Monotron Delay

Ever since Andy tipped me off about the new Korg Monotrons (Delay and Duo), it seems I’ve been waiting next to forever for my Delay to arrive. Given all the out-of-stock Delay delay (hehe), I’m apparently not the only one anxious to get this toy.. But arrive it did (finally)! If you’ve kept an eye on my tweets, you probably noticed me rejoicing this some days ago.

I never was much of a fan of the first Monotron (better analogs at hand aplenty ;), but the Delay sure caught my attention. Fun little effect-oriented gadget (LPF + delay) to play with and all that..  What gives the Monotron product range an extra edge is that Korg has released the schematics to the public. This makes the series a inviting platform for circuit-bending or modding.. And it’s not much of a surprise, that also my intent is very much just that :).

Not that I have the time to dive straight into modding just yet, so instead I decided to first have a good look at the circuit and its sub-sections. Sort of set a framework for “what could be done” and let the ideas simmer. With this in mind, I’ve circled some points of interest to the schematic below. These are mostly spots where signals could be sourced, but I’ve also marked the diodes connected to opamp IC4C and some of the potentiometers. There are also two unused ports on 74AHC14 logic inverter (IC3). The diodes are probably responsible for the delay feedback clipping, thus replacing them with some other types could provide different sounding distortion. The potentiometers could be replaced with light-dependant resistors (LDR), allowing you to control the value by waving your hand.

Monotron Delay schematic.

One additional thing I find somewhat interesting is the clock out (pin 5) on IC5,  the PT2399 echo processor ASIC-chip. Despite this clock seems to be in the megahertz range it might be off some use. I’m guessing this output is meant to be used for syncing multiple chips as the clock frequency changes according to delay time. The range is from 2 to 20MHz and the longer the delay time, the lower the frequency.

With more extensive modding in mind, it’s also good to verify how much the internal DC/DC converter (as in, PSU)  can actually handle. If active electronics (ICs, transistors and such) end up being added, these will of course increase power consumption. The power supply is built around a single chip buck boost converter, a TPS61071 by Texas Instruments. This can generate up to 200mA current @ 5V from a 3.3V battery supply with 90% efficiency.

Doing a few tests with lab PSU, my Delay seems to draw between 0.27W when in use. Thus with a input voltage of 3 volts, the current draw is around 90mA. The synth starts introducing a mildly audible tremolo effect over the output when input voltage drops around 2.5V. I assume it’s the LFO LED turning on that makes the internal operating voltage duck, in turn causing a dip in the overall volume.. At least the tremolo is firmly tied to LFO speed ;). From here, going down to below 2V starts causing other random behaviour, like pitch warbling on high tones, and the synth dies entirely when input voltage hits 1V. Tampering with operating voltage is of course one of the easy circuit-bending tricks, if random behaviour is what you’re after ;)

Shortly returning to PT2399, one good thing about it is that it’s rather common. With this in mind here’s one good recycling tip: If cheap delay effects are what you’re after, keep an eye out for binned DVD players that have a ‘karaoke’ function (mic input + delay). If you spot one, GRAB IT! It’s almost 100% certain that the karaoke delay function is implemented using a PT2399. Good chances are also that it’s installed neatly on its own separate PCB and has most of the support circuit implemented by directly copying the application note schematic.

If so, remove this board, add your own connectors, PSU and case, and you’ll have a delay effect in no time! I’ve built one prototype like this (which reminds me, should write about it!) and must say perfectly usable delay effects don’t get cheaper than this :)

Ok that’s all for now, actual Delay mods to follow some time (way) later..