for music synthesizers.
This project arose out of a need for a small mixer that preserved the D.C. component of signals, at audio frequencies and below, specifically for the hobbyist synthesizer builder, though it's applications do not stop there. Its compact size means it can easily be included along with other circuit boards in modules that do not have enough inputs of their own, such as some VCAs or VCFs.
It can just as appropriately be used as a module in its own right, as shown in the photo to the right. In this case there are six inputs, two "normalized" through switched jacks to + and - 7 volts, allowing the mixed signals to be shifted up and down. The seventh knob works as a master level, though including this control is optional.
The circuit design is quite conventional, using two op-amps in a unity gain inverting configuration. The first stage mixes the signals from the six inputs. The output signal from this op-amp is inverted. The second op-amp, which could be considered a single input "mixer" simply re-inverts it.
The resistor (*B) can be replace by a variable resistor of 50k if a master level control is required. As the value of this resistor reduces, the swing at the output of the op-amp also reduces.
The 3k3 at the output simply offers some protection to the op-amp in the event of a short circuit, which happens frequently when patch cords are being used. The second 3k3 (*A) allows a second output to be easily implemented, and gives some isolation between two loads, thus reducing interaction. These resistors can just as easily be 1k, bringing them more in line with what seems to be a standard for synthesizer modules.
The two 10mfd capacitors give some power supply isolation. Power rails can be almost anything from +/- 6V to +/- 15V. Asymmetrical rail voltages will work, but the mixer will behave as if it was running on supply of +/- of the lower of the rail voltages. E.g. supply rails of +12V/-6V would cause the mixer to behave if it was running on a +/- 6V supply.
Only a single example of an adjustable input is shown on the circuit diagram. Simply repeat as needed.
The overlay for the original PCB, showing optional components and external connections.
Parts List (PCB only)
The back of the mixer module. Panel-work will vary with the individual's needs.
Article, art & design copyright 1998 by Ken Stone