Friday, June 25, 2010
How to modify a Korg Monotron
So here's how to modify Korg's new Monotron analog ribbon synthesizer. Perhaps this is the first document of Monotron mods in Europe since it still hasn't been released here.
It's actually a very easy machine to modify because for whatever reason Korg decided to label all the interesting points on the bottom side of the PCB. This may well have been for testing units at the factory/service centers. Perhaps though it could have been that the designers anticipated this little machine would be hacked, much like the Gakken which pretty obviously inspired this piece. Incidentally in the magazine that comes with the Gakken there's a picture of them showing it to Korg, that was in 2008, go figure.
Modification wise everything is simple enough except for the gate. Unfortunately the gate point affects the pitch somewhat. There's a few solutions, the first of which is to use a single pitch cv line into the gate socket. This will work but you wont be able to get really low notes which is less than useful.
After a little research I came across an idea to use a dtc114 transistor to trigger the gate (soldered at a different point). I tried this but didn't have much success. Also that method will result in the LFO not re-triggering which is bad since the LFO is the closest thing to an envelope on the machine.
By far the simplest solution is to use something like Silent Way by Expert Sleepers (see my previous video on this software) or Volta by Motu. After calibration there's no issues whatsoever and the monotron tracks over 5 octaves nicely.
Using an adjustable dc power supply is a good alternative to the batteries and is also useful in getting the Monotron to a decent tuning range, just don't dial in more than 3 volts. I built my power supply based on this schematic
If you intend to use both battery power and dc power you should include some diodes in your design.
And lets not forget the filter, the side of the packaging says "the legendary MS-20 filter" while this is true design wise it would probably be more accurate to say the MS-10 filter since it's only low pass and the MS-20 has low and high pass filters. How does the filter sound? actually very good! I've repaired some MS units and the Monotron really does have some DNA from the MS series, even down to the small noise hiss when the resonance is high but not yet self oscillating.
A couple of things I noticed about the filter input, if you drive the input signal too hard the resonance almost disappears, so be sure not to overdrive it too much if you want the full character of the filter. Also the input jack is stereo which means if you use a Y cable you can run two signals into the filter comfortably. I tested to see if the left side input leaked to the right input and it doesn't at all, which means it really is a true stereo input to the filter which is very nice.
As for the output, even though it's a mono output via a stereo jack the manual states that you shouldn't use a mono jack on the stereo output, so it's probably best to use a stereo jack plug but leaving the ring floating to get a mono signal into your daw/mixer. The output is a little noisy due to the fact that its really a headphone out and the output level is a little low so you will probably want to use a noise gate to remove the noise floor when it's not playing. (this is the same with the Gakken)
That's about it, here's the diagrams for modding, as you can see it's very simple indeed and should be no problem for anyone with soldering skills.
The Monotron should be available to buy in Europe from around the 30th of June for around 50-60 euros.