Korg MS-20 LFO stripboarding


Remember actually making music, not just fiddling with a resistor and a voltmeter and swearing? No? Never mind. I liked the MS20 modulation generator, the skewing of the waveform is quite fun. Here’s the schematic.


Which turns into this (via Marjan Urekar’s additions)


which came out like this


Which works. After the obligatory tracking down of a few shorts, anyway. Here’s the ramp up on the square wave of an Analogue Systems RS-95 oscillator, it’s just as you’d expect. It’s really loud, turn it up so that it scares the shit out of the cat.

I need to do a bit more experimentation with the sizes of the capacitors. The smallest 1nF cap goes well into the audio range, but there’s something odd that happens to the waveform as it gets past a certain point. The 1uF cap allows the cycle to last for nearly two minutes. It’s just under 6v peak-to-peak, I’m wondering whether to boost it to 10v-pp. With the larger capacitors, the oscillation reset is revealed as being quite slow. On a slow speed setting with a ramp you get a fast slide down, like a zap, rather than a hard fall.

After scouring the s-diy archives for more on the MS20 LFO, hoping for a reset hack, I find that Tony Allgood had already done it on his Little LFO using a DG403 switch. Which renders my careful stripboarding a bit superfluous, but anyway. Taking the chance to do a bit of learning, but not that much, I’ve been experimenting with making it resettable. The accepted method for resetting this kind of circuit seems to be to short out the capacitor. Through some fiddling I learned that just shorting it wasn’t enough, it needed to be shorted to a negative voltage to make the waveform start from the bottom. Someone had previously recommended looking at the ARP Odyssey LFO for inspiration, this thing :


I’ve been using a 2N5460 for the JFET at Q2 and a BC550C at Q1, and pretty much just glued the stuff around the opamp on to the first opamp in the MS20 circuit. From experimenting a bit, I replaced the resistor at R9 with a pot so I could dial in the right voltage to reset to. So it works, or near enough. On a ramp down, it sounds ok – here I’m stabbing away at the (noisy) System 100 keyboard to reset the LFO.

But on a ramp up, it’s not quite so good.

Rather than getting falling instantly to -v, it does a sort of zap as though it is falling from +v max, rather than a smaller zap down from wherever it is at the time. Making C2 in the Odyssey circuit a bit smaller seemed to help a bit, I had it at 15nF for the experiments above. Making it too small just killed the LFO. I’m in two minds whether to keep fiddling and try and learn exactly what the circuit is doing to try and fix it, or just say fukkkkkkit, done, move on.


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.