Friday, April 26, 2013

Midi2CV module via Midimplant board

UPDATE 8/19/13 -- I'm working on a new version of this module with buffered outputs.  Stay tuned

 This is a module I built based on the Midimplant board.  They are capable of different configurations which you can upload via midi Sysex.  The website has a configuration widget here, which makes things incredibly easy.  One of these options will convert midi to CV and Gate signals for 2 independent midi channels.  My eventual goal is to have a 4 channel midi to CV module, so I hope to be getting another board when they are available.  With that in mind, I designed the module and the PCB to be ready for my future configurations.

Here's the board. I soldered a header on it, to make it easy to play with.

Here's the pinout.  Something to keep in mind, is the gate signal is whatever the Positive voltage input is.  my system is a standard +/-15V.  All my modules seem to work fine with 15v gate, so I didn't really worry about adding a voltage regulator, but it would be easy to implement if that was a requirement. 

Thursday, April 11, 2013

Korg Mono/poly polysix SSM2044 Low Pass Filter chip module

UPDATE!!! 8/19/13 I'm not sure how I feel about this project.  I've had the module for a while now and I'm not a huge fan of the end result.  I guess it is what it is. 

I've owned a Korg Mono/poly for 8 years now.  It's an excellent synthesizer and I've never been to compelled to buy any other expensive analog synth because it's such a workhorse.  I was thinking about selling it recently, because All I ever play these days, is my modular.  I thought it might be cool to build a filter module based on the filter chip inside the Mono/poly--the SSM2044.  Did a quick google search and found these plans.
You can find these chips for like $20 on ebay and I thought that was a minimal investment for a fun project.  I ordered one and some other parts.  Like all projects, I breadboarded it first to make sure it worked.  I had a hard time finding a tempco 1k PTC thermistor.  All I could find was NTC thermistors, so I used one of those.  That was probably a mistake.  I finally found some PTC 1k thermistors which I ordered today.  When they come, I will replace the NTC.

So,  this wasn't the most satisfying of all projects. The first thing I didn't like was the range of the Cutoff knob, so I played with that for a while.  I also didn't like how the Resonance knob only seems to kick in--in the last 30 degrees of rotation.  I messed with values and I just kept getting farther and farther from a solution.  I realized later that it probably has more to do with the fact that I was using a log pot instead of a linear.
  I did some more research and turned up a few more values to try, which is the primary source of the modifications above.  A modification that I made, which I was surprised hadn't been made before has to do with which input you use on the SSM2044 chip.  It has a + signal in and a -signal in.  I determined that the - signal in was an "inverting input".  I chose this to be the main input because there is an inverting amplifier on the output.  This makes the output signal the same phase as in input signal.  I also switch out the 150k resistor for a 120k. I did this because I was having a horrible time trying to tune the thing to a 1v/oct.  I never really got it to a place I was happy with, but it's good enough I guess.