AW: PAIA Hex VCA circuit

[Mark Smart]

(In German accent) THE TIME HAS COME TO DRAW.

Here is one-sixth of the PAIA hex VCA circuit in question. You guys pretty
much guessed it:

Control in >---////--+--////-------+------+
0-5V           4.7k  |  100K       |      |
                     /             /      /
                100K /        100K /      / 4.7M
                     |             /      /
                    gnd            |      |
                                V+ |      |
                                |  |      |
                                /  |      |
                            50K /<-+      |
                                /         |
                                |         |
                               gnd        |
                                          |
                           +-------///----+
                           |       4.7M   |
                           |              |
           22K             |              |
Audio in >--///--+---------+-----o<|------+
                 /               1/6 4049
                 / 270 ohm
                 /
                 |
                gnd


Here's the single mixed output:

4049 pins 1& 8
|     
|     
|     15K
+-----///-------+
|               |   15pF between 1 & 8 on op-amp
|     |\        |
|     | \       |
|-----|- \      |
      |   \     |   22uF
      |    >----+---| |-----output
      |   /         +  -
 gnd--|+ / LM301
      | /
      |/

I tried running 0-15V pulse and square waves into it, and when the
volume is down really low, the output looks like:

_____        ______
     |      |
     |      |
     |______|
     |
     | 

And sounds just as bad. This happens even if you just run one square wave
into it at a time, so I don't think it's a just crosstalk problem. I tried
changing the 4.7M resistors to 2.2 Megs, and changing the voltage divider
on the audio input to 100K and 470 ohms. This seemed to 
help a little, but did not get rid of the problem on all channels.
Also I tried putting 68uF caps on the audio inputs to remove the DC offset
of the square waves. No good. Also tried removing pin 1 from the output to
the op-amp (this introduces another transistor into the circuit which
doesn't really need to be there. I think), no good.

I think maybe this circuit is just too nonlinear to have much dynamic
range. I would appreciate suggestions on how to improve it, but I've pretty
much decided just to build a bunch of ARP VCA's, which aren't all that
complicated. 




************************************************
*     Mark Smart                               *
*     Electronics Engineer                     *
*     And Musician                             *
*     smart at medusa.nn.com                      *
************************************************