Doepfer on Acid

Close-up

So now that I decided to relegate my DIY-101 module based on Doepfer DIY in a box, it is time to say farewell.  I previously wrote that I will certainly regret the filter.  This is why.

The following track is entirely based on the Doepfer DIY filter.  There is no other filter involved.  All sounds are produced by the modular, except the drum part (909 samples) and an overdub on the pads (Korg Polysix emulation).
The base of the track is made of a single PWM output from the Doepfer A-110-1 VCO, into the Doepfer DIY filter.  Each of the filter output (LP, BP, HP) is treated in a different path and recorder separately.
Low pass takes the role of the bass, band pass gives some overtone and high pass gives the high pitch percussions.

The modular synth suitcase as featured in this track

The Korg SQ-1 sequences the lot.  There is a single sequence in two modes : zip of 16 steps on intro, and bridge, 2 lines of 8 steps (one for low pass, one for high pass) for the verses and outro.  High pitched percussions are driven by the Turing Machine, hence their random aspect.  Filter frequency is driven by two envelopes and manipulated by hand as well.

Wind effects are pink noise from the Random module to Doepfer DIY filter, mixing HP and LP before going to the VCA.

Lead synth is a mix of saw and square wave from Doepfer A110-1 VCO through Doepfer DIY filter.  Portamento courtesy of Befaco Rampage.

Pads are a mix of the modular and the addition of pads from Polysix emulation from Korg Legacy Collection, with heavy use of chorus.

Drums : 909 samples in NI Kontakt 5

DAW: Reaper

Effects : TAL Dub II, TAL Chorus-60, TAL Tube, TAL Reverb II, NI SuperCharger, Thomas Mundt LoudMax

Synth in action

DIY-101 synth voice - part 2

This is the second part of the making of my Doepfer DIY synth based module.  You can find the first part here.

Finished module in action.

Now is the time for some cabling.

There are 7 connectors on the circuit, numbered JP1 to JP7.  Moreover, I have to connect the 5V and common ground.
With the synth, I  ordered the set of dedicated flat cables to ease the interconnection.

I started with Ground, 5V and all connections that were not linked to a specific connector.

Before cabling connectors.

Pin 1 above

Then I did cable connector after connector, cutting the wires, splitting the flat cable, stripping with my nails and finally soldering … One wire at a time, in order not to lose track. 
I started with JP2 and JP3, as I reckoned they connected mostly to pots and jacks below the circuit and that would prove difficult if I finish with them.
All ground pins on those connectors were ignored as I chose to have a common ground from the 5V power connector.

Pin 1 below

Thanks to the schematics from M-19, I noticed the dot indicating the first signal (red line) on the connector was sometimes above or below in the Doepfer documentation tables.   I fell in the trap on my first schema.

Ongoing cabling

I also verified twice each wire connection before soldering.  And I did well. I noticed a couple of mistakes on my schematics this way.  I annotated my drawings along the way.

 

 

Annotated schematics

Bob checks a solder joint

 Bob managed to get some useful Quality Control time. 

My first tests showed I made some minor mistakes :
- switches pins are reversed with regards to the switch handle position;
- a couple of signals were wrongfully soldered on the switch pin of the jack, instead of the tip;
- I detected one or two bad solder joints that escaped Bob's control.





Here is the finished product.

Finished

Bob fixes knobs.

Finally, I let Bob place the knobs.









Tuning the VCO proved problematic.  Even after I let the board heat up for 20 minutes, I really had a hard time approaching the 1V/oct slope.
I'm not 100% sure yet, but the VCO is not very stable and slowly drifts, even with the tempco option installed.  Is the circuit defective ?  Is the design problematic ?  Was it damaged during my early tests with a home made power supply ?  I don't know.  The rest works like a charm.

Tuning the beast

Finally, all the Eurorack power flat cables I got for my DIY modules were 16-pin to 16-pin.  And JP1 is aligned in a way it is not possible to use a 16-pin connector properly.

I had to divert the 10-pin to 16-pin cable from Rings to put the module in place.

Time to buy some 16-pin to 10-pin cables next time I guess.




Finally in place.   Ready to wiggle.

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.

Repairing my VCO

A-110-1 standard VCO

Maybe you followed this blog for a while and wondered why I didn't write more about the Doepfer A-110-1 Standard VCO that is shown on some photographs.
Or maybe you don't care.

