Saturday 19 December 2015

Korg Monotron (Duo) - Buchlarized

I'm resurrecting some of my old Korg Monotron hacks for a new job - adding an extra voice to a Buchla Easel. The easel is a wonderful synth and I'm always looking for ways to expand on it.

The Duo Monotron is very small but is fully analog & has 2 VCOs, Filter & a X-mod circuit
borrowed from the Korg Mono-poly. (VCO cross modulation). The Aux jack allows you to input external audio.

The LP filter is based on the MS10 & MS20 filter design. Sadly there is no HP filter.
The second VCO takes the place of the LFO from the original monotron. 

It should be relatively easy to hack this again. 
The Monotron seems like a prefect candidate for this job. I'm using the Duo Monotron.
If this is successful I will try the same on the Monotron Delay & Original.

The Monotron-Easel BOB (break out box).
The box is a $9 Jaycar job (Australia).
The plan is to be able to plug this into the 208 card slot but still have the slot available for additional
expander cards.

The box was too narrow (by just a few mms) so I had to cut the sides of the card. This makes me cry.
I might in the future etch some smaller 56 slot edge connectors for jobs like this. It's a shame to damage these beautiful expander cards. 

Lots of SMDs. The Red button is for cycling through different scales of the ribbon controller.
the ribbon is quantised to chromatic, major, minor and continuous scales, with chromatic mode the default on power-up.

Both of the Duo's oscillators have square & pulse outs. VCO 1 also has a saw out.

On the back of the PCB are various points labeled gnd, VCC, gate,square 1&2, Saw1&2, PS 1&2, Virb, Vbias, etc

Vrib is the CV input.

In the Monotron DUO the +3.36V is labeled VD and serve as Digital power source (Vcc), it's not the value of Vbias. Vbias is the +1.35V "middle" (or virtual ground) voltage between GND and VCC used in the analog section.

It's a bit like the AUX expander.

Now the real fun starts.
For now I'll leave the ribbon alone. Its essentially a linear 10k pot.

Before I start linking the BOB with the monotron, I thought it would be useful to add some additional mods.
The above patch card (from Cobramatic) illustrates some of the cool mods available.

1. CV in (Blue Banana) with an addition on-off-onn switch to gain access to 2 step mode & the
    Seq random No. of steps. 
    You need to add 120k resistors to both of the outputs on the card to properly utilise this feature.

2. Next are the Pulse Outs for each sequencer step (red bananas). 
    For now I've omitted adding the LEDs. I might do this at a later stage.
    Note, these un-attenuated outputs are "hot". I'm reading 15V pulse triggers so do be careful
    when interfacing with external gear. I will prob add some resistors later to cool things down a bit.

3. Now for the Pulse Trigger In’s (yellow bananas) to:
    a. Sequencer,
Random CV generator rate    
    c. Pulser
Envelope Generator.
    This should be extremely useful for syncing with external gear
    You need to add 390k resistors to each jack. 
     I might make some permanent connections between some of these inputs and the monotron.

4. On the right side panel I'm placing the LPG inputs on the top.
    These comprise 2 blue banana jacks with 120k resistors and a switch to add random CVs
    They will step through the different modes : LP Filter, Combination & VCA.

5. Complex Oscillator
    Two CV ins (White Bananas) with 120k resistors & switches to add random CVs
    These connect to the modulation inputs for wave-shapers 1&2.
On the vintage easel card, the complex osc has marked a "key" and two "WS" waveshaper inputs.
The modern BEMI easel cards names them "Key", "t.lev" (timbre level), and "tws" (timbre wave shapes - tri/sq/saw). I understand these are the same so modulation of the left WS input should affect the level of waveshaping, and the WS on the right should morph between tri/sq/saw).

6. Modulation Oscillator
    Two CV ins (Purple in) for modulating the waveshaper &
    the Modulation Mode (FM, AM, none).
    For each input I've also added a simple on-on switch to swap inputs between random CVs and
    banana jack CVs. Again, 120k resistors are needed for each input.

It's a tight squeeze.
Finally, we can add the monotron.

 I'm using just 2 control inputs in this prototype: CV in for the main VCO's pitch & the filter.
The gate and the second VCO inputs didn't respond well to the CVs from the Buchla.
I'm also taking a mix of 5 CV outputs from both of the Duo's oscillators (the red bananas on the front panel).

Some pics of the inside
120k resistors are the magic value for interfacing the buchla
Cvs with the monotron.

I'm using random, sequencer & envelope CVs from the Buchla to modulate the monotron.

++ DIY schematics (original monotron)- Muffs 
++ DinSync (hacking the original monotron)
++ Adafruit
++ Korg's official schematics for the original monotron are here.
++ Duo schematics
++ Muffs - Monotron Duo CV Mod?
++ ElectroMusicDotCom 


  1. Wow this is awesome this is inspiring me to do something with me easel. Thansk for sharing.

  2. Cool Chekmate,
    Let me know what you come up with. The Easel program card is a wonderful invention. It opens up the world to your Buchla