Korg Monotribe LFO output

…but why?

A friend asked me to make a LFO output for his Korg Monotribe. I had some experience with synth modding, so I though its the job just for me! On Muff Wiggler, topic “Korg Monotribe modifications thread” you can read, LFO signal (fed to VCO) is avaliable at pin7 of IC15. It should be easy.

 

Measurements

First of all, I disassembled the Monotribe, soldered temporary wires and got to measurements: for three MODE positions – FAST, SLOW and 1-SHOT, three WAVE settings – saw, triangle, square and three RATE potentiometer settings – maximum, middle and lowest. The TARGET switch was on VCO (LFO signal on pin7 is absent if TARGET = VCF) and sequencer was in PLAY mode.

Connections for Korg Monotribe waveform measurements
Connections for waveform measurements: YELLOW wire – LFO output, RED wire – +5V (not used now)
Connections for Korg Monotribe waveform measurements
GND was caught somewehere on one of many potentiometers; good joint, melts easly 🙂

Waveforms for SLOW setting looks very good, with almost constant amplitude (1V p-p) and offset (Vbase between 1,0 and 1,3V). A dream for scaling and biasing. For this mode, the highest frequency is around 19Hz, lowest – 0,05Hz (1 cycle per 20 seconds).

Original Korg Monotribe LFO waveforms
The original LFO waveforms for: mode SLOW, waveforms: saw/triangle/square (columns) and three RATE positions: max, middle and lowest (lines)

Seems like a little worse. For FAST mode and maximum RATE setting (over 5kHz frequency, far into audio range), the waveforms are almost unrecognizable, but there is hope for middle RATE setting, where everything looks as it should. As the frequency goes down, the amplitude drops drastically. In the last case (squarewave) the generator stops (which is strange, as LFO signals are told to be generated digitally).

Original Korg Monotribe LFO waveforms
The original LFO waveforms for: mode FAST, waveforms: saw/triangle/square (columns) and three RATE positions: max, middle and lowest (lines)

The 1-SHOT waveform is visible only with certain RATE settings, corresponding to sequencer TEMPO setting. The amplitude is lower and offset greater then for SLOW LFO setting. It is around 0,5V p-p with 1,75V offset (or 1,5V of Vbase).

Original Korg Monotribe LFO waveforms
The original LFO waveforms for: mode 1-SHOT, waveforms: saw/triangle/square

 

Circuit design and prototyping

The idea was to make a scaling/biasing circuit, to get 0-5V output from LFO. As the waveforms levels and offsets varies, I decided to design the circuit for “average” LFO output waveform with 1V p-p level and 1,5V offset. Finally I got to this circuit (designed with help of: SLOA097 – February 2002, “Designing Gain and Offset in Thirty Seconds” by Bruce Carter, Texas Instruments”):

LFO output scaling circuit schematic
LFO output scaling circuit schematic – one IC and 4 resistors. Somehow biasing voltage divider resistors values are matching opamp amplification setting ones 🙂

First, I prototyped it on breadboard. The circuit is using MCP6002 rail-to-rail opamp, to get full 0-5V LFO output swing from 5V supply voltage.

Disassembled Korg Monotribe
Disassembled Korg Monotribe, testboard and sheet of calculation

Then, I moved to protoboard. The circuit is so simple, there was no need for dedicated PCB. It’s size is  22×25 mm.

LFO scaling/biasing circuit made on protoboard
LFO scaling/biasing circuit made on protoboard

The signals on the installed LFO output looks as following. For SLOW mode, output looks almost perfect, with usable range and stable waveforms.

Korg Monotribe scaled LFO waveforms
The scaled LFO waveforms for: mode SLOW, waveforms: saw/triangle/square (columns) and three RATE positions: max, middle and lowest (lines)

For FAST mode, the problems are as in original – distorted waveforms and often unstable for maximum RATE.  At moderate settings – frequency-related glitches happen sometimes, but still signal is very useful as modulation source. For minimum RATE setting, the offset is greater due to scaling circuit compromises, the levels are also not satisfying – it is better to use LFO in SLOW mode with maximum RATE.

Korg Monotribe scaled LFO waveforms
The scaled LFO waveforms for: mode FAST, waveforms: saw/triangle/square (columns) and three RATE positions: max, middle and lowest (lines); (*) as for original waveform, squarewave was absent at lowest RATE setting – the one presented here is for around 1/4 RATE potentiometer rotation

1-shot waveforms look very promising. Good-looking shapes, unfortunately over 2,5V of Vbias. Note, that waveform shape is related with sequencer TEMPO.

Korg Monotribe scaled LFO waveforms
The scaled LFO waveforms for: mode 1-SHOT, waveforms: saw/triangle/square (columns) and three RATE positions: max, middle and lowest (first 3 lines); for the 4th line, the sequencers TEMPO was lowered in comparison to 3rd.

It took over 5 hours to make. I wasted 1 hour because somehow I calculated 1*5=25, which totally wasted my resistor calculations :/

Installed LFO output on the side of Korg Monotribe
LFO output on the side of Korg Monotribe (socket type: CLIFF FC681374V). MIDI mod (not mine) is also there.

 

Summary

Good:

  • easy to make and cheap,
  • useful ~0-5V range,
  • adds nice LFO to your setup.

Bad:

  • works only for TARGET in VCO (VCO+VCF) mode and sequencer ON,
  • does not align with sockets on the back of Monotribe :/

 

Generally I’m happy. The only thing that could be done better is output range. The Monotribe is powered by 6x AAA, which give more then 5V and it could be possible to power scaling circuit with it. Maybe next time.

And remember: use this schematic/info in your own responsibility.

Cheers

Jack

One thought on “Korg Monotribe LFO output

  1. Bonjour, j’ai lu avec attention la modification du monotribe.
    J’ai deux korg volca FM et sample, j’aimerai les réunir dans un seul et meme boitier et si c’est possible mettre dans ce boitier une TR08.
    A votre avis est ce que c’est possible de tout reunir l’interieur des machines dans un seul coffret.?

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