This module is the first module I bought new  It's also the first I fried : I plugged the connector in reverse during one of my trials, before I have a case.

The module began to act odd.  Plugging its output to Rings input halted the later.  It became very hot.
I stopped everything and began to investigate.

 -12V pin of the module connected to an unconnected pin of the bus board, presumably close to ground voltage.  Gnd pin was at 12V and 12V pin at 0V.   I headed to the A-110 service manual from Doepfer to find out which parts could have been stressed.

I spotted two components that would certainly suffer to be powered in reverse (inversion of the 12V and gnd pins) : the CA3046 transistor array (IC3) and the 78L10 10V regulator (IC5).

The two components I changed.

I reckoned the two TL074 op-amps wouldn't have been stressed as they were powered between 0V and a floating voltage that would most certainly be close to 0V.  I removed them and tested them on a breadboard : they were OK.

I also checked the TL431 Reference Voltage and it gave me the proper 3V it should.  So it was OK as well.

Maybe other transistors were stressed but I have no idea how to verify without desoldering them.  The plan is to change the CA3046 and 78L10 first and test the module.

Calibrating the module.

After some difficulties to get a CA3046 (they are reported as obsolete in DIP14 package by main distributors) and de-solder the 78L10 (I guess my de-soldering braid was oxidized), I finally replaced the two parts. 
Et voila.  A quick check at the oscilloscope showed all outputs seemed fine.  Maybe only these two components were affected after all.

Finally, I re-calibrated the module, following the service manual, skipping step 1 though (too complex).


I can finally use my VCO.  End of the story.

The case

I finally made it.

I got a case for my modular synthesizer.

The finished case

Even though I had identified the necessary pieces of wood available at home, I dismissed my initial idea of having a full custom 8U wooden case.
I got inspired by this thread on Muff Wiggler to use a basic tool case to host my first modular synth. 

I reckon my initial idea of 2U 84HP utility panel would have taken me too much time.  So starting from an existing box seems a better idea.

As usual with me, I progressed step by step.

The box is a standard tool case from the local DIY store (Sencys from Brico). External dimensions are 46 x 33 x 15 cm.   This is great for hosting a 6U 84HP Eurorack modular synthesizer.
I'll keep the lid from now.

The Doepfer A100 DIY kit is perfect for the job : four rails, two bus boards, a power supply is all you need to start..

Checking everything is in place ...

I started by checking that everything could easily fit into the box.  And from there I evaluated what would be needed to mount the kit inside.

Apart from the box, I exclusively used pieces of woods already available in the house.  Mainly remnant from a 6mm thick MDF wood panel..



I decided to keep the foam inside.  I only had to tighten the corners and cut out the part where sides were recovering the bottom part.




I bought some 6mm x 6mm square nuts to fit in the rail, as the standard hexagonal ones have a tendency to rotate and block into the rail.  As I miscalculated my needs, I kept 4 hex nuts per rail, two on each end, for a total of 21 nuts per rail.  I reckon this will suffice.




The back of the structure is a bit more than 4 cm deep.  It gets to support the power supply placed vertically and can accommodate 2 8HP modules turned horizontally.  Not as good as the 2U utility panel I initially envisaged, but this will do.




For the bus boards, I decided to fix them from below an horizontal wood piece.  These slats were a bit thick.  The box is not very deep.  This way I keep a full 7 cm clear.  The bus boards are blocked by the foam.  And yes I checked if, by any chance, this would be conductive … which it is not, obviously.

The connection of the power supply to the 240V-15V AC transformer is done via an appropriated connector on the front panel to avoid piercing the box.  No trouble thanks to the parts I collected when I started to make guitar pedal kits.


A green LED connected between the -12V rail and the ground (with the help of a 1k resistor) indicates when the case is powered.

Both fit into a custom panel, drilled with some ventilation holes.





I made sure the power wires would stay nicely on the side and I double checked the connection with a voltmeter.

Before fixing inside the case

Finally, I secured the lot into the box thanks to four 5mm carriage bolts.

Finished.  It's time to add modules.

First family photograph

You might have noticed that this blog is a little bit late with regards to the real progress of my synthesizer project.

This post is, for once, right on time.

My first 3 DIY Eurorack modules are now finished.
It's time for a family photograph...

And under the hood...

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....