Tuesday, 10 April 2012

Euro to Buchla CV Interface Module

I was commissioned by a client to design and build an interface module that allowed his Euro synth to modulate his Buchla synth and vice-versa.

Format Jumbler?

Euro uses 3.5mm mini jack sockets and Buchla uses 4mm banana sockets (which are colour coded). However this design is not just a 'format jumbler' because aside from the connector differences the signal ranges are different.  Buchla uses CV signals in the range of 0V to +10V and Euro uses nominally ±5V (well actually anywhere between -11ish Volts to +11ish Volts).  My client wanted 4 channels of Euro to Buchla and 1 channel of Buchla to Euro. 

Design Spec

The design spec I settled on was ±5V in (Euro) gives 0V to +10V out (Buchla) on channels 1 to 4 and 0V to +10V in (Buchla) gives ±5V out (Euro) on channel 5. As you can see the inputs and outputs both have 10V swings and so a gain of 1 is required but with a 5V offset (-5V one way and +5V the other). The input impedance of 100k and output impedance of 1k was used as it's fairly standard.  I used clamping diodes on the Buchla outputs so that an input more negative than -5V didn't cause a negative output to the Buchla. Because there might not be a common ground reference between the two synths a GND socket was also added to the design. Note that Buchla colour coding uses black for CV outputs and ground connections, blue for CV inputs.


As the job was for just one module I used one of my standard 8HP front panels (order code 8HPP) and a new DIY prototyping PCB (order code DIY3) to build the design onto. 

The 8HPP front panel has 10 3mm pilot holes which had to be opened out to fit the 4mm and 3.5mm sockets plus a new hole for the GND socket. I settled on the position for the GND socket equidistant between the four lowest connectors really just for best spacing; I looked at putting it below the lowest two connectors but decided (incorrectly) that there would be not enough space to get fingers in and also avoid the mounting rail.

8HPJ (left) and 8HPP (right) Front Panels

The DIY3 is a bigger version of the DIY2 prototyping board and allows more complex circuitry to be realised due to the larger breadboarding area, power rail support for op-amps and A-100 Bus support (it is effectively a hybrid between DIY1 and DIY2 boards). The idea behind DIY3 was to be able to do more with jack-socket only designs such as buffered mults and attenuators, amplifiers, logic and gate/trigger circuits etc.

DIY3 Prototyping Board

The First Module

Here is the first module which I'm pleased with. It showcases the DIY prototyping board's capability nicely. The client was happy too.

The Second Module

On the back of the first order another client expressed an interest in the same functionality but preferred the GND socket to go below the lower two connectors. There was enough room to clear the mounting rail but pushes it closer to the channel 5 connectors than is ideal but still looks ok.

Further Modules?

I have another two expressions of interest for these modules especially if they were production rather than prototype versions. One suggestion was for front panel graphics to be added using the same font/style as Buchla which I like the idea of. Another thought is that not just CV translation is useful, gate/trigger translation would be useful too (as they are not compatible between the two 'standards'). I could also provide these as very basic kits (all the bits you need, a circuit diagram and a layout pattern). If I can get a few more interested parties to make it viable I would consider a small production run with purpose designed PCBs and a screen printed front panel. So if there is any interest out there please make your thoughts known to me.