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.

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  1. 8th January 2011plasticanimal says:

    On the muff site they think maybe the noise is from the filter because it seems to be same with the MS-20 as well. Only the MS-20 has a VCA to kill the noise after a note is released. My solution was to disable the gate on the VCO and VCF while leaving the LFO’s rigger/gate and turning it into a one shot so it acts like an EG. Then try to send the LFO signal to the output amp to control it’s volume using a vactrol. I’ve seen this done with analog chorus to get tremolo effects. The muff guys didn’t think it was possible but you seem to have gotten the gate disabled. So if you can keep it on the LFO and figure out the amp part then why not?Let me know what you think.

  2. 8th January 2011ua726 says:

    My MS-20 is **much** quieter than the Monotron. I’ve just a had a quick test on the headphones really loud, and there is background noise, but at a much lower level than the Monotron. I’m guessing this is due to there being much more headroom in the MS-20. I’ll do a couple of similar sequences with them for comparison in another post.I managed to abate the clicks by using an external VCA and ADSR generator from my modular, although the general hiss still sticks out, especially when playing notes with the VCF tuned low.I was interested in the VCA that Beatnic had put together for his Monotron++ (in http://beatnic.jp/monotron/source/mono++_audio-schem.PNG), I was half planning on building that and then routing the LFO to it or building a simple envelope generator based on a 555, but your idea of using a vactrol is interesting – I’ve never used one myself, all I know is what I’ve read about them, so you’re way ahead of me there. I’m not sure how I’d place it, but could be worth an experiment.I’ve still not found the one-shot LFO as EG mod details, think that masa921 might’ve come up with that, and his Infoseek-based site is now gone, let us know if you find them.

  3. 9th January 2011plasticanimal says:

    I’m still looking for those as well. As for using a vactrol for control check out this page http://experimentalistsanonymous.com/board/index.php?topic=1578.0 He explains how to send the LFO to the op amp of the Small Clone chorus. I plan on trying it with the Monotron if I can find some free time.

  4. 9th January 2011ua726 says:

    Finally found the LFO as EG mod here:http://translate.google.co.uk/translate?hl=en&sl=ja&tl=en&u=http%3A%2F%2Fhoushu.at.webry.info%2F201006%2Farticle_3.html

  5. 10th January 2011ua726 says:

    Found this thread on Muffwiggler, suggesting how to use a vactrol to control an opamp as a VCA: http://www.muffwiggler.com/mobile/thread.php?topic_id=14918…links to some suggested circuits for voltage to resistance here, could be worth a go:http://thesquarewaveparade.com/images/VtoR.jpg

  6. 10th January 2011plasticanimal says:

    Found this site but they refer you to the, now defunct, "masa921" page. [url=http://denhaku.blogspot.com/2010/05/korg-monotron.html]denha monotron[/url]Every pic on the page manages to just miss part of the mod. And there is a pice of tape over part of it. I’m going to poke around and see if I can work it out.

  7. 10th January 2011ua726 says:

    Just now tried the houshu version of mod, which uses a blue LED to clip the LFO:http://translate.google.co.uk/translate?hl=en&sl=ja&tl=en&u=http%3A%2F%2Fhoushu.at.webry.info%2F201006%2Farticle_3.html…which kind of works, but it seems the LFO doesn’t fall all the way down, the LED for the LFO is still slightly on after it has run.The chaps on that Muffwiggler thread also pointed out this on Beatnic’s site, which I’m thinking is same as the masa921 version:http://translate.google.com/translate?hl=en&sl=ja&u=http://beatnic.jp/monotron/monotron%2B.htmlHaven't tried this yet, it needs rather more hacking.

  8. 13th January 2011ua726 says:

    Just to confirm that the suggested LFO as EG mod on Beatnic’s Monotron+ page works – tried this last night, and found that a bog standard 0.01uF (103) ceramic cap works as C1 in his diagram. Will write it up later.

  9. 18th January 2011plasticanimal says:

    I had a chance to play around with the vactrol idea. I’ve got the LED side connected to the LFO point with a 100 ohm resistor. For some reason this slows the LFO rate. Not too much of a problem since I’m going to use it as an EG with a switch back to LFO.The real problem is I can’t find a suitable place in the amp section for the photo cell side. It seems like the best place would be at the volume pot, but I can’t get it to work. I can hear the LFO having some effect but only when the volume is turned up. Also you would think that shorting the pot’s in to out leg would cause it to play at full volume. Nothing happens there either.Oh well. I’ll have to sleep on this one.

  10. 18th January 2011ua726 says:

    I’ve not had a chance to have a go at the vactrol thing yet, but I was thinking of taking the audio out from the volume pot to a breadboarded LM324 to experiment with it, and then if worked out, recreate the setup on the internal LM324 when I’m done. Or just stick with the external amp perhaps – I’m not trying to squeeze it all into the Monotron case.

  11. 19th January 2011plasticanimal says:

    OK, so I wasn’t getting good contact on the volume pot legs. Now they short out and give full volume. The problem was my photo cell has to much resistance to open the volume. Have to hold it close up to a lamp to get it to work. I can turn the volume up to give it a sort of attack,sustain effect though. Not satisfied yet.

  12. 20th January 2011ua726 says:

    Nice, sounds like you’re getting close. Probably just a bit of fiddling with resistor values by the sounds of it.

  13. 20th January 2011plasticanimal says:

    I got tired of poking around at the volume pot so I just cut the trace leading to it and stuck the vactrol there. It works really well. If you turn off the other EG mod you get a nice tremolo effect. I may play around some more to see if there’s a better place for it. At first I thought about putting it before C25 and C26 but it was easier before the pot. Should give the same effect either way I guess. Now to clean it up, find a proper switch and fit it all inside this tiny box. And I still have to get the CV mods in there. :/

  14. 10th August 2012SiD says:

    Sorry…where I can find the detail of the LFO as EG mod?Thanks in advnaceCheersSid

  15. 10th August 2012ua726 says:

    Hey Sid

    Here’s what I did, hope it helps. (check the comments too).


    To avoid clicks on the output, I put my Monotron into a droning state (by disconnecting the gate input and ribbon keyboard) and used the filter to gate the oscillator. plasticanimal attached the LFO to the volume pot via a vactrol for extra points.hope that helps

  16. 13th August 2012SiD says:

    Thanks, Highly appreciate it!<br><br>Will try and let you know<br><br>Thanks again MAn<br><br>Rgds<br><br>Sid

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