My First Modular Synth!

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I’d better qualify that. Today I received some Eurorack hardware and one module, not a whole synth. I believe it is the first Eurorack module I’ve ever seen, even though (as danbri delightfully put it) I’ve been around electronic music synthesizers since the time of the Queen’s Silver Jubilee (God Save…).

I’ve assembled and tested things, I’ll go more into details soon, but want to get some first impressions down. Capture a gleeful moment.

This is what I got :

It’s a Tiptop Audio Happy Ending Kit Black EU (€129) and a Behringer 150 Ring Mod/Noise/S&H/LFO (€88), both from Germany-based company Thomann who I’ve found very good over the years.

The rack hardware/PSU feels outrageously overpriced (and I had to replace one of the screws with a longer one for it to hold), the module very inexpensive.

Recent History

Why did I get these specific things?

I’ve been playing with an ESP32 microcontroller for various things, including sound generation. I should put my things in nice boxes, perhaps even try to make a bit of money from the hobby. Eurorack is the de facto standard for synth modules, but new to me.

A few months ago I ordered a Behringer Wasp synth as a present for a friend. It took a few weeks for it to occur to me – I would like one of those myself! (His has arrived, mine due next week). That is a complete synth, and can be de-cased to fit Euroracks.

The Wasp isn’t exactly modular, it’s set up as a fixed very trad subtractive synth chain (keyboard in, LFO, VCOs/Noise, Envelopes, VCF, VCA). It only has output from it’s two VCOs, a mix input before the rest.

But despite Eurorack dimensions, it wouldn’t satisfy my need to get hands-on with something like a more typical module, see how the rack physically fits together etc.

So when looking for a first Eurorack module I wanted something fairly minimal and something I could use around the left-hand side of a chain and which could be fun with the Wasp. As cheap as possible. When looking through the thousands available, this one jumped out :

Ok, I shouldn’t assume everyone knows these abbreviations :

Sample and Hold (S & H)

Not to be confused with other kinds of sampling.

These things appeared to interface a piano-style keyboard with analog synths. The most common way of doing this was to have a switch on each key attached to a long line of resistors, acting as a voltage divider. Press Middle C you get (say) 5v out, press C an octave higher you might get 6v. That voltage gets fed to a Voltage Controlled Oscillator (1v/octave is the most common standard). But you also want to do something about when you let go of the key and the voltage could be Ov or floating… So you also want a trigger message from the keyboard, note on/note off. Add another switch per key (or a bit of circuitry to snag the change). But you may want a long decay on a note, after the key is released. So you want to grab (sample) the voltage and hold it. Is monophonic, one note at a time.

These things are also useful for generating arbitrary sequences – in the 150 module it can be switched to the noise or LFO source, so it’s grabbing/holding a random or ascending/descending value.

I do have a traditional use for these. A few years ago I had a MIDI keyboard die on me. At the time Australian singer/songwriter(/model!) Julian [note to self : remember his surname and link to youtube stuff] staying here so I videoed him smashing it up (should be on a backup drive somewhere). I kept the keyboard, has been on shelf in the workshop since, should be able to repurpose a couple of octaves.

Low Frequency Oscillator (LFO)

As named. Typically …

Wait! this is meant to be a First Impressions post…

Ring Modulator

Epitome of evil synth. Daleks!!!

A very long story in themselves, but in a nutshell, they are ‘four quadrant multipliers’. Think of a graph in the chart sense, x & y.

Voltage controlled amplifiers (VCAs) just turn the level up and down, 0-11, a positive number times whatever. Ring mods do a + times +, + times -, – times +, – times -.

The maths does get a bit funky, but the effect of mixing two signals this way is that you get the sum & difference of their frequencies. Interference, like tuning a guitar : if one note is 440Hz and another 445Hz, you can hear a 5Hz beating. I’ve not read any papers on this, but my guess is that the human auditory system (at a cognitive level) tends to block this effect.

What’s notable is that more often than not, the sum & difference of frequencies aren’t harmonically related to the inputs. You more typically get a piercing discordance (Exterminate!). (Note that if you put the same in both inputs : sum = 2x freq, an octave higher).

The natural world isn’t very linear, including the eardrum, this kind of artifact must be ubiquitous, but considered in evolutionary terms doesn’t offer much new information (unless you’re tuning a guitar or doing radio decoding). Bang a bit of metal and you may well hear such interactions.

The effect in practice depends a lot on the design of the ring mod, seriously, you could write a book on these things alone.

The Wasp synth has the outputs from the two VCOs, an external in.

The Future

TBD.

Funny coincidence, the size of my current prototype ESP32 circuit & the 150 module :

I only anticipate wanting about 1/2 the sockets & controls that are on the module, so I guess I have to re-learn PCB design.

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