Quad VCA, AC or DC coupled with normalized input. www.haraldswerk.de

Motivation

This is one more module for my quad system. It contains four VCA with normalized input, so you can distribute one input signal to four output channels. With each channel under voltage control you can move the input signal through the room dependent on the control voltages. I have two versions build one AC and the other DC coupled. Beside using this VCA's for signal distribution you can use every VCA independently. The gain to voltage ratio is linear. This is done because it is easier to predict the performance. The distribution of the signal must be steered through the control voltages. The implementation follows the recommendations from an article at Open Music Labs from October 3, 2015 (Minimizing distortion in OTA's)

Specs and features

Four VCA with normalized input
Usable as four independent VCA as well
Linear gain to voltage response
Individual Volume, CV and Bias controls
Low distortion implementation
Runs on +/-12V and +/-15V (with minor resistor value changes for best performance)
Power consumption around 35mA each rail

Implementation

Schematic

Description:

The linear controlled current source for Iabc is straight forward and implemented as usual. An elaborate description for the VCA part is given here

Quad VCA DC schematic control board — Quad VCA schematic: Control board DC

Quad VCA AC schematic control board — Quad VCA schematic: Control board AC

Description:

On the left side are the potentiometers for volume control followed by the input buffers. The only difference between AC and DC coupling are the capacitors. On the right side are the CV input potentiometers and the bias potentiometers followed by the voltage adder.

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Calibration

Zero adjust
- Set all potentiometers to zero.
- Measure the output at MP_1 (MP_2, MP_3, MP_4)
- Adjust trimmer TR_1 (TR_4, TR_7, TR_10) to zero Volt
Second Harmonic
- Apply a sine wave of about 1kHz/10Vpp to the input
- Turn the volume and the bias all up
- Measure the THD and adjust TR_3 (TR_6, TR_9, TR_12) for lowest THD
- If you can't measure the second harmonic you can adjust the THD by eye and ear. Apply a 1kHz/10Vpp sine wave. Listen and have a look at your scope while adjusting the trimmer. The input and output sine should look the same on your scope and you should not here any distortion.
Output DC offset (Only when DC coupled)
- Set volume and CV potentiometer to zero.
- Measure the output voltage while moving the bias potentiometer
- Adjust trimmer TR_2 (TR_5, TR_8, TR_11) so that the DC output swing is symmetrically to zero

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Building hints

The LM13700 differ in quality from part to part. You might want to select the IC for lowest DC output swing when DC coupling is used.
Match capacitor C12 with C13 (C16 with C17, C20 with C21, C30 with C31)