Modification gives Rev. 3 Prophet 5 more fatness

[Haible_Juergen#Tel2743]

Hi!

No, I won't draw comparisons to the Rev. 2's here anymore.
(Most probably I was wrong about the mixer overdrive in the 2's;
the resistor that mixes Noise is connected differently in the 2's
and 3's; so the 2's probably haven't got enough gain in the mixer
stage to clip, either.)
After having said this, here is what it is all about:

* The pots that mix OscA and Osc B on a Prophet 5, Rev 3, are
only used to set the relative level between the two oscillators.
Ok, you can also change the total volume of a patch if you
don't set them at maximum, but you also loose SNR that way,
so I always have at least one of the pots at maximum.
But the point is that the mix level doesn't change the sound.

* Now I want to add some overdrive in the mixer stage.
After the mod, as long as you keep the Mix pots at middle
position or below, everything is just as it was before.
But as soon as you have higher settings, the mixer stage
clips. This changes the sound of two slightly detuned VCO's
dramatically: You loose the much of the amplitude component
of the beating (no more fading ...), but you still have the thickness
of the detuning. The synth sounds more "brutal", and fatter than
before.

* This must not be mixed up with filter overdrive. Overdriving
  the CEM3320 sounds rather awful, IMO. But if the mixer stage is
  clipping, the distortion products are filtered by the VCF.

This is how it is done:

(1) Increase the gain of the osc mixer amps by factor 3
==============================================
This is achieved by increasing the amplifier bias currents.
 -> On PCB3 (Processor board), solder 15k resistors in parallel
     to R322 and R328 (both 33k). (best to be done on component
     side; you don't even have to remove the board.)
     These two resistors affect all 10 mixer VCAs.

After this step, you can overdrive the VCF/VCA section. I have not
checked if clipping occurs in the VCF or in the VCA, but for sure it
sounds very unpleasant.

(2) Make the osc mixers clip earlier than the filter
=========================================
This is achieved by inserting resistors between the OTA outputs
and the filter input current summing node.
 -> Cut the copper trace from Pin 12/13 of U464 (CA3280) to Pin 1
     of U469 (CEM3320). Then close the connection by inserting
     a 220k resistor. (best to be done on copper side)
 -> Do the same thing for voices 2 to 5.

(3) Re-wire the 100k resistors for noise
=================================
After (2), the summing node is not at U464, Pin 12/13 anymore, but
the noise is routed there. To avoid further scatching of copper traces,
replace the 100k resistors completely:
 -> Cut out R4367 (100k), and solder in another 100k resistor that leads to
     Pin 1 of U464 directly. (best to be done on copper side).
 -> Do the same thing for voices 2 to 5.

Summary
========
* You need 12 resistors that should cost you less than 1 dollar.
* Your Prophet can sound the same as before if you only use the
   first half of the mixer pots' range. (You do *not* loose SNR, because
    you have the same if not better signal stength as you had before.)
* You can achieve much more "fatness" (or was that "thickness" ??)
   than on a unmodified Rev. 3 Prophet.

Hope you found this interesting.
As usual, I guarantee for nothing, so don't blame me if you fry your synth!

JH.