CGS multi-mixer

CGS80 the CGS multi-mixer module is a multi-purpose mixer that can be used for both synth-level audio and control voltages. There are eight input channels, each which has AC coupled and DC coupled inputs and a level control. The AC inputs are primarily for audio use, but can be used to remove any DC offset from a control voltage. Only the fast moving portions of the control voltage will be passed.

The module will work on +/-12 volts.

The eight inputs are then mixed into three different types of output stage.
 * 1) DC Summer. This is the standard type of DC "mixer" used with synthesizers. The voltages on all inputs are added together, the sum being present at the output. An inverted output is also provided (mirrored around 0V). Optional DC offset and cascade inputs can be fitted. The problem with this sort of mixer is that the output can easily be pushed beyond the usual voltage range of a synthesizer. E.G. 8 x 5V = 40V. Of course the output will clip long before this, and in extreme cases, may lock up until power is removed. The Master Gain for the output stage can be used to scale the sum to a useful level.
 * 2) AC Summer. This is an AC coupled summer. All DC offsets will be removed from the mix. The output will be the sum of the AC portions of the signal. An inverted output is also provided (mirrored around 0V). Optional DC offset and cascade inputs can be fitted. This output is primarily for audio use, but may give interesting modulation possibilities. Its functionality overlaps that of the AC coupled inputs. A Master Gain is provided for convenience, but will not have the same impact as with the DC summer.
 * 3) DC Mixer..... well, read a bit more about this below.

A little on how it works
The Multi-Mixer is basically an array of inverting summers. Inputs are buffered (and inverted) by the op-amp on each input stage. The outputs of these buffers are fed to three mixing buses. The AC summer and DC summer busses are then fed to their summer stages, where they are also re-inverted. The inverted output is provided by an extra inverting buffer.

The resistive mixer bus is buffered by a voltage follower before being fed to a two op-amp output stage identical to the others. It is this voltage follower that prevents the output stage from summing the voltages on this bus.

Note: DC Mixer. The idea was to provide a different mix to the DC summer, but if my brain is finally working correctly, the output of this will merely be the sum of the inputs divided by the number of inputs (8) - which is no different from the DC summer with its output turned down to one-eighth. So with that taken into account, it is rather pointless. It can however serve as a separate "partial mix" channel, or a second output buffer if a person wants to divide this into two smaller mixers. Leave out the TL071, link pin 3 to pin 2, and put a link in place of resistor RH.

If you don't require this channel, simply leave out all associated parts. These are the resistors marked RC that are connected to the mixer bus, and the buffer amplifier marked "mixer 1" on the PCB.

If you do want to add an odd mix, just to fill the panel holes, connect the output of the DC Summer (OU) via a 200k to inverting input of the mixer buffer TL072 (Pin 2) and install every second mixer bus resistor marked RC. These will inputs will now subtract from the output total instead of adding to it.

Construction
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.

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 planning to use this for audio mixing, the DC offset pots should be omitted.

If you are planning to drive the mixer to saturation, it may be wise to substitute op-amps that do not suffer from phase-inversion when their common mode range is exceeded.

Resistor identification
Most resistors are shown on the board as identifiers. The values given here are recommended values. Resistors sharing the same functions in each channel share the same identifier, making it easy to substitute different values if required. In the default configuration, most of these resistors are 100k.

If you wish to achieve greater than unity gain, reduce the values of RC and RD.

Pad identification
Each channel has its connections grouped on the PCB. Each output stage has its connections grouped on the PCB. To distinguish between each group, look on the PCB for the corresponding label.



Setup
There should be no setup required. Depending on the quality and tolerance of the pots used, you may find that you do not have unity gain with a pot set to its maximum position. This can be corrected by tweaking the values of other resistors in the circuit. The easiest solution is to pre-test your pots and select suitable ones.

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

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