Together with the Power Supply Unit, Voltage Controlled Resonant Filter and ADSR, another modular's primary building block is the Voltage Controlled Amplifier (VCA)!

A VCA is a fundamental modular synth's gusset because, in it's most common use, it opens and shuts the otherwise continuous wave coming from the (filtered or not) oscillator, accordingly to a variable control voltage (generally generated by the ADSR or directly by a gate signal). It's function is not a simple, final stage amplification then, but a very important sound sculpturing one.

In this article I will show you how to realize a simple but effective VCA for your modular system. I will also share with you a PCB layout to speed up the assembly.

Circuit Design

The circuit I adopted is a discrete VCA using a classic differential amplifier. It's mainly based on the work of Kassutronics, which in turn took inspiration by Yusynth (who didn't?).

With respect to Kassu's work this design adds an additional control voltage input (you never have enought!).

What to some extent differentiate this design is the adoption of a NPN transistor pair in a single SMD package. This has the advantage of an intrinsic good matching of the transistor pair and a less tendency to drift over time thanks to a far better junction temperature pairing (they are in the same package, after all).

By using good quality metal thin film resistors in the differential pair part of the circuit (the two 10K resistors referencing the transistors to V+) should be enought to delete the otherwise necessary transistor pair testing and selection phase (it was for me!).

No improvement comes without a price, unfortunately: the chip (BC847BS) is in SOT363 package, which means it is almost invisible (0.65 mm pitch!). See next step for some soldering tips.

The circuit works on +/-12V dual voltage, but should also work on +/-15V.

The circuit is inverse polarity diode protected between the +12V and -12V lines.

There are two potentiometers to control CV amount and gain respectively. A 100K trimmer is used to tune the gain initial level. This should be adjusted to make the output silent when the gain is at zero, but I find it more practical/useful to rise the gain a little bit to compensate for circuits volume losses and then set the trimmer to just mute the output. If your oscillator output is too HOT and you are using this VCA as final stage before a mixer's line input, you can easily lowen it to the expected 100mV to 1V max.

Please, notice that I have tested this VCA with a digital oscillator outputting a 1Vpp signal, but the circuit is well documented and adopted all over the net, so chances of a fail with higher voltage signals should be close to zero.

Soldering tips

To solder the microbe (A.K.A. BC847BS) you have to adopt some strategy. My suggestions are:

- use a thin iron to solder the chip and a scanning electron microscope (just kidding! A magnifying glass will suffice)

- use some soldering flux. I use a cheap "no-clean" Rosing Mildly Activated (RMA) pen with excellent results

- soldering method: melt a little amount of solder on the board pads, place the transistor pair over it and then remelt the pad-solder with a clean iron tip while applying a veeeeery small pressure on top of the IC. This is the method that works best for me.

Having it well soldered could ask for some trial and error at first, but believe me: if I can do that, you can. As a precaution and giving the limited cost of such IC (at today 10pcs costs less than 1 euro), buy a little bunch of them.

I placed some vias close to each feet of the IC: use them for continuity checks with chip's feets.

Notice that the chip can be soldered in both directions, therefore it is useless to lose diopters in search of the right one :)

PCB and Components List

The board is intended to be mounted perpendicular to the front panel. If your case is not deep enought, you can lock the board on the case bottom and use wires to gain electrical connection with elements (potentiometers, buttons, jacks and so on) on the front panel.

The circuit is very simple and asks for a limited amount of components. All components values are silkscreened on the PCB to make assembly easier.

Here follows the list

Capacitors

1x non-polarized capacitor 100nF

2x electrolitic capacitor 10u

IC's, diodes, transistors

1x diode 1N4004

1x op-amp TL072P

1x BC547 NPN transistor

1x BC847BS NPN general purpose double transistor

Potentiometers, trimmers and resistors

2x 50K 16mm Potentiometer

4x 100K ohm resistors

5x 10K ohm resistors

1x 47K ohm resistor

3x 220K ohm resistor

2x 1K ohm resistor

3x 22K ohm resistor

1x 100K ohm trimmer

Others

1x 10 pin IDC connector, male

https://kassu2000.blogspot.com/2015/10/vca.html