Oberheim Mini-Sequencer

Schematic for the Oberheim Mini-Sequencer

Theory of Operation - Quantizer

The counter's lowest six bits feed the DAC, while the seventh bit forms A7 & A7* to toggle between the two sequencer CV signals in the multiplexer. This way only one DAC is needed for each sequencer CV, that's why there's a two channel multiplexer. The output of the DAC is a cyclic ramp waveform with discrete steps, which can be reset by the sequencer clock. The counter's clock at pin 1 comes from the two NOR gates and I assume it's a much higher frequency than the sequencer clock.

Now look at the multiplexer. A7 & A7* select which sequencer CV goes to the opamp input. The opamp compares the selected sequencer CV to the DAC signal. When the ramping DAC signal is greater than the CV, a positive spike is generated (via AC coupling on the opamp output) which enables the NOR gates controlling the CMOS switches. The DAC signal is also routed to the CMOS switches, thus the *instant* the DAC signal is greater than the CV, the switch is turned on for a very short transition and charges the cap on the switch's output. While the switch is off, the output is held constant by the charging cap that is part of the sample-and-hold circuit. Therefore the output of the sample-and-hold is the quantized signal.

This cycle repeats for each CV at a much higher frequency than the sequencer clock.

When the DAC ramp cycles back to zero, the diode on the output of the opamp shunts negative transitions, thus the CMOS switches are only turned on with positive spikes.

That's how the quantizer works. The DAC's ramp waveform is the heart of the system. If you wanted to change the scaling of the quantizing you could just range and scale with the trimpots on the 1458s on the output of the R-2R network.

Very clever circuit. Flexible too, lots of I/O points, even a CV for controlling the sequencer clock. You can even transpose the sequence CV with the keyboard CV. And not a single obsolete IC anywhere. A very good candidate for a DIY project.

Michael Caloroso

Thanks go out to the following people:

Schematics/Seqs/MiniSeq (last edited 2007-02-12 03:03:50 by TomArnold)