Analogue Synth Modification ?

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Gadget999

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i have an analogue synth that responds to midi note information, but not midi sys ex info

i would like the knobs on the synth to transmit and respond to sys ex information

can i hack the Synth so the knobs transmit and receive midi ?

i am assuming the pots are simple single turn pots

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in order to do this - do i need to make the pots feed a voltage to a teensy and then get the teensy to output voltages to the synth instead

I can intercept the signals and convert them to midi messages ?


is there an easier way to do this ?
 
I think you mean MIDI CC messages? SysEx is for manufacturer specific messages such as saving/loading complete patches. You need to read the voltage off each pot with a multiplexer into an adc and also send the voltages back to whatever takes the pot voltage, via a dac and something that can hold the voltage at that level - just cutting the PCB traces and inserting the circuit I think. There are particular ICs for this purpose, I saw one the other day and forgot what it was, I think it was from Maxim. This is how old polysynths worked to store and recall patches.

What synth is it out of interest? Behringer? (urgh...!! ;))
 
I think you mean MIDI CC messages? SysEx is for manufacturer specific messages such as saving/loading complete patches. You need to read the voltage off each pot with a multiplexer into an adc and also send the voltages back to whatever takes the pot voltage, via a dac and something that can hold the voltage at that level - just cutting the PCB traces and inserting the circuit I think. There are particular ICs for this purpose, I saw one the other day and forgot what it was, I think it was from Maxim. This is how old polysynths worked to store and recall patches.

What synth is it out of interest? Behringer? (urgh...!! ;))

Hi - Thanks for the reply

it is indeed a Behringer TD3 !

I have some old analogue stuff from the 1980s but the TD3 is a recent purchase, I bought it 2nd hand to modify it. They have done a good job at emulating the TB 303 - but none of the tweaks to the filters are transmitted

would be nice to modify it and make it the synth it should have been - the programming interface is the same is the old TB303 and leaves a lot to be desired in 2021 ! - but i guess that is where the quirks of interface add to its charm

there are lots of mods to change its sound, but none yet to add to its midi performance
 
Maxim do a range of digipots that you can use for this. Also Microchip MCP42050 is popular.

Discussion on subject in another forum might be of interest....
https://modwiggler.com/forum/viewtopic.php?f=17&t=245549&p=3477479#p3477479 cheers, Paul

thanks for the link - it would appear that 7 bit resolution will make the control a bit too steppy - however unless I try it I will never know

i have a Red Sound Darkstar that uses 7 bit resolution (at least i think it does) it has some steppy filtering but that is part of its character
 
14 bit resolution for CC is perfectly possible - it takes more bytes on the wire of course which can be an issue,
and the MIDI sender has to know to use the 14 bit extensions for the controls in question.
 
I think digipots are the wrong way to go, as you want to control the synth with the on-board pots and allow control by MIDI via the Teensy. All control will go through the Teensy. Something like below. The multiplexer connected to the DAC needs some sort of sample and hold, so that it holds the output voltage until it cycles around again. Can anyone suggest a suitable cheap IC for this? For scanning the pots a 4051/4061 MUX should be ok. A T3.2 is 5V compatible and has a 12bit DAC. You might want to open your TD3 and measure the voltage across the pots first. How many pots and possibly switches do you want to control?

Capture.JPG
 
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