Synth Module Board---Second Time Around

First Posted December 19, 2011
Feb 10, 2012

Blackmer VCA

    I thought it would be fun to experiment with the Blackmer VCA.  JH used to use a version of this VCA in things like glide controls, etc, where he needed low CV feedthrough.

    By the way, this is the first drawing I have done with KiCad.  So far I am finding it clumbsy to use, but I am going to keep at it a bit before I give up.

December 19,2011

    Back around 2000, I posted the web page for my rather ambitious synth module board.  I finished this synthesizer around 2005.  I vowed to never make anything like this again...well, here I go again.  There were a few things I did not like about this synthesizer and I would like to fix them.  After thinking about if for a  bit, I decided to go ahead and just rev the board design.  Although, this is goin to be a rather major revision.  Here are some of the things I did not like.

1. I used TL074 quad opamps for most of the circuits.  One problem this part has is that the power supplies are, from my point of view anyway, backwards.  The new board will be using TL072 or TL052 opamps.

2.  The VCOs used my first try at an electronic temperature compensation, and it was not entirely successful.  The new board will use the new circuit which stays in tune quite well..

3.  The four pole filter was OK, but I am going to use the four pole filter that I did that has LP, BP and HP outputs.

4.  The MIDI to CV converter used two QUAD DACs.  Even though all of the DACs have almost the same gain, they are not identical.  The new MIDI to CV converter will use a single DAC and an 8 channel sample and hold.

5.  The MIDI to CV converter also used an AT90s8515 microcontroller.  While at the time, this seemed pretty fantastic, it is really dated.  I haven't decided on what platform to use.  I am, at a minimum, considering an ATxMega, or maybe I will really take the plunge and use a Cortex M0 or M3.

6.  I thought an interpolating scanner would be fun, but I found the one I did just not that exciting.  I am thinking of substituting a diode function generator instead.

7.  I used my favorite OTA.  But the CA3280 ceased production about the time I completed this thing.  I still have quite a few of them, but I am using them only in places where they are needed the most.  I am using NE5517 in all of the non critical places.

    The board will still be the same form factor, since I want to just plug the new one in where the old one was.  Well, the same within reason.