DIY-101 synth voice - part 1

I bought a Doepfer DIY synth nearly two years ago.
It was lying on my shelf ever since.  
This month I made a eurorack module with it.  I'll document it's making in two parts.

Close-up of the finished panel

The Doepfer DIY synth is an assembled printed circuit board that includes all the necessary functions to build a basic analog synthesizer: VCO, VCF, VCA, EG, LFO, you name it.  You can design your own synth.  All you have to do is to provide a case and all the connections to the mandatory controls : jacks and potentiometers. 

Before I could get to a finished product I'm proud of, I went into a lengthy thought process.
My first idea was to make a semi-modular synth whose functions are as close as possible to an Roland SH-101.

I tried several panel designs before deciding on the final one.

XAOC Karl Marx Stadt module provided the necessary faith that I could build a 42 HP (about 21 cm) module to house the Doepfer DIY synth.

Moreover, I was inspired by the Motherfucker-19 work from YokoBoko.  It helped me making some decisions on jack placement and avoiding some mistakes during cabling (not that I didn't do any…).  You can find description and instructions on the module modulagrid page.

As usual, I prototyped the panel on cardboard to check my placement hypothesis.  It seemed critical here.  I wanted to specifically check  the density of jacks on the low part and the room for the spacers to hold the board.

Back side of the early cardboard prototype.

Annotated paper and cardboard proto

I used my traditional way for panels : punching to locate the holes, drilling, then printing the panel, laminating and glueing it.

This time, I chose to drill the holes with a wood block beneath the panel.    It proves to be way more comfortable than my usual technique of maintaining the panel over the edge of the bench.  The panel is firmly clamped and I move the drill freely to get to the next hole.  I know, I should get a drilling press…
Holes are 6 mm for LEDS, jacks and switches and 8 mm for pots.  It is a bit large for the pots.  But it proved helpful later.

Bob cleans-up

This generated a lot of chips and dirt. Fortunately, Bob helped me to clean the mess.

Given the panel is 3U 42HP, or 212 mm x 128,5 mm, it fits nicely on an A4 sheet of paper.
It's a large sheet.  So placing it parallel to the panel edges is essential.  A small deviation could give an awful result.
I'm quite happy with the final result except the paper is one millimeter too much on the right and could have been half a millimeter lower.

Bob cuts holes

I cut the holes with an X-Acto knife.   Here again Bob gave me a hand.

This is the time to place all pots, jacks, switches and LEDs.  There are 51 of them (23 pots, 23 jacks, 3 switches and 2 LEDs).
As the holes for pots were a bit larger than needed I corrected the most obvious alignment problems by moving the pot half a millimeter here, a millimeter there.

Sorted potentiometers

Here is the final panel before cabling.

Final before cabling

Go to part 2.

Do I really look like a guy with a plan ?

Nowadays, as you start a project like a modular synthesizer, you look for resources on line.  You are soon overwhelmed by the possibilities.
It's easy to get drowned into the 'mare modulae'.  In a pinch, you imagine a system that will take dozens of modules and require tons of space and money, with little purpose.

So it's important to remind the fundamentals :

• start small.
• some sort of a plan is necessary.

I intend to go one step at a time, at a steady pace; leapfrogging from stable state to another stable state to have some fun and learn at each phase.

With a modular synth, I believe it's a good idea to start planning with the type of sounds you have in mind. 

I tried to imagine my first patches.  How many modulation sources will I need to achieve a fluid ambient patch ?  How many mixer tracks for a drum machine ? Do I need to input a guitar into my FX setup ?  What could be the minimal viable system that permits me to achieve my goal ?

My plan for a modular synthesizer came after I made some rough sketches about two simple synthesizers I thought I could build as first Synth DIY projects.

At first, I wanted to build a noise machine, with some whizz, some bangs, some crsssshh.  Something different from the sound of my Anushri.  I intended to build it all with the help of the schematic from the Yusynth project.   I wasn't too sure of my ability to tune a VCO though.  So I was happy that this project didn't involve tuned VCO.  The main sound source was  a noise generator, plus a couple of VC-LFO that could eventually extend to audible range.

Early sketch of the noise box

Then, I thought I could build after that a basic synth with roughly the same sound architecture as a Roland SH-101.  Nothing revolutionary.  But I would have built it myself, once again thanks to the resources from the Yusynth project.

Is it really simple ?

When I saw that both projects shared the same basic functions, I decided to merge them in a single modular synth.  The Eurorack format seems the most practical and has a great community.  I settled for 2x 84HP rows, partially because Doepfer sells a simple kit to start with.

So here we are.

I'm still not very confident to build and tune a complete VCO, plus I needed a good example of Eurorack module to serve as mechanical reference.  I chose a Doepfer A110-1 Standard VCO as my first and main VCO.
To bootstrap myself on the basic synth voice, I picked a Doepfer DIY synth.  The assembled board has all the necessary functions : VCO-VCF-VCA + Enveloppe generator & LFO.  I intend to fit it in a semi-modular 42HP module.

The Doepfer boys chilling on the bench.

Time to think about something to build from scratch.

I'll give a go at the Sample&Hold/Random module from Yusynth (for the noise source).
Regarding effects, I want to try mounting regular guitar pedal kits into Eurorack format.
Finally, I plan some utilities (multiples, inputs, outputs, inverters, …) in a 2U space between the two rows.
And as a starter, a simple module : a basic audio/CV mixer.

Early case sketches

I wanted to experiment by making the case myself as well.  After having sketched several versions of angular cases : 30°-45°-60°, 0°-30°-60°, etc… , I finally decided to keep it simple and have a basic 8U box.

Basic plan of the basic 8U case

White modules are in the making.  They constitute the minimal viable synth depicted above.   When I'll be there, we'll see.  Greyed out modules are envisaged later on, depending on what I feel is missing.

I'll still need some kind of controller or sequencer though.  A Korg SQ-1 or an Arturia Beatstep maybe.

Mmmh let's dive into more demo video....