Korg Monotron – LFO as one shot EG and gate to trigger LFO reset


I’ve got quite far with the Monotron fiddling, mostly to try and stop the audible click that happens when the gate is turned off.

I’ve put switches on it to disconnect the ribbon keyboard and the gate input, so that the Monotron drones constantly, then doing Beatnic’s mod to turn the LFO into an EG, which means removing the 1k resistor at R33:


…and soldering a couple of wires to the pads of R33. Treat the wire to the top pad as B and the bottom pad as A, and have a look at Beatnic’s diagram – this is what you’ll need to recreate. I just used a small bit of stripboard, but I’m sure you could wire it point-to-point and keep it all in the Monotron case. I used a 0.0047uF cap for C1. (see update at bottom of post)

There might be a cleaner/better way of doing this, but when it’s set up for external CV/gate control, the rightmost switch is set up so that when it’s droning (external gate input off) the gate is passed through to the LFO reset point through a replication of C9/R24 to turn it into a trigger, otherwise the LFO won’t reset until the key is released (see LFO reset point on Korg Monotron for a demo). The simple gate-to-trigger mod is the red section added to the schematic:


Here’s a quick demo – the middle switch of the three is the LFO/EG switch, the right hand one is the gate input on/off switch.

At 1m10s I switch the gate input on and you can hear the click at the end of the note that I was trying to get rid of. It’s more obvious when the filter is tuned low. It’s still really hissy and the pitch knob is ferociously twitchy, but it sounds pretty good. It would be nice to build in some fine tune adjustment for the pitch somewhere.

Update – May 2014

Beatnic’s schematic is long since lost to his website change a while back – it’s my own fault for trying to link back to the source. As people still seem to be interested in how to do this mod, I’ve recreated the schematic from the stripboard in my fiddled-with Monotron. Here it is :

Monotron LFO as one-shot

The resistor I’ve labelled Rx essentially replaces and does the same job as R33 when switched in. The 4.7nF cap goes to ground, and the 100k resistor goes to Vdd, which I think is 5v. Let me know if this works in the comments…


LFO reset point on Korg Monotron

I’m doing a lot of guessing here – I’m thinking that the C9/R24 arrangement converts the gate into a trigger pulse. When I try the bottom end of C9 on the circuit board, I just get hiss, no oscillator.

I thinking I might need to look at recreating the C9/R24 arrangement off the Monotron circuit board, or alternatively use something like Ken Stone’s gate-to-trigger converter to reset the LFO on each keypress. Haven’t killed the thing yet, which is a bonus.


Korg Monotron hacking



Sorry for shouting, but it’s really good fun. Like every other synth freak out there I got bought one of these for Christmas. I’ve had it next to my MS20, and the filter sounds really similar to the low-pass in the older synth.

Here’s a picture round the back of the Monotron, see how the patch points have been clearly annotated by Korg.


Lots of people have already tried to mod their Monotrons…

Dinsync has written up lots of notes on hacking in cv and gate inputs.

Korg have released the schematics for the Monotron.

Rude66 is offering to build the Dinsync mods into your Monotron over at www.monotronmods.com.

Beatnic has gone all out on the Monotron++, adding a square wave and control of the pulse width to the oscillator, a switch for converting the LFO into a single shot EG, and MIDI and cv/gate inputs.

There’s another suggested MIDI interface over at fengland.org, which includes an arpeggiator, which is always good.

I think it’s masa921 that put together this impressive 8-step sequencer for the Monotron, sadly the site with the schematic has gone, though I’m guessing it’s something similar to the Baby 10 sequencer. denha also demonstrates a two octave mod and the Beatnic LFO as EG modalthough I can’t find the details for that one – now found this, which uses an LED to clip the LFO at houshu.at.webry.info, and Scott Willingham pointed out the Beatnic/masa921 version of the mod on the original Monotron+ page.

Some people over at Muffwiggler are looking at making the Monotron into a Eurorack module. Tim Stinchcombe has heroically put together a handy annotated photo of the circuit board with the component values overlaid.

This whole post is going to look bad if I’ve not had a go, so – here’s my hack so far, built into a small lunchbox:


Going off all the stuff on Din Sync’s blogpost and the Muffwiggler thread, I got something working, which is the thing above. This bit is always quite exciting. People in internet-land say these things are possible, but actually having the thing sequenced by my CSQ-100 and squeaking away in 4 octaves-or-so on my desk was great. This doesn’t terribly well tuned in, but here’s an example:

This is what I’ve done so far – bear in mind I’m reasonably clueless at this electronics stuff:

  • drilled a single hole in the base of the Monotron to feed all the wires out into my sandwich box
  • wired CV output from sequencer to “pitch” point, with a 33K resistor and 10K multi-turn trimmer in series
  • wired gate output from sequencer to “gate” point, with a large-ish resistor and a 100K pot in series – this is (hopefully) to limit the gate voltage going into the Monotron, which could be up to +15v, although I’d probably be better off with a voltage clamp using a pair of diodes
  • wired filter CV from external sources to “filter” point, with 200K resistor in series with a 10K pot, which has a resistor across it to make it into more like a 2.5K pot to make the range useful
  • removed R11 with a soldering iron and a knife to stop the gate voltage affecting the pitch. This does rather bugger the keyboard ribbon, so attached a switch across the pads of R11 with a 1K resistor in series to replace R11
  • added a switch between the gate output pot wiper and the “gate” point. This seems to give a gate-on forever effect, although it also stops retriggering the LFO (could possibly change this for an on-on switch and wire the other point to the LFO reset area, maybe between R24 and R30? – update 10/1/2011: I had a fiddle with this here – LFO reset on Korg Monotron)

Lovely though it is, there’s a couple of problems. Although you can’t really tell from the onboard speaker, it’s quite hissy. This has been covered on the brilliant Muffwiggler thread, with Scott Willingham replacing the filter output op amp (an LM324) with an OP462, which brought the noise down. Because it’s all surface-mounted, I’m not sure if I’d fancy this. There’s a post on the Unofficial Monotron Forum which shows a possible place to take a line out, I’ve not tried this, but there’s some doubts as to whether this will make much of a difference.

The other problem is the massive click after every gate ends, here’s a really short example to illustrate this:

…and here’s the waveform, look at the state of those clicks after every note.


It’s less of a problem when the filte r is open, but it’s still pretty horrible. Here’s another bit from that demo, starting with the gate on, then I switch the gate to forever a few seconds in. Also I couldn’t resist adding some moody spring reverb and some sample and hold filter CV modulation:

Final demo – ‘scuse the dodgy keyboard timing – using an external envelope to shorten and lengthen the gate from the CSQ-100, and a slow external LFO modulating the filter cutoff:

Not bad for fifty quid and a bunch of pots and sockets. Still hoping that someone can hack a way to stop the clickiness, but I imagine it’ll be rather involved.