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.
Page 2 is the same as page one but with different part designators.
I used 475 ohm parallel with 220 ohm to get the 150 ohm resistor value I needed but did not have in my drawers.
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.
|1||LM13700 Dual gm OpAmp||LM13700||U2|
|1||TL084 Quad Op Amp||TL084||U1|
|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|