CGS analog logic voltage processor

CGS26 the Analog Logic voltage processor module is the analog implementation of some basic logic elements. Instead of dealing with binary inputs, the "logic" is applied to whatever voltages are present on the inputs. When the AND element is fed several voltages, the output will equal the the lowest input voltage. The NAND output will be the inversion around 0 volts of the AND output. When the OR element is fed several voltages, the output will equal the the highest input voltage. The NOR output will be the inversion around 0 volts of the OR output. Apparently the AND and OR functions are the same thing as "peak" and "trough" on old Serge synthesizers, though they are implemented somewhat differently.

How to use this module
Connect the inputs to several voltage sources feed the outputs to a module that requires a CV input. Unused inputs will automatically set themselves to a voltage where they will not interfere with the operation of the logic elements. The modules have an operational range of +/-10V.

A little on how it works
The elements along the left side of the schematic are essentially "perfect diodes", that is diodes with a voltage drop of 0 volts. For the OR/NOR module, the diodes are forward biased, while for the AND/NAND module they are reverse biased. Pull down resistors on the inputs of the OR/NOR module set any unused inputs to the lowest voltage possible, so that those inputs will not affect the output. Pull up resistors pull any unused inputs of the AND/NAND module to the highest possible voltage for the same reason.

In both modules (AND/NAND and OR/NOR), the next op amp is wired as a voltage follower, to buffer the voltage from the combined inputs. Following that are traditional unity gain inverting buffers, the first giving the inverted outputs and the second giving the non inverted outputs for their corresponding modules. There is a trimpot in series with each of the output buffers, allowing the modules to be precisely trimmed for 1/V per octave on at least one of the inputs.

Two transistors wired as emitter followers give the (approx.) +10 volt and -10 volt levels as used elsewhere in the circuit.

Construction
Note that the v1.2 board has the following issues: OR and NOR outputs have their labels swapped with each other. AND and NAND outputs have their labels swapped with each other.

Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges.

When you are happy with the printed circuit board, construction can proceed as normal, starting with the resistors first, followed by the IC sockets if used, then moving onto the taller components.

Take particular care with the orientation of the polarized components, the electrolytics, diodes, transistors and ICs.

When inserting the ICs in their sockets, take care not to accidentally bend any of the pins under the chip. Also, make sure the notch on the chip is aligned with the notch marked on the PCB overlay.

Ideally, the diodes should all be matched for voltage drop, so all inputs can be set to track 1/v per octave. I did not do this on my prototype, instead selecting one input to be accurate, and accepting the others as they came, as they were only being used for varying LFO voltages, or other voltage sources for which any variation could be trimmed out elsewhere.

To match diodes, try connecting them in series with a 10k resistor across 10 to 15 volts, and measuring for identical drops across the diodes themselves. It would be best to use the same supply and resistor for all measurements, and try to do all measurements at the same temperature. Avoid handling the diodes while testing them to prevent body heat changing their temperature. Note: I did not do this for my module, so this is untried. Alternatively, you could use the diode test function present on most digital multimeters.

If you are going to match your diodes, it would be worth the extra effort to hand-match the 10k resistors too.

Diodes and resistors need only match each other within each gate. There is no need for the OR/NOR parts to match the AND/NAND parts.

When assembly is complete, the trimpots can be adjusted so that the output voltage on each module accurately tracks one input of the corresponding module. This is to counteract the attenuation of the input networks. It would be possible to substitute the input resistors for 1k, and the pull down and pull up resistors for 1M, for less attenuation though as this is countered by the gain in the later stages, it is not really an issue.

Euro and Frac rack builders can cut off the mixer section of the v1.2 board if required.

The value of the pot used in the processor will affect the response of the pot. The pot itself MUST be linear. Other values of pot will vary the degree of the curve. For example, a 50k will be slightly anti-log.
 * 10k – Anti-log response – Greater sensitivity near center of rotation. Finer control towards ends of travel.
 * 100k – Linear response.
 * 1M – Log response – Finer control near center of rotation. Greater sensitivity towards ends of travel.

Parts list
This is a guide only. Parts needed will vary with individual constructor's needs.

CC-BY-NC
Readers are permitted to construct these circuits for their own personal use only. Ken Stone retains all rights to his work.

Suppliers

 * CGS26 Analog Logic Voltage Processor, revision 1.21, Elby Designs