Korg Monotron Euro Module DIY

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Scott Willingham
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Post by Scott Willingham » Fri Feb 18, 2011 10:37 pm

Oran wrote:Thank you teatanic, Scott & th0mas, for your help!
I really appreciate it!

I've got it all worked out now, and finished two Monotrons this morning!
Glad you got it all sorted. Sounds like you're having fun, which is the Monotron's forte.

Now it's probably a good time for a ... :guinness:

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studiokpg
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Post by studiokpg » Sun Mar 06, 2011 6:26 am

phono1337 wrote:
4mspedals wrote:This guy added CV jacks to his:

http://www.dinsync.info/2010/06/how-to- ... otron.html
That was me :lol:
Congrats on the mention on Collin Cunningham's Make video:
http://blog.makezine.com/archive/2011/0 ... cking.html

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Tim Stinchcombe
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Re: Update on noise mod

Post by Tim Stinchcombe » Wed Mar 09, 2011 5:50 pm

Scott Willingham wrote:
What time I have spent on this over the last week or so has been on trying to understand how the pole(s) of the op amp interact with the poles and zeroes of the filter, and how R74/C22 'helps', but the algebra approaches the damn near intractable at times. Finally I think I have an approximation that'll give me what I seek, which is specifically the frequency-dependence of the resonance, and then what happens when using a higher-bandwidth op amp.
Sounds like an interesting challenge. I look forward to reading more about it.
Well I have been away from this thread for far too long! My investigations proved a little more protracted than I would have wanted, and I had a couple of enforced breaks from it all, plus a few whereby I kind of lost interest (some of you will probably say 'not surprising' when you see it! :eek: ), but I have finally just finished writing up lots of stuff and drawing lots of pictures, and here is the final result, a new page dedicated to the Monotron filter poles:

http://www.timstinchcombe.co.uk/synth/m ... /mono.html

I'm reasonable sure I'm happy with it, so I will probably do any minor tinkering later in the week, and remove the 'under construction' banner, and also link to it from my main synth page.

All comments and suggestions are welcome, particularly if I have made any right howlers there!

Tim

Oran
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Re: External gate-in mod

Post by Oran » Wed Mar 23, 2011 1:38 pm

Scott Willingham wrote: Image
Hi Scott!

So, I'm modding a few more Monotrons and I think I'm interested in utilizing your new gate/pitch fix method you've outlined. My previous method of using a switch on r11 worked very well, but I'd like to avoid the additional switch. Masa's other method seems questionable to me as it sounds like it affects the LFO. Your new method sounds like the best solution to me, but I just have a few quick questions first.

1) Have you (or anyone else, for that matter) physically tested this method yet?

2) If so, are there are side-effects?
(e.g. Does the ribbon remain functional?, etc.) I've done some research on the Schmitt trigger inverter in order to try to understand how this fix would work, but I don't entirely understand it, thus I'm still a bit confused about using an existing ("spare"??) part of the circuitry as a fix. Will this not affect anything else?

And somewhat related:
3) When I have an effective gate/pitch fix enabled (even as simple as removing r11) am I correct in assuming that I do not need the additional pot as described in DinSync's schematic?
Upon testing, this pot seems to only be necessary in the case that the gate is allowed to affect the pitch, in which case fine-tuning (e.g. calibration w Silent Way) of the cv pitch is necessary. With a fix in place it seems that the pot on the back of the Monotron is sufficient, and two separate pots seems like overkill.

Thanks!!
- Oran

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Scott Willingham
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Re: Update on noise mod

Post by Scott Willingham » Thu Mar 24, 2011 11:15 am

Tim Stinchcombe wrote:... but I have finally just finished writing up lots of stuff and drawing lots of pictures, and here is the final result, a new page dedicated to the Monotron filter poles:

http://www.timstinchcombe.co.uk/synth/m ... /mono.html

I'm reasonable sure I'm happy with it, so I will probably do any minor tinkering later in the week, and remove the 'under construction' banner, and also link to it from my main synth page.
Looks great, Tim. This helps elucidate a whole area of the MS filter design that has not been previously explained (the "extra" feedback components).

