Poly Modulation for OB-8

The Story
 I did not intend to buy an OB-8, when I was looking for a "real" analogue polysynth some years ago. When I  was a student, I could only afford digital emulations of analogue sounds (on a Kawai K1 or Roland D50), or an analog/digital hybrid like the Ensoniq ESQ1. Then, when I got a job, I was looking for the "real thing", and that was, by then, a Prophet 5.
Then I saw this almost new OB-8 at a local dealer, at about the same price as a Prophet. I was offered to take it home and test it for a week or two, and that was when I fell in love with this synthesizer. I forgot the Prophet, bought the Oberheim, and did never regret it. (Even now as I have a Prophet, too, the OB-8 is still my favorite.)
That was many years ago, and I still find new interesting sounds on this machine every time I play it. But in comparison with the Prophet 5 or with a Memorymoog, there always was one drawback: the lack of "Poly Modulation", especially audio rate filter modulation. Meanwhile I had built a clone of the Minimoog, and I loved the sound of oscillator 3 modulating the filter with a saw wave at audio rate. That's why I decided to retrofit such a thing into my OB-8.
The saw wave of VCO2 of each voice should modulate the VCF of the same voice, with a modulation depth controlled by one CV for all voices.

Description of the Modification
All you need to get the desired effect in the OB-8 is an additional signal path from the VCO2 saw to the filter control node. The OB-8, as well as many other synthesizers, has an opamp that provides a virtual GND summing node for adding separate filter control voltages. For a first test, I connected the saw output of  VCO2's CEM3340 with this summing node over a resistor in the 100kOhm range. Everything was fine. In order to make the effect variable, and to turn it completely off, a voltage controlled resistor would be needed. As one end of this resistor is connected with a virtual GND node, the resistor can directly be replaced with an OTA like the CA3080. That's the way I finally implemented it: One CA3080 soldered directly on top of the filter CV summing opamp of each voice, with the 3080's supply pins directly connected to the opamp's supply pins, and the 3080's output pin directly connected to the inverting input of the opamp. Two resistors for a voltage divider at the OTA input, and a short insulated wire connection to the 3340's saw output pin complete the new signal path.
The control currents for the 8 OTAs (for 8 voices) are provided by one exponential converter which I located beneath the Modulation / Bender section. I have two sources of control voltage for this Poly modulation: The first is an external Pedal input, and the second is derived from the Modulation potentiometer in the Bender block. (The potentiometer behind the Modulation knob, not the Lever.) This potentiometer is originally used to control the 2nd LFO modulation, but it is only actice when pulled up. So I could use it as a source of PolyMod CV. First I wanted to have a switched operation (knob in original position -> PolyMod, knob pulled out -> LFO mod), but then I found that I didn't even had to care for switching. What I have now is:
No Modulation knob turned to ccw end position
Poly Modulation only knob turned cw, not pulled out
LFO Modulation only use the Lever ...
Poly Modulation and LFO Modulation knob turned cw, pulled out
This is a good compromise for me, because there is still the other, programmable LFO for a fixed / manually set amount of LFO vibrato on the OB-8's main panel section, and I used the local "performance" LFO only together with the Lever, anyway.

For more information, please contact
Juergen Haible



All drawings copyright J. Haible (C)1996