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MODS - Midi

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Adding Midi to the Transcendent 2000 Module

Well, since it's been module-fied, I knew that sooner or later i'll add internal midi to it, rather than running it off from my Midi-to-CV converter (Kenton Pro-Solo in this case).

There were a few things to consider when it comes to adding midi, and i had to plan ahead slightly beforehand.
- VCF Control needs -12V - 0V - +12V, so any simple Midi-to-Aux CV won't do, an additional circuit is required.
- The Gate/Trigger is Pulled-Up S-Trigger - it's ok with my Kenton Pro Solo, but the S-Trig Out from say a Kenton Pro-2 would not cut it. So either a Midi-to-CV Converter that can do pull-up S-Trig or some additional circuit is required.  (Pull-up is not difficult tho, it's just adding a 10k resistor wired to the +ve - although it can also be done using transistors).
- How to patch in the Aux CV to VCF - if patched to the Filter Frequency Pot directly, the Pot would become useless (as the Aux CV would add voltages to it constantly). If patched to the Foot Pedal Control, you'll lose the foot pedal input function (if you have the foot pedal - i do have one)
Since i have the External CV routed so that the onboard Portamento would still work, basically I would just need a Midi-to-CV that does Pitch Bend and a couple of Aux CVs.
I happened to come across a 2nd hand (cheap!) Doepfer MCV4 - the same one i put inside my Korg MS-20 - which is kinda perfect for this job!

Main Pitch CV (with Pitchbend) + 3 Aux CVs, Gate can be configured to be V-Trig, S-Trig, Inverted or Non-inverted. Gate can also be configured to run at the power supply input voltage - which in this case, a pull-up is not neccessary.

This one is actually an older RED model...



External Aux CV to VCF "Tuner"
Problem: The VCF requires -12V to go from centre to minimum and +12V from centre to max. Normal Midi -to- Aux CV only provides either 0 to +5V or 0 to +10V, and so making Aux CV not able o control the Filter Frequency properly.
I actually emailed and asked Kenton about this. Their advise was to buy a Kenton Pro-4 (!) which gives -10V to +10V. However, they're nice to provide me with a small schematics which would convert 0 - 10V to -10V to +10V...

I've redrawn what they've sent me:

Click to Enlarge!

I've done a stripboard for this:

Click to Enlarge!
I've decided to use the Doepfer MCV4, and use CV2 (the Midi Aftertouch) for Aux CV controlling the VCF.
Problem is the all of Doepfer's Aux CVs are 0 to +5V. So for this to work, I would need to make some modifications - like doubling the 1st feedback resistor to double the initial gain.
Somehow, The circuit kept giving me -4V to +8V, instead of -10V to +10V...  I was too lazy to refresh my electronic theory to recalculate the stuff, so i just tried swapping some other values for the resistors and eventually got it giving something around -10V to +10V, which is close enough to the required -12V to +12V.
Here's what i eventually come up with:

Click to Enlarge!

So now that i have -10V to +10V for the VCF, i need to think where to tap it to...
I mounted the stripboard next to the External CV Tuner board:

There are a couple of ways of doing this. Looking at the schematics, centre of the VCF Frequency Pot (RV17) is the perfect place for this, but since the Aux CV will constantly supply voltage to this point, the Freq would then become totally External Control, so you loose the function of the pot.
Well, you could add a switch to select internal/external, but i didn't wanna do that. I'd rather the external CV acts like the Foot Pedal.
Tapping to the foot pedal input is another option, but it requires slightly more thought - i do have the T2k Foot Pedal, and i don't wanna loose the pedal input after this mod.
Problem is, the VCF Pedal Input Socket is NOT a switching jack socket (on my T2k anyway) - so in the end i changed this into a switching stereo jack socket.

Original VCF Pedal Input Socket:

Changed to a switching socket:

The modded Aux CV only needs to go to the RING of the socket, so without any jack inserted, it would go directly to the VCF Control point. (I probably added a 2k7 resistor there, but it's not too important).
Adding the Midi-to-CV Convertor
Basically the rest is just mounting the Doepfer MCV4 into the case, and also, redirecting the Config switch and status LED to the front panel.
The MCV4 is powered by +12V/0V tapped from the PSU PCB. The jumper inside the MCV4 is configured to +U (which means Gate voltage equals power input voltage) otherwise it won't quite trigger

MCV4 Mounted:


Midi LED and Config Switch mounted at the bottom right of the front panel:


And obviously, Midi In and Midi Thru at the rear panel:

Note: The MCV4 is not configured as S-Trigger. it's actually INVERTED GATE (Program Change #8).



What's Left To Do...
Actually there could be more done. I have 2 more Aux CV's left on the MCV4 (1 being Midi Volume CC#7, other being a freely assignable Midi CC#). These maybe routed to other areas for further controls... But I think i'll leave it like this for the time being.
Well, now at least I have a fully midi-ed Transcendent 2000 Module :)




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