There is a spurious reference to the TL074 in the first paragraph, but that's the only correction I've noticed.

-Scott-

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Scott Willingham
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Re: External gate-in mod

Post by Scott Willingham » Thu Mar 24, 2011 11:37 am

Oran wrote:So, I'm modding a few more Monotrons and I think I'm interested in utilizing your new gate/pitch fix method you've outlined.... Masa's other method seems questionable to me as it sounds like it affects the LFO.
If you mean the method of adding a bipolar transistor to pull down pin 9 of IC2C, that fix should work in the same manner as mine. It's the same method with different implementation.
Have you (or anyone else, for that matter) physically tested this method yet?
Yes. My Monotron has the mod I sketched up. No problems.
If so, are there are side-effects? (e.g. Does the ribbon remain functional?, etc.)
The ribbon still functions as normal. Either the ribbon or the external input will gate the sound and both also retrigger the LFO. When there is no external input plugged-in, the Monotron operates as stock.
I'm still a bit confused about using an existing ("spare"??) part of the circuitry as a fix. Will this not affect anything else?
The Schmitt-trigger/inverter IC contains six identical circuits; only four are used in the Monotron. Two of the inverters have their inputs tied to VCC or ground and their outputs are unused. Making use of these inverters does not affect any other circuit.

When I have an effective gate/pitch fix enabled (even as simple as removing r11) am I correct in assuming that I do not need the additional pot as described in DinSync's schematic? Upon testing, this pot seems to only be necessary in the case that the gate is allowed to affect the pitch, in which case fine-tuning (e.g. calibration w Silent Way) of the cv pitch is necessary. With a fix in place it seems that the pot on the back of the Monotron is sufficient, and two separate pots seems like overkill.
You are partially correct. The pot on the back of the Monotron can be used to calibrate the scale factor. There are a couple of drawbacks. First, the pot on the back is less convenient, especially if you need to periodically adjust for temperature drift. Second, the pot on the back affects the scaling of the ribbon controller, so you will have to adjust it back and forth if you sometimes use the external input and sometimes the ribbon.

If you use Silent Way, you should not need the external pot. The software will calibrate the scale gain and offset automatically, gate-fix or not.

-Scott-

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Re: External gate-in mod

Post by Oran » Sat Apr 02, 2011 11:27 pm

Thank you Scott, that all made perfect sense! I appreciate you putting up with my partial understanding of some things.

The reason I asked about the pot is that I'm stuffing the mod into the Monotron, so of course anything that I can leave out is helpful. It seems clear to me that the additional pot potentially serves a valuable purpose, and since my local shop has a shipment of inexpensive 10k multi-turn trimpots coming in, I might as well include it. Now that I think about it, unless I can affix a knob it won't be very convenient...but at least it'll be there.

Thanks!
- Oran

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marmad
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Korg Monotron Euro Module DIY

Post by marmad » Tue Jun 21, 2011 11:27 am

Scott - thanks so much for your mod circuits! They're great - and I'm incorporating them into my newest Monotron rebuilds.

You asked back in January about cleanly incorporating switches into the Monotron - well, I'm here to provide you with the info. You can (relatively) easily fit up to 3 SPDT slide switches (Farnell equiv. #1437714) into the cover - and then use connectors of your choosing to the mods (or you can hardwire them if you don't mind the cover staying fixed).

Here are some photos from my first Monotron mod from last year (I have a newer, much much smaller one almost finished). This one has just 2 switches, but you can fit another one at exactly the same place on the opposite side of the ribbon. I use 2mm countersunk-head screws and nuts to mount them:

Added specs to Monotron:
EG/LFO switch
2-octave ribbon switch
10 pin connector for breakout box (Pitch/VBIAS/GND/Rate/Cutoff/Gate/VCO/VCC/LFO/Aux In)

Breakout box specs:
External trimmer: 10K for Pitch CV
Switch: Connects Cutoff CV and Pitch CV
Inputs: Rate / Aux In / Cutoff CV / Pitch CV / Gate / DC power 1.5-5.5V (will override batteries if plugged in)
Outputs: VCO /LFO
Breadboard for testing and adding circuitry.

