Analog Shift Register
for music synthesizers.


This module is an analog control voltage shift register, three stages in length. It duplicates the functionality of the Serge ASR, but goes about it in a very different way. It is great for producing "arabesque-like" sequences, trills etc., when used with VCOs. Of course it's use is not limited to VCOs only. Unlike ASRs produced using "bucket brigade" techniques, the CV outputs are constant from the beginning of the cycle to the end - there is no "charge period" in which a brief glide is heard. The CV input/output range of the ASR is approximately +/- 10V.

How to use this module:

Connect the CV input to a voltage source such as a keyboard or sequencer. Connect the three CV outputs to the three modules to be controlled. Connect the clock input to a LFO or other low frequency trigger source. To cascade the unit with others, connect the clock inputs to the same trigger source, and connect the third CV output of the first ASR to the CV input of the second ASR. Unlike the Serge ASR, there is no need to used delayed trigger pulses.

A little on how it works:

Click here for the Schematic

The ASR is nothing more than a 4 stage analog memory. Each cell of this memory is a capacitor, followed by a buffer. Addressing each stage of this in turn is half of a DG409 4-pole analog switch. As each cell is addressed, it is forced to follow the incoming voltage. The cell remains connected to the incoming voltage for a full clock cycle, giving it ample time to reach the correct level. At the same time as this is happening, there are three other 4-pole analog switches that are pointing to each of the other three cells. The first of these points to the cell filled the previous clock cycle, the second to the cycle before than, and the third to the one before that. As such, once each cell is filled, it remains that way for 3 clock cycles, then is filled with a new voltage on the fourth. The other three cells are cycled in turn to the 1st, 2nd and 3rd outputs. The TL071 and 4024 form the clock pulse to address converter.

The remaining circuitry is an input buffer and three output buffers designed to adapt the voltage to that suitable for switching by the DG409. The input range of the ASR itself is approximately +/- 10V.

Construction:

The component overlay.
Click here for an enlarged printable version.

Click here for a 3D Model

Constructors should refer to the printed Component Overlay for any specific comments regarding the board assemblies, the Bill of Materials for the current value of all components and General Construction Notes for general pcb assembly guidelines. You are advised to check all of these documents on our website to ensure you have the latest copy.

Calibration:

The CGS34 will operate without calibration however you may observe differences in the output voltages at each of the 3 stages. If this is of concern then you should apply the following calibration process.

You will need a good digital multimeter with at least 4-decimal-point resolution

  1. Apply a steady clock pulse to [CLOCK-IN]. A few Hz will be fine
  2. Apply a stable DC reference to [CV IN]
  3. Monitor U201_6 and adjust its trimpot to match the DC reference. It should be possible to get better than 0.2% accuracy (for a 1.00V reference the error should be less than 2mV)
  4. Repeat for U202, U203 & U204

An alternative method requires a VCO and a suitable clock source such as an LFO

  • Connect a constant voltage to the input.
  • Connect one output to a VCO
  • Connect an LFO or clock running at a few hertz to the Clock input.
  • You should get a constant tone.
  • If there is a stepped warble, one of your ICs has a different offset to the others. You can work out which chip it is by holding your finger on the pins of each chip in turn. Swap the affected chip for another. Repeat if needed until you get a constant tone.
Notes:
  • PCB info: 6" x 2" with 3.2mm mounting holes 0.15" in from the edges.

Parts list: