Modifying "rev B" CrowBX voice cards to use the AS3310

March 2020, by Jacob Vosmaer.

The idea of this hack is to take a working CrowBX rev B voice card and retrofit an AS3310 onto it. Maybe you want to match rev B voice cards with newer ones that use the AS3310, or with original OB-X voice cards that use the CEM3310.

This mod leaves as many parts of the non-3310 EG's in place as possible, to make it easier to revert the mod for whatever reason.

Bill of materials

Per voice card:

The 47n capacitor and the 24K resistor control envelope timing. The 1n capacitor is there to automatically create a trigger signal when the gate goes up; the 3310 has a trigger input but the OB-X only has a gate signal so the trigger must be derived from the gate. The purpose of the 22n capacitor is not clear to me.

The 680R is a current limiting resistor for the negative rail, which is internally regulated to -6.5V by a zener inside the 3310.

VCA

Remove:

When removing R43 and R128 it may help to also cut R166, which is part of the VCF envelope and which is not needed anymore if you install an AS3310 on the VCF.

Connect:

Use wire wrap wire.

Install:

Note that the AS3310 is facing the wrong way, and it has its pins 1, 2, 15, 16 floating to the right of the socket. Pins 5 and 6 should not be socketed.

Fly:

Use wire wrap wire.

Special:

Both of these have one component leg in the board, and one leg flying to an AS3310 pin.

VCF

Remove:

Connect:

Use wire wrap wire.

Install:

Note that the AS3310 is facing the wrong way, and it has its pins 1, 2, 15, 16 floating to the right of the socket. Pins 5 and 6 should not be socketed.

Fly:

Use wire wrap wire.

Special:

Both of these have one component leg in the board, and one leg flying to an AS3310 pin. Put insulation (heat shrink) around the 680R resistor because it has to bridge a long distance.

Why does this work?

Here is a map of the AS3310 pins to the TL074 pins in this mod. "Ax" refers to "pin x of the AS3310" and "Tx" means "pin x of the TL074 socket".

AS3310 pinsocketdescription
A1 hanging outside socket 47n cap to ground via flying wire
A2 hanging outside socket envelope output via flying wire
A3 T8 unused, mechanical connection only
A4 T9 gate input
A5 bent out 1n cap to A4
A6 bent out 680R to -15V
A7 T12 ground
A8 T13 22n cap to ground
A9 T2 sustain input
A10 T3 24K resistor to A2
A11 T4 +15V
A12 T5 decay input
A13 T6 release input
A14 T7 ground
A15 hanging outside socket attack input via flying wire
A16 hanging outside socket unused

If you look closely at the CrowBX schematics and the original OB-X schematics then this should all add up.

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