Monotron runs exactly as default when disconnected from breakout box (except with the added specs mentioned above).

If you'd like more detailed photos (or measurements) of the switch placement, just let me know. And thanks again for your circuits in this thread!
Attachments
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Scott Willingham
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Post by Scott Willingham » Tue Jun 21, 2011 7:26 pm

Marmad:

:woah: Your setup looks awesome. The breakout "dock" is done so cleanly and the internal breadboard setup looks handy. Your switches are also very clean work. I can do a lot with circuits, but I don't seem to have the patience to do such sweet mechanical work.

Please post pics when your new, improved mods are complete!

-Scott-

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Paradigm X
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Post by Paradigm X » Wed Jun 22, 2011 6:53 am

Excellent work :yay:

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Post by pulplogic » Thu Jun 23, 2011 6:58 pm

Great idea on the dock, looks awesome.

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th0mas
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Post by th0mas » Sun Jun 26, 2011 11:59 am

:O beautiful dock!

one question: is the only physical connection to the dock at the connector? Not sure how to do it to stay as nice but I'd suggest a second connection at the bottom of the monotron to take pressure off that connector.. would hate to see it break.

Then again, the monotron's so light, maybe the connector's all it needs?

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xtrmnt
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Post by xtrmnt » Wed Jun 29, 2011 1:22 am

The dock looks very cool. How can I get one of these :)

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marmad
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Post by marmad » Thu Jun 30, 2011 10:46 am

@everyone - Thanks for all of the kind remarks!

@th0mas - You'll notice 8 holes in the top of the dock (in the photo with the Monotron removed). 4 of those line up with the 4 screws that hold the Monotron together - and can be used to remove the Monotron's cover without having to remove it from the dock. The other 4 holes use small parkers to keep the Monotron attached to the dock. You just open the bottom of the dock to access.

@xtrmnt - I actually built the dock for a client - I'm modding 3 Monotrons for him to be used in tandem (BTW, it's really easy to sync many Monotrons together so that they make a single polyphonic analog synth - no need for the Monotribe :hihi: ). But I'm willing to upload a parts list and dimensions - or even make more of them if there's interest.

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Post by nightmayor » Tue Jul 19, 2011 11:29 am

marmad

Awsome mod Marmad.

Would you be interested in doing a small custom job on 1 monotron?

You can PM me.

Thanks

nightmayor
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what connectors to connect a new ribbon to monotron?

Post by nightmayor » Thu Nov 17, 2011 3:30 pm

what connectors to connect a new ribbon to monotron?

I want to add a new softpot ribbon to the monotron, ideally
with something easy to disconnect/reconnect.
I would like to have a longer cable so the ribbon can be attached
to a piece of plexi or wood outside the monotron.

Thanks

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Post by fortunato » Fri Nov 18, 2011 5:41 pm

hi. i bought a monotron a couple of days ago. really great sounding little thing.
i wonder if there is some simple mod to run it from the +5 volts of a usb.
i dont know anything about synths and very little electronics.
will it work if i just wire the vcc point labled on the pcb to the +5 volts on the usb and gnd to gnd?

thanks in advance
some mods here seems realy cool O.o

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Neutron7
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Post by Neutron7 » Fri Nov 18, 2011 9:54 pm

my local music store has them for $20 or something, i might get a bevvy of them and just use them as a filter bank.

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corex
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Post by corex » Fri Mar 09, 2012 1:23 pm

I've finally got around to planning out my monotron mods. I plan to do a complete re-housing for bigger knobs, bigger ribbons, and 1/8" jacks with Euro-compatible signal levels.

First, I plan to remove the switching supply and power it with my own +/-12V PSU, and I'll use an LM7805 to create a 5V rail for the monotron.

