Fixing the random module

This Random module, with its noise generators and sample&hold, is one of the first modules I made in 2017.  It's based on a design by Yves Usson for his Yusynth.

For some time, I had been under the impression that the output of the sample&hold had no influence on the modulations I assigned it to.

After some investigation, it appears that the input jack is at fault.  This jack has a switch so that if no plug is connected then the internal noise feeds the sample&hold. When a plug is connected, it opens the switch and the input signal feeds the sample&hold function.
 
In this case, the jack is of bad quality and its switch no longer closes properly.
 

Access is not so easy

Once replaced, everything works again.
I even changed the clock input jack because I thought we were using the same internal/external function management principle. But it wasn't necessary: the switch is outside.

Fixed !

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...

Yusynth's Random Module.

This is the story of my build of a Yusynth's Random Module in Eurorack format.

I learned a lot by building this.  Read :  "I made a lot of mistakes."

Finished module

I started from the schematics from Yusynth website.  It is clear and quite well explained.

Once again, I patiently captured the layout in DIYLC.  I used Yves' own layouts to guide me. 
I chose to make the two PCB version, one on top of the other, as I was sure one PCB would be too large for the Eurorack format.

I added two reverse polarity protection diodes.  And I chose Schottky diodes this time, to limit the voltage drop.

Anyway, I reproduced the mistake of having the power connector horizontally and not vertically.  Certainly due to the fact it was oriented that way in Yves'  layouts.
Here are the layout I used.

The layout of the two boards

I was mistaken in one of my part order and ordered some 2-Watt 10 Ohms resistors.   Nothing wrong done, as I had enough space to accommodate them, but they are total overkill.

Work-in-progress

As I was soldering the parts, I scrupulously followed each and every wire and parts, highlighting the schematics and my layout at the same time.

Despite those efforts, nothing worked at first.  Except the white noise output seemed to output ... some noise.

I quickly diagnosed that the oscillator did not oscillate.
Board inspection found nothing.  Without an oscilloscope, I was blind.  You cannot debug by ear a non audible LFO.

I finally bought a used Tektronik oscilloscope.  It confirmed the diagnostic, but still, no clue.
Oh, what's this ?  An unconnected pin on R5 ?
Soldered.  First problem fixed.

Oscillator debugging.  First success.

I began inspecting each output systematically and saw nothing at the random output.
It uses the pink noise output as reference.  And nothing on the pink noise output as well.
A wrong connection this time, due a misleading layout : because of the bad layout drawing, it was not clear which pin of the Q4 transistor to connect R20.  Of course, it was clear to me at layout time but I chose the wrong one at solder time.

Now white, pink and random outputs seemed to work.  More on this later on.

The sample&hold did not to hold.

Not holding properly

I couldn't understand why.
I reproduced the circuit on a breadboard with spare parts, so that I could better debug it and learn how it works.

Part of the circuit on a breadboard

I could reproduce the behavior : not holding well with signals from the noise module : white or random, while the circuit hold well with other signals. 
I switched capacitors, changed resistors, exchanged transistors to not avail.

Out of idea, I gave up and asked Google about it.
Finally, the solution was given by Yves Usson himself.  In a forum he detailed why it might not work :

Here are the possible causes for a holding fault :
- too high white noise signal (>8Vpp)
- a dead BF245C (Q5)
- a dead TL072 (U2)
- a dead diode (D3)

The last three could be excluded as the circuit behaved properly with another (about 1Vpp) input signal and I had the same behavior after having reproduced the circuit on the breadboard.  So dead components were out of the question.

Noise source was effectively way too high : about 20Vpp.  
In fact, noise saturated both output opamps so R34 and R27 should have been lowered.  I didn't want to lower my noise sources as much as 8Vpp, so I chose a compromise.
I chose R34 and R27 as 220k, both because hasty measure and crude calculation would give me about 16Vpp and I had some in my drawers.

As the circuit seems to be sensitive to the input voltage, I decided to add a passive attenuator on the input.

More debugging

Finally, my Random module works as expected.
It took some time and effort but the journey was worth it.

And it does a little bit more than blinking its LED...

My first DIY module

Start small they say.

My first DIY Eurorack module is a simple DC mixer.

I made that choice because I reckon it's easy to build and I found tons of examples on the Net.  It can serve the purpose to mix any type of signals, CV and audio.

Schematic was inspired by Yusynth's with some tips from Ken Stone's and Dintree websites.  It's not much, but you have to start somewhere.

Schematic capture on DIYLC

You'll notice that the first stage is at unity gain and I chose to have the second stage with a 2x gain so that I can use the circuit to boost the signal if need be.

I used DIY Layout Creator (DIYLC for short) to capture the schematic and to trace the layout for a perf board.    Layout is OK, but capturing the schema was a pain.

Layout of the simple mixer : power at the bottom, inputs on the left, outputs on the right..

Warning : the schematic and layout are given for illustration purpose only.  Use them wisely and at your own risk.  Don't forget that I don't really know what I'm doing.

I used a lot of space.  It's a luxury I won't have anymore for the next modules.

I did a mistake though in placing the power connector parallel to the bottom side of the module.  It seemed a good idea at first to have the power lines aligned with the IC.  But once placed into the rack, the flat cable will be crooked.  I didn't respect the de-facto standard to have the -12V at the bottom.  My bad.

Before finalising the cabling, I assembled the mechanical parts to check if all fit.

Recto

Verso

I  then started with the cabling that did not concern the board : herebelow the input attenuators. 

Cabling pots

I chose to have PCB connectors on the board to ease with the mounting and un-mounting.  I did it on the next module as well.  It takes a lot of space and does not bring so many advantages I think.  My third module will be different.  I'm still undecided on this.

Fully assembled

Here is my first DIY Eurorack module, alongside the A-110-1 VCO and the next one : the Sample & Hold and Random module from the inspiring project from Yves Usson.   The two DIY modules have a temporary annotated paper facade.

The first three

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....