Dual Log/Linear VCA (Works from +/-9V to +/-15V)

Ray Wilson authored this content while he was actively running MFOS as the founder and resident genius.
We retain the content because it reflects a valuable point of view representing that time and place.

Article by Ray Wilson
This is an intermediate to advanced project and I do not recommend it as a first project if you are just getting started in synths or electronics. Only the circuit and some explanation are shown here. A lot of project building, troubleshooting and electronics experience is assumed. Additionally, electronic equipment ownership (scope, meters, etc.) is taken for granted. If you are interested in building this project please read the entire page before ordering PC boards to ensure that the information provided is thorough enough for you to complete the project successfully.

Minor Bias Network Change (Mar 2010)

Previous Version for Reference
Original Version for Reference

Features

Introduction

VCAs are essential to your synthesizer. They provide ring modulation effects, stereo panning, delayed vibrato, not to mention the normal envelope shaping of your sounds. This VCA works nicely with 10V PP (+/-5V) signal levels. If you are using +/-15 volt supplies you will probably need to mess with some values to get this to work for you.

Dual Log/Linear VCA Page 1 PDF

This VCA uses the LM13700 transconductance amplifier as the gain control cell. The input voltage is converted to a log response by U1-C and associated transistors Q1 and Q2.

The summed control voltage appearing at the output of U1-C is sent to U1-D for reamplification and application to the LM13700 via R9 and S1. S1 selects the current generated by either the log version of the input voltage (LOGA on the schematic) or the linear version (LINA on the schematic) and applies it to the "amp bias in" pin 1 of the LM13700 thus controlling the current through the device and subsequent amplitude of the resulting voltage generated across R16.

The LM13700's built in buffers are used to drive the output. They work quite well.

The Offset adjust is used to null the gain for whatever control voltage you are using to drive the VCA. You must readjust the null when you switch from log to linear (or visa versa). I have noticed that for control signals that are operating about ground (i.e. + and - 4 to 5 volts) the pot is usually toward the left when set to linear response and usually toward the right when set to exponential response.

The TRIMA control is used to set the voltage divider which feeds the non-inverting input of the LM13700 (pin 3). Adjust it for 50mV peak to peak signal at the maximum input voltage. It permits adjustment so that voltage from about + and - 3VPP to + and - 5VPP can be accommodated.

You can use just about any general purpose quad BIFET opamp for the TL084 and you can sub any of these (LM13600, NE5517, AU5517, NTE870) for the LM13700. Just in case you are wondering, I have tried to use the bias inputs to forward bias the distortion reducing diodes in the chip but always find that it completely bugs up the biasing throughout the circuit.

The new bias adjust injection point applies an offset voltage via 2M resistor R46 (R48) to the base of the darlington output transistor. With no input signal use the bias adjust trimmers to zero the output voltage.

Dual Log/Linear VCA Page 2 PDF

Page 2 is the same as page one but with different part designators.

Adjusting Trimmers

I will describe the trimmer adjustment using the designators from Page 1 (R18 and R45).
Adjusting the trimmers on page 2 is the same (but the pots are R41 and R47).

Triangle Wave Modulation in Linear Mode

Triangle Wave Modulation in Log Mode

Dual Log/Linear VCA PCB Parts Layout (Parts Side Shown) PDF

Dual Log/Linear VCA PCB Part Values View (Parts Side Shown) PDF

Dual Log/Linear VCA PCB Bottom Copper (Parts Side Shown)

Dual Log/Linear VCA PCB Top Copper(Parts Side Shown)

Dual Log/Linear VCA PCB Populated

I used 475 ohm parallel with 220 ohm to get the 150 ohm resistor value I needed but did not have in my drawers.

Dual Log/Linear VCA Front Panel and Wiring PDF

Jason Proctor took the time to make a nice Front Panel Express design and share it with all of us.
If you use it make sure you take into account any differences related to the wiring diagram.

Visit Front Panel Express

Dual Log/Linear VCA Project Parts List

Qty. Description Value Designators
1   LM13700 Dual gm OpAmp   LM13700   U2  
1   TL084 Quad Op Amp   TL084   U1  
2   2N3904   2N3904   Q1, Q3  
2   2N3906   2N3906   Q2, Q4  
14   Resistor 1/4 Watt 5%   100K   R1, R2, R4, R5, R11, R12, R19, R23, R24, R26, R27, R33, R36, R40  
2   Resistor 1/4 Watt 5%   120K   R14, R34  
2   Resistor 1/4 Watt 5%   150 ohm   R20, R42  
2   Resistor 1/4 Watt 5%   1K   R17, R39  
4   Resistor 1/4 Watt 5%   20K   R8, R15, R30, R35  
6   Resistor 1/4 Watt 5%   2K   R3, R7, R13, R25, R29, R37  
2   Resistor 1/4 Watt 5%   2M   R46, R48  
2   Resistor 1/4 Watt 5%   30K   R9, R31  
2   Resistor 1/4 Watt 5%   4.7K   R16, R38  
2   Resistor 1/4 Watt 5%   470 ohm   R6, R28  
2   Resistor 1/4 Watt 5%   470K   R21, R43  
2   Resistor 1/4 Watt 5%   47K   R22, R44  
2   Trim Pot   100K   R45, R47  
2   Trim Pot   2K   R18, R41  
4   Ceramic Capacitor (S)   .1uF   C2, C3, C5, C6  
2   Electrolytic Capacitor (S)   10uF   C1, C4  

Miscellaneous