Korg Monotron schematic

[StephenGiles]

http://www.korg-datastorage.jp/Manual/monotron_sch.pdf

Although this is a synth, the VCF and LFO look interesting!

[tommy.genes]

The VCF is supposed to be copied from the MS20. Cool of Korg to make this schem publicly available.

Lots of people coming up with mods for that reason...
http://www.synthtopia.com/content/2010/06/29/we-love-monotron-movie-version/


-- T. G. --

[Mark Hammer]

The VCF is supposed to be copied from the MS20. Cool of Korg to make this schem publicly available.

If it is, I'm not seeing it.  The only thing I see in common with the MS20 filter is the use of a pair of clipping diodes in the filter output (hence resonance) path.  the rest looks quite different.

Where the heck is the VCA?  I'm looking for something familiar, but I'm not seeing it.  Any clues?

[slacker]

The filter is similar to the MS10 and early MS20 filter not the more familiar OTA based one. There's some good info here http://www.timstinchcombe.co.uk/synth/korg.html
According to the blurb on the  Korg website it hasn't got a VCA. That's a bit of a strange omission especially as a few Korg synths  had a very simple transistor based one.

I've been meaning to play with the filter for a while but haven't got round to it yet, apparently it can do high pass as well.

[Mark Hammer]

Okay, that clears some things up.  Even though I've pored over the MS10 and MS20 schematics for years, I had never seen the Korg35 module, only the buss numbers.  Other sites that have posted clones of the "MS20" VCF had posted OTA-based circuits, so I naturally assumed that's what this one ought to have and that's what I looked for.  Having looked at what Tim Stinchcombe posted, though, you're absolutely right, Ian.  It's the original MS10/20 filter alright.

As for VCA, my guess is that somehow the actuation of the note is integrated into the VCO/keyboard somehow; i.e., wherever the LFO circuit connects to.  Remember, it doesn't really have a "proper" envelope generator either.

[slacker]

Yeah I think you're right about the VCA. Looks like when you press a note it sends a voltage to the VCO to set the pitch and it also goes to a comparator (IC2C). The threshold is set to just above ground so the output is low when no key is pressed then when you press a key the output goes high. This then goes to something called "VCO enable" which I guess does what it says on the tin and it also resets the LFO and does something I can't figure out to the VCF.
Resetting the LFO is quite cool, as that will give a pseudo envelope effect, especially if you let go of a key before it has time to repeat.

Looks like a fun little toy, and that's before you plug a guitar into it, wonder how much they are.......

[Mark Hammer]

I think they're well under 50 pounds.

[Salvatore]

If it had midi, I would buy 2 of them.
Without midi I mostly see it as a neat toy, or modding paradise.

Here in the Netherlands they sell for 59 euro's.

[Strategy]

There are a number of add-on kits that could bring midi for this. You'd have to put the midi jack in a sidecar-type enclosure. Maybe a Kenton MIDI add on, or www.highlyliquid.com - there are others.

I am going to get one and do CV/Gate inputs - apparently very easy as all necessary pads are on the accessible bottom side of hte pcb and labelled There is a diagram online for this.

I played with one last week. A bit flimsy, I am strongly considering a rehouse if I get one. Very cool sounding and fun.

Strategy

[Salvatore]

Thanks for the links Strategy.
I hardly considered midi>voltage converters cause most I saw where many times the price of the actual synth.
This one is not so expensive, just 45$, would this one be ok for the monotron vco and vcf handling ?

http://highlyliquid.com/midi-decoders/md24/

[Strategy]

I would not consider the MD24 - that is more for just MIDI to trigger signals, like you might want to convert an old Drum Machine to MIDI.

I am guessing this could work:

http://highlyliquid.com/kits/cs01-midi/

or

http://highlyliquid.com/kits/umr/

Or these:
http://www.synhouse.com/originalmidijack.html
http://www.kentonuk.com/products/select/rfit.shtml