DIY Eurorack Compatible Paraphonic Modular Synthesizer

In 2021 I started the adventure of designing my whole synthesizer. Given the prototype character of the project - based on schematics from the net, but not a carbon copy of any project - going "modular" was the most obvious choice.

Module after module, I ended with a MIDI ready, 4-voices paraphonic synthesizer.

Modules, while being 90% Eurorack compatible, were "affected" by an original sin: the use of 1/4 of an inch jack connectors in place of standard 1/8" jacks. This made modules a bit larger (not that big issue by itself) which in turn forced me to leave some feature out of some modules (ok, this is not good!).

Well, I was starting to feel like "The Princess and the Pea" (no kidding me please...) and then I decided that it was time to smooth this corner and give panels their "final" (the last famous words...) eurorack dress!

In this instructables I will show you how the front plates changed in this redesign process. I will also share with you links to Gerber files to have them manufactured at the best price around.

Please notice that only panel boards have been modified: all circuit boards were left unchanged (the choice of mounting them perpendicular to the faceplates paid!).

If you are interested in the circuit PCBs you will find the relative links below.

Let's go!

Any module here cited has already received a detailed description in a dedicated Instructable. Dedicated Instructables include PCB BOMs, so they will not be reported here (continue reading for links to those Instructables).

My whole synth panel took, in addition to PCB components:

• 96 1/8" connectors
• 48 knobs (MF-01A)
• 4 ON-OFF-ON latching switches
• 5 ON-ON latching switches
• 1 panel mount push button
• 4 LEDs

Face panel components are obviously a function of the number and type of module and will likely be different in your synth.

Knobs pictured are MF-01A plastic knobs. They have been replaced with smaller and coloured knobs in the name of practicality.

I realized two oscillator modules: an Arduino wavetable oscillator and a CEM 3340 based VCO.

A new version of the first one is still on my to-do list and the panel has never been realized. This is because I still want to improve some aspects and give it some testing before. This oscillator is on hold at he moment.

The CEM3340 oscillator is the module that paid the jack dimensions choice the most. I had to left out all the oscillator sync options because of space limits.

The circuit board I layed down for the CEM3340 module is massively based on previous work by Eddy Bergman, which in turn is based on Digisound 80 design. Main differencies between the design adopted and those suggested on the CEM3340 datasheet are the use of buffers at (already in-chip buffered) waves out, the amplification of the triangle out signal (lower than the others wave outs immediately after the chip output) and some component value.

Take a look at my original article for more informations about the circuit board.

By redesigning the faceplate I was able to gain 2HP (the new faceplate is 14HP, the original was 16HP) and include ALL the sync options of the integrated circuit. Two HP could seem a limited dimension reduction, but considering that I have four of these modules in the synthesizer, it's a 8 HP gain ... not bad at all!

The four stages envelope generator circuit is based on a previous work by m0xpd. The original project outputs a classic ADSR envelope, in two different modes: "normal" or looped (a.k.a. retrigger). In addition to these I coded two new operation modes ("biased semi-invertion" mode and "biased quasi-invertion") and added an amplifier for "conventional" envelope invertion.

The face plate was 12HP in the first version, but by using 1/8" jacks I was able to reduce it to 10HP. Being that I am in the need for two envelope generators (one for the voltage controlled amplifier, the other for the voltage controlled filter), a net reduction of 4HP was possible.

The PCB I layed down for the LFO module is built around a circuit first introduced by Nicolas Whoolaston in 2008 at Electro-Music forum. Main components are a common LM324 quad op-amp, a non polarized capacitor and only a few others elements. The result is a nice oscillator with two different wave forms (triangular and square) and control over waveshape and frequency. HERE is the link to the original post (electro-music.com).

With respect to the original project, I have included the possibility to change the frequency range of the oscillator, a feature present in most commercial LFO modules. The maximum frequency is determined by the non polarized capacitor (C1 in THESE schematics) and it was a simple matter of adding other two caps in parallel and a 3 positions switch to include this welcome feature.

Only few components for a very effective LFO module.

In the first face plates iteration, I realized two different version of LFO panels. The first one was a full featured 8HP LFO, with selection of the extension range of oscillation frequency. The second one was a reduced 6HP version, limited to a single oscillation frequency range. Needless to say that, even if you could have a preference between the three frequencies ranges, you will end missing those left out.

Problem solved with the jack connectors dimension reduction: the new panel is 6HP and has all the features the circuit delivers.

Take a look to the dedicated article for additional information.

Combining three modules into one was already an attempt to reduce panel space. In particular, this is a combi module with two multimode resonant filters a-la MS20 and a voltage controlled amplifier in series.

The filter section is a Korg MS20 MK II multimode resonant filter clone. The original Korg filter was built around now obsolete LM13600. Here I adopted the LM13700, a direct replacement for LM13600, and high performane amps as buffers in the audio path.

The VCA section is built around a NPN transistor pair in a single SMD package (BC847BS), which has the advantage of an intrinsic good matching of the transistor pair and a less tendency to drift over time thanks to a far better junction temperature pairing (two transistors packed in the same, small, IC you know...).

