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Voltage Controlled 3-Input Audio Mixer


Jump to Voltage-Controlled Mute daughter module


A versatile 3-channel voltage-controlled audio mixer, giving three input channels - with individual post-fade outputs and signal status LEDs - and one master channel, again with signal status LED. There is also a facility for expansion by adding a daughter card, the first of which will be a voltage-controlled mute for the three input channels.

  • All four channels (three input, one output) have:
    • 510Ω output impedance (protects against accidental shorts)
    • 0-10V control voltage input with reversible attenuator
    • manual gain control
    • gain range 0dB down to -100dB (see table below)
      • using external CV the gain can go up to +10dB
    • signal present/peak LED

  • three input channels, with:
    • 33k audio input impedance

  • output channel has an auxiliary input for future expansion

  • short-circuit protected output buffers
  • input signal levels up to ±5V

  • operates from ±12V to ±15V supplies
  • the PCB can be built with either:
    • MOTM/Oakley-compatible 0.156" (3.96mm) 4-pin header
    • Doepfer/Eurorack-compatible 0.1" (2.54mm) dual-row 10-pin header
      (Note: due to poor design of the Eurorack power system it is recommended to replace the two 22Ω resistors with BAT42/BAT85 diodes mounted appropriately to protect the board from accidental reverse polarity connection.)
  • signal pads suitable for single-row 0.1" headers, or direct wire connection

  • high quality fibreglass PCB, through-hole plated, soldermask, silkscreen legend, with full ground plane
  • 80mm x 100mm (3.15" x 3.90") PCB with 70mm x 70mm (2.75" x 2.75") fixing centres


Block Diagram

VC Mixer Block Diagram


Documentation

For interested parties the following documentation is provided.
Please note that all these documents are Copyright © 2011 Neil Johnson.

Notes

  • Correction: the pot orientation for the manual controls (CN1,2,7,8) should be the other way round: connect CW to pin 1 and CCW to pin 3. The connections to the reversible attenuators (CN5,6,11,12) are correct.

  • Correction: resistors R38 and R40 should be wire links, not 22R as stated in the schematic and parts list.

  • The gain range is set by R1,R2,R17 and R18 (one for each channel). The maximum manual gain is 0dB, while the minimum gain (or attenuation if you drop the '-') is show below:
    Resistor ValueGain Range
    510Ω0 to -100dB
    3k3 0 to -60dB
    5k1 0 to -50dB
    Note that the external CV can drive the gain up to +10dB. The exact voltage necessary to achieve this will depend on the gain range as selected above and the manual control setting.

  • The CV sensitivity is set to 10V full scale. For smaller systems this is not sensitive enough. To change the sensitivity to 0-5V add a 100k resistor between pins 1 and 8 of each of the resistor networks RN1-4. (For the more skillful constructor you can cut the trace between pins 1 and 2 and link pin 2 to pin 8).
    Trimmers R3, R6, R19 and R20 will need adjusting accordingly.

  • The op-amps have been arranged so that it is possible to substitute higher-quality ones if you desire. The audio-path op-amps are IC3, IC4, IC6 and IC7, and the control path op-amps are IC1 and IC5. For example, the OP-275G is a very good low-noise low-distortion audio-grade op-amp that would fit well into this module's audio path.

  • Components marked 'DNF' are Do Not Fit - I put them in for experimentation (I find it easier to not fit a component rather than to bodge it on later if I change my mind).


Completed Module

Photo of the test board during testing. This is quite a dense board so best tackled by experienced constructors.


Availability

The PCB is no longer available for sale.

The only special component is the SSM2164P (DIL package version). This is available from suppliers such as Farnell, Digikey and Magic Smoke Electronics.


Copyright © 2001-2017 Neil Johnson