Mar 4, 2019

Dual VCA

You can never have enough VCAs, a common meme in modular synthesis declares. While this module certainly cannot fill that infinite gap, it does provide two handy voltage controlled amplifiers. They are linear, DC coupled VCAs, suitable for both audio and CV processing.

Front panel and render of the PCB

The control inputs have dedicated attenuators, and each VCA can be manually opened with the Bias control. Finally, the module has a built-in mixer, summing the output of both VCAs and one auxiliary input into the Sum Out jack. This way, for example, multiple units can be daisy-chained to form a multichannel voltage controlled mixer.

The module is built around the AS3360 dual VCA chip, a clone of the CEM3360. This chip is easy to implement, reasonably priced and has decent performance. The rest of the circuit is implemented with 0805 SMD parts.

Circuit description

The CEM/AS3360 is an easy-to-use VCA chip, and requires few external components for basic operation. However, like the other CEM chips, it was originally designed to be integrated in ``normal'' analog (poly)synths. In a modular synth, where the inputs and outputs are exposed directly to the outside world, some external scaling and protection circuitry is needed.

VCA core

Let's first describe the basic configuration of VCA channel 1 (channel 2 is identical). The VCA itself, U1A, is a current-in, current-out amplifier. The input signal voltage is converted to a current by R18, and the output current is converted back to a voltage by R22. Choosing these of equal value gives the VCA a nominal voltage gain of 1. The value of 51k is chosen such that a standard +/- 5 V modular signal has a peak current of 100 uA, which I found to be a good trade-off between signal-to-noise ratio and distortion. The AS3360 can handle at least 300 uA of signal current, so there is plenty of headroom at the input side (+/- 15 V min.), however the output swing is limited to around +/-10 V. The output signal is buffered by U4A, which is a unity-gain buffer. The network R2, R14 and C16 allows the opamp to handle significant capacitive load arising from long patch cables.

The VCA core itself has an exponential control scale (pin 3). The chip however includes a log converter (pins 5 and 4), which accepts a linear control voltage between 0 and 1.7 V (nominal) and generates the appropriate exponential voltage for the VCA. The VCA block further includes power supply connections (with decoupling capacitors drawn at the bottom of the schematic) and a decoupling capacitor on the internal reference voltage generated by the VCA on pin 8. This reference voltage is not used in the external circuitry, but is used internally by the log and exponential converters.

CV signal conditioning

The module is designed to accept a useful CV range from 0 V to 4.5 V (with fully opened CV attenuator), which must be scaled to the 0 V to 1.7 V range expected by the AS3360. The incoming CV must therefore by attenuated with a gain of 1.7/4.5 = 0.38. The module must also safely accept any voltage in the +/- 12 V eurorack power supply voltage, whereas the AS3360 absolute maximum rating for CV is -2 V to +2.5 V.

The required scaling and clipping is performed by two inverting opamp stages, U2A and U2B. U2A first sums the external CV with the bias voltage set by RV3, and amplifies it by a factor -1.96. The output of U2A is clipped to about +/- 10 V, which corresponds to a CV input range of +/- 5 V. U2B then attenuates the CV by a factor -5.1, giving the required overall gain of -1.96/-5.1 = 0.38. Thanks to the clipping action of U2A, overvoltage on the CV input is limited +/- 2 V at the output of U2B. R15 and D2 further clip the negative part of this range, keeping the voltage well within the AS3360 absolute maximum spec.

For good measure, the two CV processing stages both include a roll-off capacitor limiting high-frequency CV signals. The CV bandwidth (3 dB attenuation) after both stages is 22 kHz, fast enough for audio-rate modulation.

R20 gives the control voltage a slight negative bias of about 25 mV. In testing, I found this helps to ensure the AS3360 turns off completely.

Mixer section

U5A and U5B form a standard mixer with unity gain. U5A sums and inverts the two VCA output signals and an external (Aux In) signal together, and U5B inverts them again to restore normal polarity. Using C10 and C11 the mixer is band-limited to 22 kHz, and U2B has a series output resistor R31 in the feedback loop to isolate the opamp from capacitive load.

Build documentation

The PCB for this module is available for sale. The build documentation below contains everything you need to know to complete a successful module. Please read it through carefully before starting!

No comments:

Post a Comment