T984 4x4 Channel Mixer

Introduction

This module works as a mixer matrix for 4 input channels, which can be routed and mixed to 4 output channels. So there are 16 combinations possible. Each output channel contains a sound boost function for bass and treble.
If you do a cascade of the mixer stages, you can integrate other modules or external effect processors in the patch before final output, which makes the sound design even more flexible.

Although this seems to be a quite clever module concept I've not seen this module very often on pictures of Moog System 55 modular systems. Perhaps people wanted to save the money and do final mixing and sound processing externally.

I would say this is an appropriate output module for a modular system, although the outputs are mono. But why not using two channels for stereo output.

T984



Below: Patch example taken from the Moog modular owners manual (the signal routing "arrow" should be the other way round though):
The input signal is connected to input 1 and provided at the output of mixer channel "A" to be processed by an echo unit. The echo signal is connected to input 2 and is mixed together with the original input signal by mixer channel "D". The result is provided at the "D" output.



Module views


Click to enlarge
Click to enlarge
Click to enlarge
Click to enlarge
Click to enlarge


Frontend / Human Interface

The four mixer stages (A to D) are arranged horizontally above each other. Each stage contains four level pots for the input lines (1 to 4) which can be mixed in each stage. Beside them you can find the tone control pots (bass and treble) and an output volume pot. The output connectors are located at the bottom right of the front panel, the four inputs are located on the bottom left.

Circuit

Schema of a single mixer and tone control stage: Attention: clicking the schema means accepting the disclaimer on page bottom!

Rough circuit description of the T984

Mixer: The four inputs (P1, R3 (only one is listed here)) are connected to the mixer stage ac-coupled via C1. The input buffer Q1 reduces the input signal amplitude to a level of about 3%. The inverted output of Q1 (collector) is connected to the first amplifier stage of Q2 via voltage divider R8, R9 and C4, C11. This stage does another inversion and an amplification of factor 5. Q3 serves as output buffer and levels the output signal to double amplitude of the input signal. C12 serves as output decoupling capacity. P4 works as output volume pot of the stage.

Tone control: The circuit contains an active tone control (boost). The non-inverting output of Q2 (emitter) passes a lowpass filter (P3, C7, C8) and a highpass filter (P2, C5, C6) in parallel via C10 and R17 and the "results" are fed back via C11 to the base of Q2, which leads to an additional amplification of those frequency bands.

The circuit does an overall amplification of factor 2, if input and output pots are open and the tone controls are set to "neutral". But if you boost bass or treble there can occur output levels of factor 5 or more, which can lead to distortion effects. So be carefull and use the tone control and level pots moderately.

Scope pics (click to enlarge)

This is an example of a square wave processed by the T984:


Square Input
Most neutral
tone settings
Tone controls on
mid position
Total sound boost
Treble boost

As you can see, a real neutral pass-thru of the input signal is not possible. The corresponding scope pic reminds of the baviour of the T904a lowpass filter.
The center position of the tone controls do a slight highpass filtering like the T904b highpass filter does.
The boost pics show the needles of the emphasized higher frequencies and show how fast the amplitudes can run "out of the rudder".

Component changes

I've replaced the following comonents compared to the original circuit:

Boards of the T984 Clone

I realized the module with two boards. Each contains two mixer and tone control stages. The first board contains an additional distributor of the four input channels to the four mixer and tone control stages.

  • Board size: 120 x 100 mm
  • Left side: Power supply, input distributor
    In contrast to the original concept of the Moog modulars I supplied the board with a +/-15V power supply. Voltage regulators on the board convert it to +12V and -6V. Benefit of this is a higher stability of the power supply.
  • Lower side: mixer and tone control stage A
  • Upper side: mixer and tone control stage B

Click to enlarge
Click to enlarge
  • Board size: 120 x 100 mm
  • Left side: Power supply
    In contrast to the original concept of the Moog modulars I supplied the board with a +/-15V power supply. Voltage regulators on the board convert it to +12V and -6V. Benefit of this is a higher stability of the power supply.
  • Lower side: mixer and tone control stage D
  • Upper side: mixer and tone control stage C

YouTube Video



Please send questions or remarks to:
Carsten Toensmann

Home



Disclaimer:
No warranty for completeness or correctness of technical descriptions, schemas or any information provided on www.analog-monster.de/* pages.
Contents of linked pages are in responsibility of the page owners and analog-monster dissociates from them.
All tracks provided on www.analog-monster.de/* pages is in ownership of www.analog.monster.de (except especially marked) and for private use only. All rights reserved.