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Building a Modular Synthesiser Part 3: Modulation

Building a modulation module with sample and hold and a white noise generator.

This is the third part of my series of posts on building a Modular Synthesiser. I have already done the groundwork, sorting out a case and making up some power cables. I have also built my first module using the Beaglebone Black and Bela to provide a versatile, programmable module that I can adapt to be either a sound source or a modifier.

Control Voltage

One of the interesting things about analogue synthesisers (and modular systems in particular) is that the sound sources and modifiers can be altered not just by hand, but by voltage control. It gives real flexibility to a modular synth and provides the potential to make unique and interesting sounds.

Control Voltage (CV) was put into practice in the 1960s by Bob Moog, who developed a system where 1v equaled one musical octave. This has been widely adopted for control interfacing by many electronic musical instrument makers and was incorporated into the Eurorack standard by Doepfer. As one volt represents one octave, so the pitch produced by a voltage of 2v is one octave higher than that produced by a voltage of 1v. Although CV as a means of controlling electronic musical instruments has been somewhat superseded by MIDI in recent years, it still has a vital role to play in the world of electronic music-making.


One of the prime uses for CV in a modular synthesizer is to control modulation. Modulation modifies your source signal, varying its pitch or volume, for example. This could be done manually by knob-twiddling or key pressing, but using a CV generated by a Low-Frequency Oscillator (LFO) to take care of it opens up all sorts of interesting possibilities. For example, a regular variation in amplitude controlled by the voltage from an LFO produces a tremolo effect, or if it is applied to pitch, a vibrato effect.

A modulator is a vital component of any modular synthesiser so I needed to build one for mine.

Building a Modulator

I had chosen the Erica Synth Modulator II DIY. It features a single LFO with simultaneous triangle and square wave outputs, alongside a very stable and clockable Sample & Hold (S&H) with external CV input and noise source.

The kit came with 3 PCBs — the main board and two versions of the noise generator: a simple, transistor-based one and a more complex Zener diode-based one that can produce full-spectrum white noise. The kit included a very nice front panel and what Erica describe as the “rare, high quality” S&H chip.

I put together a BOM of all the other parts which is attached to this article.

Starting with soldering the smallest components, diodes, and resistors, it soon started to take shape.

Once the capacitors, DIP socket, and transistors were added, I soldered the S&H IC chip in place. Then added the potentiometer and jack sockets, inserting them in the PCB and then placing the front panel in place over them and fixing it in place with the jack socket nuts, to ensure everything lined up as it should before carefully turning the whole thing over and soldering the components in place.

I could then remove the front panel for the next stage which was to solder the resettable fuses, 10-pin power connector, electrolytic capacitors and connectors for the noise generator on the other side of the PCB.

The final thing to do on this board was to add the LED. It needs to be just the right distance from the board to peep through the front panel. I inserted the LED taking care to get the polarity right then, before soldering I fitted the front panel, fixing it with the jack and potentiometer nuts. I could then adjust the position of the LED so it was just right before soldering it.

The Noise Generators

Now to build the noise generators. As I mentioned above, the kit comes with a choice of two noise generators. I started by making the comparatively simple transistor-based one.

Then I was ready to test it. The Pulse and Trigger outputs worked without any trouble, but initially I was not getting any signal from the S&H. After a couple of helpful emails from Erica Synths' support, I tracked down the problem to the noise generator. They very helpfully sent me details of the test points on the PCB and I could use these to look at their outputs using my oscilloscope. I could hear (and see using the oscilloscope) the noise output but it was not anywhere near the 10v range it needed to be. I replaced a 2.2M resistor on the noise circuit with a 2M one and re-soldered one of the capacitors, and that fixed the problem.

I then built the more elaborate diode-based noise generator and when it was completed, I could definitely hear the difference. The latter version producing a denser white noise.

Using the Duowave project on Bela Pepper I could now control the pitch of the two oscillators from the Trigger and Pulse outputs of the Modulator.

I could also use the modulator to control the playback speed and the pitch variation of Pepper’s Granulator Project:

Conclusion and what next

My synthesiser is now starting to take shape and, although not exactly musical, I am getting some interesting sounds from it. My next step is to build a filter which will be my first experience of soldering surface mount components – so that should be fun! I am then going on to build my most ambitious module, which incorporates a Voltage Controlled Oscillator (VCO), Voltage Controlled Filter (VCF) and an Envelope Generator – this will be the heart of my synth.


I currently look after production at AB Open. I have a background in the arts, environmental conservation and IT support. In my spare time I do a bit of DJing and I like making things.

4 Dec 2019, 10:54