So I plan to use TL071/072/074 opamps to buffer and scale the input and output jacks. The inputs and outputs will all need offset biasing as well, right?

I imagine that an input will need attenuation (scale down) and add the BIAS signal (incoming signals use 0V as reference). This seems straightforward. I'm not sure about circuit protection on the inputs though -- any advice there?

The outputs are giving me a little conceptual trouble -- the output signal will need gain (scale up) and subtract the BIAS signal (the monotron uses BIAS as reference but I want 0V on output). I don't think I know how to subtract the BIAS from the signal. I could use a capacitor in series to block DC, which should block BIAS, but then my jack for LFO out (for example) wouldn't be able to pass DC. How could I accomplish that in a better way?

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Monotron delay mod

Post by Bitmat » Tue May 21, 2013 12:12 pm

Hi, firstly thanks for the info contained in this topic, it's much appreciated.
I have modified a monotron delay by putting it into a jetfighter control panel, and included a digital synth based on catweazles fraktal pic chip synth from the electromusic forum.
It sounds great, but the md unlike the monotron has no transistor to gate the filter as it would stop the delay from working after releasing the touch strip.
So the filter hisses when swept with no vco triggered and the output is still fairly noisy when the cutoff is right down.
I tried adding a resonance pot as the md doesn't have one, but there was still a hiss even at low resonance.
So i replaced the dc-dc converter with a 5v regulator as suggested here which has reduced the noise slightly.
I checked with my scope but as scott pointed out, taking an output before the headphone amp i.e. before the volume pot gives the same noise level.
So next i'm going to replace the r79 equivalent on the headphone amp cct to increase output level but this will presumably also increase the noise floor.
Does anyone have any ideas for an alternative single supply soic op amp with less noise than the lm324 but without the offset voltage of the op462?
I will post some pics and any developments soon,
Thanks,
Mat

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Post by conscious » Fri Jun 21, 2013 11:39 am

Hi,

any chance to still get these conversion kits?
I cant find the right thread, i think I remember there was someone here who sold all the needed parts to convert a monotron to eurorack. Please point me the way.

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Post by ian-c » Sun Jun 23, 2013 3:38 am

i think this was the euro conversion.

http://erthenvar.com/store/euromodules/monotrone

he stopped the diy kits 5 minutes before i heard about them. i mailed him a couple of times, but to no avail. :ripbanana:

if you have any joy then let me know.

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Post by mcghee » Mon Jul 01, 2013 12:20 pm

Hi,
Not sure if this is the right thread but I'm having some issues with the input and output on my monotron delay, is it possible to replace the jacks, or convert it to 1/4".
Any info would be great.

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Post by pricklyrobot » Wed Sep 04, 2013 5:02 pm

Finally got around to making a little breakout box (just some jacks and 100k linear pots for attenuating the CVs) for my Monotron.

Controlling it with my MiniBrute. I've got my breakout box pitch CV pot dialed in to where I can get the Monotron to track about 2.5 octaves fairly accurately. Trying to see if there's any relatively simple way to get a larger pitch range. Read back through the thread, but can't discern if there was ever a consensus reached on the best way to do this.

I did remove R11. But haven't modified anything else (just soldered wires for Pitch, Gate, Cutoff, and LFO Rate In, plus LFO Out from the board to a regular 5-pin DIN/MIDI plug which plugs into my breakout box). Monotron is still running on batteries at the moment.

Thanks in advance for any suggestions.

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Post by RTB » Thu Nov 21, 2013 6:54 pm

Hi all,


self confessed electronics dunce here who has a question about the Korg Monotron analogue synth...I know, it's been a while since this thread has been active and I should know better!

Basically, I have a Korg Monotron analogue synth and also a theremin which has a CV output, however the CV output is as of yet not configured.

I was planning to use the theremin with the Korg but don't know what kind of voltages I should program the CV output to give so that the Korg would be usable.

The theremin has a max output of 0-5V.

The theremin has an octave range of around 5 octaves so what kind of voltage drop per octave would be useful?

Once again, apologies for asking daft questions!



Roy

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