Excerpt from letter from Serge on this board

Also, I have enclosed the MDAC PC board. This is essentially surplus to us, since Serge Modular no longer produces the module (N-Voice Controller) in which it was used. I am enclosing the parts stuffing diagram, also. Unfortunately, while preparing our soon-to-occur move to a new space (at the same address), I am sad to say we lost all copies of the schematic.
However, it is easily explained:

Altogether, there are 16 buffers (T.I. TL082 dual op-amps) each with a holding capacitor fed by one of two CD4051 RCA CMOS analog demultiplexer chips (each routing I input to S outputs according to a 3 bit control, with an "enable" control input which functions to disable all switches). The demultiplexers are fed by an 8 bit d-to-a chip (Analog Device AD7523) plus associated parts (TL082 buffer, TL431 voltage reference, trimmer). The 4012 and 4049 chips are used either as uncommitted buffers, and as a small circuit to decode the 4th bit and "enable" control necessary for the 16 channels.

In operation with a micro, a "Peripheral I/O" (PIO) chip such as a 6522 is used, with one eight bit PIO port feeding the DAC and the other eight bit port used as follows: 4 bits to derive a 1 of 16 channel selection, and a fifth I/O bit used as a master "enable".
The steps that the micro takes are:

• 1. Dis-assert the "Master Enable" - so that all channels are disabled.
• 2. Send 8 bit data to the D/A.
• 3. Send 4 bits for channel selection..
• 4. Assert the "Master Enable" so that the D/A voltage can pass through the demultiplexer and be captured by the channel's op-amp buffer.
• 5. Wait at least 10 microseconds for the capacitor to charge up.
• 6. Repeat the cycle as needed to feed other channels...

The actual card I am sending comes from working equipment and works. It was used as an 8 channel board, however. You can easily expand it to the full 16 channels by adding 5 chips. The "U" pad is the logic power supply (<+5V) which you can obtain from the micro (negligible power drain). The other power supply voltages are standard Serge... Pads ABCDEFGH are the 8 bit inputs to the DAC. Pads MM, NN, PP, are the 3 bits addressing each 4051. l'm afraid I don't know which other pad is the 4th address bit, or the Master enable (CC..DD? or LL,RR?) but that can be traced.

What I haven't said in all this, is there is quite a bit of elbow grease needed to get this project going, since you will not only have to get the board going, but also a suitable interface for the micro, the micro itself, and some means to get the micro to talk. Preferably, it should talk FORTH, since that language makes it easy to talk to D/As etc...