Most commercially available analog sequencers are rather expensive and take up a lot of space. Since I am only interested in having a small rack for testing modules that I develop, I chose to build my own simple sequencer. I bought some cheap potentiometers with knurled axles, all ready to solder vertically on to a PCB. The potentiometers are rather small for very accurate adjustments, but since I always run the sequencer outputs through a Doepfer A-156 Dual quantizer this is not a major “musical” problem. Each potentiometer-wiper is fed to an analog multiplexer, controlled by a RobotDyn microprocessor. The hot end of each potentiometer is connected to a common reference via resistors. These resistors prevent unintentional short-circuit of the reference (through the multiplexer) from one potentiometer at 100% to another at 0%. As a little extra feature, the reference can be modulated from an input on the front panel.
A few different modes of multiplexing can be selected with an encoder (blue knob), when the sequencer is not in RUN-mode:
- Scan through up to 32 steps with output to “Main”, forwards, backward or random.
- Scan simultaneously through two times 16 steps (or less), with output to two separate outputs (“Main” and “small”).
- The above scanning can also be forwards, backward or random, individually selected for each block of 16.
- Scan through the first 24 steps (or less) with output to “Main”, forwards, backward or random, AND independently scan the last 8 steps (or less) at any rate or combination, with output to “small” (eg.: used for arpeggio).
Different timing possibilities are also possible:
- Any mode can be clocked externally from frontpanel input or GATE-bus, or internally by the processor.
- Internal processor generated clock rate can be selected.
- Internal clock rate can also be controlled with input from the CV-bus.
- Each scan-step will always generate a pulse output to front panel.
- The length of each step (number of clock counts) can be selected with the encoder at runtime.
All scanning and timing selections are displayed on the small I2C display, and in RUN-mode the current step number is shown. The result is a compact 32 step (semi-)analog sequencer module, and the prototype frontpanel looks like this:
… and here you can see the assembly behind:
I had to use ALL inputs and outputs on the processor:
Here is a link to the schematic: 32step sequencer