when in reality it does not. I just made the figure up like most manufacturers do.


Updated 14 April 2021