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.

FOR HISTORICAL REFERENCE ONLY

Features

  • Log and Linear Response to Control Voltage
  • DC Coupling Throughout
  • Excellent for Ring Modulation Effects
  • Power Supply Range +/-9V up to +/-15V

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.

For Reference. New PC Layout Has R2 and R24 Holes Now
NOTICE THIS PLEASE! On pre May 2006 boards you must either tack solder R2 and R24 to these undrilled pads or drill 4 holes and mount R2 and R24 normally but solder it on the top side because some un-named person forgot to put bottom pads there. The un-named person has been sent to Alaska again for a little more sensitivity training. I recommend tack soldering the resistors. Why mess with the drill.
Click for larger view (100K jpg)

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.

If you use an alternate chip or you notice that the output signal is biased high or low you can adjust the ratio of R21 and R22 or add the
"Bias adjust kludge" 100K trimmer pot and 100K resistor I show in red. Yes you would add it to the other VCA circuit if you needed to adjust it's bias. How you kludge it it in is up to you. I did not need to add this adjustment but I had to select a second LM13700 from my parts drawer because the first one was either defective or the amps on the chips are not as well matched as you would think. Under normal (i.e. reasonably matched transconductance amps) circumstances you should not need it.

Dual Log/Linear VCA Page 2 PDF

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

 

Triangle Wave Modulation in Linear Mode

 

Triangle Wave Modulation in Log Mode

 

The Prototype Being Tested

Larger Photo

Bandwidth to spare? Huge Photo (ignore the unset date)

Dual Log/Linear VCA PCB Parts Layout (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 Top Silk Screen

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.
Click for Front Panel Express File
Click image to download .FPD File (Front Panel Express) File		 Jason Proctor did a nice Front Panel Express layout. Click for a larger image
Visit Front Panel Express
Click the image to download the .FPD File.

Dual Log/Linear VCA Project Parts List

Qty. Description Value Designators
2   2N3904(s)   2N3904   Q1, Q3  
2   2N3906(s)   2N3906   Q2, Q4  
1   TL084 Quad Op Amp   TL084   U1-C, U1-D, U1-B, U1-A  
1   LM13700 Dual gm OpAmp   LM13700   U2-A, U2-B  
2   Linear Pot(s) RS 271-092   100K   R10, R32  
14   Resistor 1/4 Watt 5%(s)   100K   R1, R11, R2, R5, R4, R12, R19, R23, R33, R27, R26, R40, R36, R24  
2   Resistor 1/4 Watt 5%(s)   120K   R14, R34  
2   Resistor 1/4 Watt 5%(s)   150 ohm   R20, R42  
2   Resistor 1/4 Watt 5%(s)   1K   R17, R39  
4   Resistor 1/4 Watt 5%(s)   20K   R15, R8, R35, R30  
6   Resistor 1/4 Watt 5%(s)   2K   R13, R3, R7, R37, R25, R29  
2   Resistor 1/4 Watt 5%(s)   30K   R9, R31  
2   Resistor 1/4 Watt 5%(s)   4.7K   R16, R38  
2   Resistor 1/4 Watt 5%(s)   475 ohm   R6, R28  
2   Resistor 1/4 Watt 5%(s)   49.9K   R22, R44  
2   Resistor 1/4 Watt 5%(s)   499K   R21, R43  
2   Linear V-mount Trim Pot(s)   2K   R18, R41  
4   Ceramic Capacitor(s)   .1uF   C2, C3, C5, C6  
2   Electrolytic Capacitor(s)   10uF   C1, C4  
2   SPDT Switch(s)   SPDT   S1, S2  
8   1/4" Phone Jack(s)   RS 274-252   J2, J3, J1, J4, J6, J7, J5, J8  

Miscellaneous