Sequencer

One of the simplest and cheapest CV/gate step sequencer is the Korg SQ-1.
 

Korg SQ-1 step sequencer

Powered via USB or batteries, the small machine is able to deliver one 16 steps or two 8 steps sequences in various of ways.

There are very few settings for CV out.  The SQ-1 can play linear, minor, major or chromatic scales with a 1V, 2V or 5V ranges (in V/Oct mode) or 8V range (in V/Hz mode).

Its biggest drawback is that there is no way to choose the root note of the scale.  So it's either C major/A minor (major mode) or C minor/Eb major (minor mode).   You'll have to transpose yourself by adding voltage (83 mV per semi-tone).  Good enough for fun, maybe not suitable for a full symphony.

Otherwise it's an enjoyable machine that I bought mostly for parameter automation.

I don't have a lot to control on the synth just yet.  But it's still possible to have some fun.

In action

Here above, the SQ-1 sequences Rings in sympathetic strings mode.  The sample and hold modules gives some randomness to the position parameter.  Odd output is untreated, while the even output is connected to the phaser.  So one every two notes is treated by the phaser and mixed a bit differently for a bit of variation.  Sequence evolution is me activating and deactivating the gates.  The SQ-1 has no pattern memory whatsoever.
All is synced to the tempo of the Korg Volca Beats, responsible for the basic drum pattern.
Additional effects : TAL DUB II for the delay, NI Guitar Rig as guitar amp simulation and Thomas Mundt LoudMax.

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.

Guitar effect in a Eurorack module

To add effects to my modular synthesiser, I had the idea to buy some guitar effect pedal kits and design the necessary adaptation circuit to include them in a Eurorack module.  One need some sort of adaptation to go from 12V to 9V power supply and to lower the audio signal level : from max 20Vpp to about 1 Vpp.   I reckon most pedals might take a line level input, so at least a ten-fold attenuation (-20 dB) was needed.

Final module : on the left, the pedal kit; on the right, the adaptation PCB.

I chose some kits from Das Musikding as I knew already they are high quality : a Chorus and a Phaser, as planned.
I decided to start with the Phaser.
Indeed, assembling the kit was a breeze. 

Then I started to design the adaptation circuit.

Breadboarding in progress.

My original idea was to have a simple dry-wet passive potentiometer with a buffer before and after.  But the attenuation in the middle position was too much.  It was good enough when used statically thanks to the boost of the second stage but you couldn't go smoothly from dry to wet.

Then I tried with having two different volumes knob, one for the dry signal, one for the wet signal and mix both.
But it wasn't the feel I was looking for.  I wanted a traditional dry-wet knob.

I finally stumbled upon this schematics from R.G. Keen.  It was spot on what I was looking for.

The three dry-wet topologies I tried.

Power supply side of the adaptation

The power supply uses a simple serial line regulator.  The implementation principles are very well explained on Sonelec website.  I chose to use an LM317 because I had some in my drawer.  These LM317 in TO92 package were a mistake in an order I made previously.  They were sitting in my drawer ever since as they can only draw 100mA.  Now is the time to put them into use.
A LM7809 would have been better suited for this job.   Some might even argue that the added components (3 resistors, 1 capacitor) are more expensive than an LM7809, but it's another story.

Cardboard panel prototype

 I wasn't too sure about the potentiometer and jack places on the panel.  So I made a cardboard mock up before punching holes in the aluminium panel.  I intend to use this technique later with the Doepfer DIY synth.  So I'd better learn.

Cabling in progress

I used PCB connectors for the link between the two PCB.
But this time, I opted to solder wires from the lower side of the PCB to the pots and jacks, instead of using  connectors. 
Note the plastic feet glued on top of the centre pot : the pedal PCB extended over it so I had to avoid unwanted contact between the two.

I cabled everything wrong at first : input on output, potentiometers the wrong way.  Every time it was possible to make a mistake, I did one.  It was a nice and clean way to connect the jacks and the pots the first time.  It was a pain to change afterwards.

Moreover, after having cabled everything, I had a more thorough look at the oscilloscope.  I decided to modify the circuit.  I removed the original 1uF capacitors in the signal path as they were eating all my low end.  Original schema might be nice for guitars, but not for the extended audio range of a synth.  I also added the compensation capacitors on both op amps, because they tended to oscillate.

Not easy to debug in this position.

Despite my early efforts on the breadboard, I didn't notice or take too seriously both behaviours.

Un-soldering and soldering parts with the circuit so tightly attached to the panel controls was a pain.  PCB connectors would have been handy.
I really need to make up my mind about them.







Final circuit and layouts are available here below.   This time, I correctly oriented the power connector.

I used Fritzing for the schematics because I thought it would permit me to smoothly go from schematics to breadboard layout.  Schematic capture is OK.  But the breadboard module is not as usable as DIYLC, so I ended up using the latter for the layout.

A bit of work for the panel marking …

… and here is the finalised module.

A word about the gain staging to finish. 
I intended to have the first stage to go from -40dB (/100) to -20dB (/10).  From R.G. Keen, I gathered that the panner circuit would divide roughly by 3 (or -10 dB) and the 51k feedback resistor of the output op amp would give an initial  x3.41 gain.  I designed the output stage to have a final maximum gain of + 37 dB (x70). This way I could more or less cover the level drop by the first stage and the panner.   I indicated +30 dB on the panel to take into account the -10dB from the panner and it looked cooler than +27 dB.

Measurement confirmed my hypothesis.
At the highest settings, with a 10Vpp input, full dry, the output stage saturated gently.  Expected : at maximum gain, the circuit would go a bit higher than 22 Vpp
But when the output stage was at minimum gain, I expected a voltage drop of -40 dB : down to 100mVpp.  I saw … 0 V at the output.   Strange.  I certainly once again overlooked a detail.

Anyway, I have more than enough gain dynamics.  I was fed up with debugging this circuit.  That would do for now.  I updated the front panel to acknowledge that particular behaviour.