Making a Synth velocity-sensitive

So I wanted to make a synth velocity-sensitive. More specifically, an analogue synth, and retain a fully analogue audio signal chain (because I have a project in mind for which that is important).

In my setup there’s two analogue synths which I had in mind for that project: the Arturia MiniBrute, and the Korg Volca Kick. I decided to start with the MiniBrute, but get a solution that allowed to work with the Volca Kick as well.

There’s several solutions possible for this, but I already identified the one using an Eurorack setup as the best one (scroll below for other potential solutions).

The Gear

Already selecting the components could prove tricky, simply caused by my complete lack of modular/Eurorack experience. There’s the list I put together.

So I needed an Eurorack case (a small one), a MIDI interface, and two VCAs. I got the Doepfer A-100MC RAW mini-case, the Doepfer A-190-2 MIDI interface, and the Dreadbox Dual VCA (the latter mainly for its low price tag).

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The Doepfer A-100MC is a 32TE single-height case, adding a wallwart PSU and a backplane with four power connectors. So you can add the MIDI interface (6TE), the dual VCA (8TE), and still have 18TE to spare for two further modules.

Doepfer’s A-190-2 interface has a MIDI input, one gate output and a total of 4 CV outputs. The CV outputs are note (pitch), pitch (from pitchbend), velocity (also from a note) and CC (from a selectable MIDI CC). There’s also a glide setting, with which I didn’t deal.

20170916_131709_RoßtalerwegFinally, the  Dreadbox Dual VCA is just that: two VCAs, each one with two ins, one CV in and one out, plus a Level knob.

Modular synths use 1/8” mono cables, so I got a set of those, plus a pair of 1/4”-to-1/8” cables.

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Assembly

Fortunately, the Dreadbox Dual VCA did come with the two screws required to mount the thing in the case – which the MIDI interface didn’t. Fortunately, some of my old computer screws (from a time when we didn’t have screwless computer housings) did the trick. Each module is then connected to the backplane, the PSU is connected – and no idea if it works yet.

20170916_133543_Roßtalerweg.jpg Configuration

The MIDI interface is configured by first pressing a button, then sending MIDI messages. What was important for my application was to set it to send note velocity on the V/V jack – and I was set.

Connecting everything…

Now, it was time first to connect the MIDI interface’s V/V to the VCA’s CV input. Then, connect the Minibrute main out to the VCA’s in, and go from the VCA’s out to the PA. And finally, connect the Minibrute’s MIDI out to the MIDI interface’s MIDI in.

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The Result

It works! Finally, the Minibrute is velocity-sensitive! Now this may not be important to you, depending on your socialization: if you’ve grown up with organs and 70s synths, velocity might very well be a strange concept for you. For me, however, who has grown up with an acoustic piano and entered the synth world in the second half of the Eighties, this is important.

There’s a small issue: sometimes when playing fast lines that change from loud to fast notes, there’s an audible click. Now this only applies for specific filter envelope settings – still, it wasn’t there before. I would assume that this has to do with the delay caused by the Minibrute-to-MIDI-out-to-MIDI-interface-to-VCA signal flow – and as such, this won’t be a problem once I control both the Minibrute and the VCA from an external keyboard or sequencer.

Summarizing

My goal was achieved. This, however, at a cost of about 300 bucks, which you may consider rather high in comparison to the cost of a Minibrute (or even the combo of a Minibrute and a Volca Kick – after all, the gear I have is sufficient to apply that trick to both).

The Future (Pandora)

The next step will of course be to do the exact same thing with the Korg Volca Kick. For that, I need to use a kind of MIDI processor to convert MIDI velocity info to MIDI CC that is then sent on another channel. Fortunately, my DAW of choice (Cubase) allows for that with its Logical Editor.

Apart from that, I’m well aware that I may have opened Pandora’s Box by getting into that modular thing. Right off the bat, I could use an ADSR envelope generator, then set the Volca Kick for minimum attack/maximum release/sustain, and that way give it a proper ADSR envelope. I could also add a VCO, connect that to the Minibrute’s audio in and control it with its CV out and turn the Minibrute into a 2-VCO-synth. Maybe even adding an ADSR to do two-operator FM with the Minibrute. And then get a bigger Eurorack case, and then…


Annex: Other Solutions for the Problem

When thinking about adding velocity to a synth, there’s several solutions that are possible.

Mod the Synth

Very well possible, but not for me. I decided not to follow this.

Something in a DAW

There’s DAW plugins called something like “MIDI Gate” which allow you to control the volume of an audio channel by MIDI velocity signals. Or you could use a simple gain plugin and (if your DAW allows for that) control that with MIDI CCs which you converted from MIDI velocity.

The most cost-effective solution – but as I am looking for an all-analogue signal chain, not the solution for me.

MIDI-controllable Effects Processor

Use a MIDI-controllable effects processor, set it up to do a “do nothing effect” (e.g. 100% dry), and control the output volume via MIDI. You may need a MIDI processor (typically a computer) to do that.

You may have an effects processor that can do this sitting unused in the basement. All of those I have are, however, digital, so again no game.

Dynamics Processor with Sidechain

Use a compressor or expander with sidechain capability. Send your synth through it, and control the sidechain with a simple synth (e.g. a basic VSTi in your DAW). Then set it up so the sidechain audio changes the gain to simulate velocity.

This would actually be both cost-effective (something like an old Alesis 3060 can be had for below 50 bucks) and does sport an analogue signal chain. Apart from that solution being rather tricky to set up, it would reach limits as release times can usually not be set very short (like, 0.1ms or so). And that’s what you would need for this.

Build our own “Thing”

Get some embedded computer with an analogue out. Build an amplifier that’s controlled by said analogue out. Then install software that receives MIDI and does, essentially, the same as the solution described here. Cool idea, but very complex for someone who is not into building electronic things. Still, I might one day build an Eurorack module that can receive and send a lot of CV signals and process them digitally in some way…

Another (Analogue) Synth

Take a velocity-sensitive analogue synth with audio input. Set it to use audio input only, bypass/open the filter, and set the amplitude envelope to minimum attack, maximum sustain and maximum release. Route your target synth through this.

Would work – but I have other plans for using my Pulse 2. Still, in today’s analogue synth market, there might actually be something that’s cheaper than my solution!

So that’s the summary of the alternative solutions I have thought about. If you found another solution – please let me know!

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