Korg Monotron Eurorack Mod

Participants	Dennis
Skills	Soldering
Status	Planning
Niche	Electronics
Purpose	Instrument
Tool	No
Location
Cost
Tool category

Monotron PCB Front.jpg {{#if:No | [[Tool Owner::{{{ProjectParticipants}}} | }} {{#if:No | [[Tool Cost::{{{Cost}}} | }}

What

The idea is to make a eurorack module out of the Korg Monotron synthesizer. The Monotron synths are extremely hackable, Korg released the schematics and a lot of people have published mods. I have the original Monotron. I want to be able to reuse the existing pots and microswitch, so they will stick through the faceplate.

Todo

Remove battery terminals from PCB
Remove audio jacks?
Remove R11, see http://www.timstinchcombe.co.uk/synth/monotron_ref_des.jpg to the left of IC2 under the volume pot. This will disable the ribbon controller but I removed it anyway. The resistor is removed to prevent gates from influencing the pitch.
Design and build a board with the desired in- and outputs and controls, probably on protoboard (called I/O board from this point on)
Build a simple board for powering the module (called power board from this point on)
Design and build a faceplate
Assemble everything together and put it in a eurorack case

Thonkiconn mono jack sockets

I will be using Thonkiconn connectors since I've got a load of them and they are easy to work with on protoboard. Datasheet can be found here: https://www.thonk.co.uk/wp-content/uploads/2014/02/Thonkiconn_Jack_Datasheet.pdf

Signal will go on pin 3, and ground on pin 1.

The I/O board

Mono jack inputs

See http://electro-music.com/forum/phpbb-files/monotron_mods_181.jpg for connections of most of this.

Pitch CV in

Controls the pitch of the VCO.

Gate in

Can turn the sound on and off, most probably influencing what's coming out of the Audio out only, not the VCO out.

Cutoff CV in

Controls the cutoff frequency of the filter.

LFO rate in

There is a pad on the Monotron PCB called rate.

Audio in

Possibly use two mono ins and hook it up to the stereo in on the Monotron PCB.

Mono jack outputs

Audio out

Alternative audio out

The speaker output might be less noisy, but "has a restricted bandwidth, about 230 Hz to 5.9 kHz" according to this link. Might still be nice to have.

VCO out

Connect directly to Monotron PCB, pad is marked VCO.

LFO out

Connect directly to Monotron PCB, pad is marked LFO.

Controls

Switch

Toggles pitch tracking of the filter cutoff frequency

Pitch CV tune pot

Cutoff CV amount pot

Adjusts the amount of influence the CV has over the filter cutoff.

The power board

This will be fairly simple. It will have a 10-pin connector for the ribbon cable to the eurorack busboard. Only 5V and ground will have to be connected to the Monotron PCB. Since the connector has a notch, there is low risk of hooking it up incorrectly but I might decide to have a protection diode somewhere anyway.

Possible further mods

Reduce noise:

"In summary, the modification consists of: 1) replacing IC4 with an OP462, and 2) removing R74. I measured the noise spectrum of my Monotron at frequencies of 2 and 8 kHz, before and after the mod. At both frequencies, the resulting noise reduction is 9dB. This is very noticable and makes listening to the Monotron in headphones much more pleasent. " from this link but it seems more modifications are needed accoring to later posts here. Let's first listen to the thing without this mods and see if noise levels are acceptable.

Links