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Dual VCA



It is a hi-quality audio VCA module designed with THAT2150A circuits.

Audio Inputs work with synths levels as well as line levels. Level "Pad" switches are provided to select input level/impedance for best performances.The audio path is 100% dc coupled, with an unusual but perfectly sounding coupling capacitors stage. on audio outputs.

The expo converter is integrated to the 2150 circuit. Just the scaled CV paths and (optional) tempcos were added. The design has been optimised for very fast and "clic-free" control of VCAs.

CV inputs range is 0 to +5V. At 0V on CVIN the VCA is closed ( approx -100dB signal attenuation). With CVIN = +5V VCA gain is 1:1. and at CVIN =+7V the VCA gain is close to +20dB. Only the VCA1 have control potentiometers : INIT GAIN and CV IN attenuator. CV inputs of both VCA are linked together, by the jacks preset legs. It is an useful wiring tip to drive a stereo audio source with one CV and one wire...

Audio performances are excellent and much better than any common synth VCA designs (ota,transistors..). The distortion amount is low and trimmable (optional). The noise floor is somewhere around -100db.

The green double side pcb is homemade. It have a hi degree of symetry . This is mostly for "stereo" applications of this module ... The top layer has been varnished before parts soldering, the solder layer has been painted after final tests of the circuit.


Schematic & panel files...

Please note : this schematic is in generic form ... this mean "the pcb contain all possibility" but many parts are not obviously required depending of what you plan to do with this VCA.... Some values may be adapted too and external wiring may occur, like with "pad " switches : they are not represented on the schematic. All audio amps gain resistors can be changed to fit any level needs. The basic design is for NE5534 but if you want to use other good quality aops you may remove or add parts accordingly. My pictured prototype use OPA604 audio aops. At the audio output stages are "chemical emulation of a fat unpolarized capacitor". This stage give a unique color (available only in big studios with very long mixing desks ;-) The pcb design allow other kind of coupling capacitors stage to be implemented too.

The 2150 is driven with symetrical CV to avoid "clic" or" tumbs" noise that may occur during extreme variations ( a pulse square forexample). CV is calibrated by a voltage divider with 1K+3300 tempco resistor ( fully optional ). Else it is a quite common CV driver stage. The zener diode is here to limit CV input range to reasonable values. When the VCA is well tuned the full CV at the 2150 chip is around +/-310mV. Other circuitry around the 2150 chip are PSU and symetry trim. This small pot is to improve distortion of the VCA core. The natural distortion of the 2150 is very low compared to many other synth VCA design. So this pot is for people who want accuracy and have a spectrum analyser or a distortion meter at home ... There are more complex circuits possible for this, but i believe they are not very important for this kind of "musical "application. The sound quality is already excellent! Trimless versions of the 2150 also exist (2180) Then it will be easy to forget this trimpot ... For CV path aops , you can use almost anything ( TL072/74, LM324, LF412., OP275... ) But it is better to keep a good quality low drift , low noise noise one for the last driver stage before the 2150....Well , in this schematic portion, i do not advise to modify any resistor values if you do not have any idea of what you do ...0.1% Precision resistors increase performance and stability specialy here, but again this is optional and for purist only !

The THAT datasheet mention possibility to drive the 2150 with dc voltages with minimal looses on ac signal. It is quite easy to do with this pcb. Just to remove all serial caps in the audio path and few resistors all around but i did not try that yet ...

Dual-VCA schematic PDF

Dual-VCA front panel FPD file for the pictured 1x5U " MOTM like" module.

Sound files

    1. DualVCA drone loop1.mp3 (8k)

    2. DualVCA fast mod (81k) A CD player is connected at the input. Dual VCA outputs are wired to PC soundcard inputs. The CV to control both VCAs is provided by the IPS8 set in "offset" mode to generate a kind of rectangular waveform in the positve area ( 0..+5V) . The IPS is modulated by a VCO waveform ( tri/saw). The rate can vary from LF to BF. All sound cuts bellow are made by the Dual VCA module, it is not a bug of your M$ Mediaplayer !


© Marc Bareille 2004