README.md
VCA


  
4-channel amplifier


Description

Control Voltage 0-5V with 5V -> Gain=1, 0V -> Gain~=0. Can be overdriven up to 6V -> Gain=2 (actually slighly less because it will clip). Output is about 9.2 dB/V, with 0V @ -52 dB. (TODO: is this low enough to be inaudible? If not, might need to adjust design parameters.)

Control voltage inputss are summed with a control knob, which adjusts CV between 0V and 5V. Control voltage inputs cascade from the previous input if not supplied, with the A-CV defaulting to 0V.



The 4 channels are connected to the 4 channels from the filter, unless an input is supplied.

The 4 channels are mixed down along with EXT-IN, an input for arbitrary external signals, such as those from another synth. In MIX mode, the result of the mix is sent to both left and right stereo outputs. in SPLIT mode, channels A and C go to the left channel output only, while B and D go to the right channel output only.

Both 1/4" TLS main and 3.2mm headphone outputs are available. TODO: power calculation

TODO: power usage

Design

The amplifier is designed around the LM13700, with an antilog converter providing current control. Calculations of component values are here.

The LM13700 acts as a linear amplifier when Vd <= 20mV. It's output is a current source given by the equation Iout=0.5*Vd/Vt*Iabc where Vt=26mV @ room temp. Iabc cannot exceed 2mA. Setting Iabc=0.5mA for unity gain puts Iout=.192mA. This can be converted back to a voltage with a resistor. Rout=26kΩsets the overall voltage gain to unity.

Tuning


Set CV-IN to 5V, and RV2 to completely off. Adjust RT1 until base of Q1A is 0V (or Q1 collectors are the same).
Set CV-IN to 0V, and RV2 to completely on. Adjust RT2 until the current through it is 0.5mA.
Set CV-IN to 0V, RV2 to completely on, and IN to 5V. Adjust RT3 until OUT is 5V (an overall gain of 1).