In this module, filters are ordered as one high pass followed by a low pass. I made it this way because, in the end, I would have used the two in this exact sequence most of the times.

In this case it was not possible to further reduce the panel dimensions (not even a single HP), but I could add two control voltages that were left out in the first iteration of the combined module. The filters PCB, infact, has a CV input with voltage limiter and another without limiter. The second one is now made available to the musician.

On the amplifier side there is now room for the CV limiter, already present in the PCB but left out from the panel's first version because of (bet it) space constrains.

The four channel mixer is a crucial module in a multi-voices synthesizer. Not only it could be used to mix four oscillators to gain paraphony, but also to mix different waveforms from a single oscillator, opening a box-full of layered sounds.

It is built around a general pourpouse, high speed JFET dual operational amplifier. Doepfer old DIY page is always a good place to start from!

In addition to that, I included a basic gain control on the first operational amplifier in order to give some room to balance unavoidable input losses coming from the first stage passive mixer.

Another feature is a handy (digital) pitch reference for oscillators tuning, like the one installed in some iconic synth of the golden era (Moog Model D anyone?).

I was able to reduce this module by 1HP only and no additional features have been added.

The ring modulator is another module that was already full featured even in the previous incarnation, but now frees 2HPs on the chassis front.

All in all, the dimension reduction helped including this module on the panel area of my synthesizer.

The circuit here adopted is very similar to the one firstly proposed by Roman Sowa in the first years of 2000, and then used by a whole lot of people with minor or no modifications at all. Take a look at Roman's Blog: you will find other circuits and modules he developed to take inspiration from.

The barebone circuit is very simple and uses a minimum amount of components: some resistor, a couple of capacitors, a quad op-amp (TL074) and a AD633 which is the main IC of the circuit.

Take a look at the original article for more information about the circuit design and choices adopted.

The waveshape monitor is a module I had a lot of fun developing.

Software-wise, the project is based on two other projects (Hagiwo's oscilloscope and spectrum analizer module and Radiopench oscilloscope pen), but with the inedit adoption of a differential amplifier to limit the incoming voltage to common microcontrollers limits. An Arduino nano is in charge for all the math and displaying tasks.

No dimension reduction could be gained here, because the built-in screen dictates them. No new features were added being that nothing was left out even during the first iteration. Despite of this, the module has been subject to a rethinking of monitor and jacks positions after assembly. This means that the shared version is even better than the one pictured!

Please notice that the waveshape monitor's faceplate calls for through hole direct soldering. This means that, in this special (and unique) case, you are forced to use fiber glass panels instead of aluminum panels for the front panel.

"Multiples" comes handy when you want to send the same signal to different modules.

With respect to active multis, passives have the drawback of an unavoidable signal deterioration when increasing the number of simultaneous destinations connected to that very signal.

On the other hand, they have the great advantage of a reduced form factor and ease of assembly, which makes them the favourite first time DIY module of choice for most musician approaching the "modular way".

These modules are used to send a digital or analog signal to two or more different destinations. The signal loss (in the case of passive multis) in practice do not affect digital signals (i.e. triggers, gates, etc). It surely affects analog signals, but these remains absolutely usable in most cases. One noteworthy analog message calling for active multiples is the pitch control voltage.

This is actually a very new module in this series. We actually have two new modules, to say the truth.

One is a 2HP module made of two different bus sections with four jacks to be connected toghether. This is surely the most common layout for this kind of module. The other is 4HP wide, has two columns of four jacks slots in the upper side and another one in the lower side. An additional lowest hole drill, sized to host a 1/4" jack socket (What?!? They are back?!?), takes his place in the lower side of the panel.

The bigger hole could be used as the latest module of your synth to the audio mixer jack input (Aaah... makes more sense now).

I don't think there are multis with such outstanding feature out there :D

To assemble passive multiple modules, you can follow these steps:

• screw firmly the 1/8" connectors on the faceplate, with ground leads facing each other in groups of two
• melt some solder to make the touching ground leads firm
• with a peeled single conductor cable, connect all the ground leads toghether.
• melt some solder to make the connection between cable and ground leads firm
• with a peeled single conductor cable, connect al the signal leads of the same group toghether.
• melt some solder to make the connection between signal leads and cable firm

Passive multiples are the typical very first DIY module people commonly deal with. If this is your case too, take your time and have fun in the process ;)

You can download my passive multiples Gerber files >>HERE<< (Github).

A note of caution about the use of passive multiples: never NEVER directly connect two modules outputs to the same passive multiple input. Outputs are low impedance lines and must go through a mixer in order to be mixed. These passive multis are as simple as possible, with all the jacks shorted toghether. You could damage your modules by connecting their outs to the same passive multi.

Links to all the PCBs I have layed down for this project (actual circuits and faceplates) are available at the links I posted in the various modules dedicated Instructables.

Modules PCBs are 100% compatible with new panels given the "flag" mounting geometry adopted and the respect of original distances between panel mounting components (potentiometers in primis).

I always upload to my Github repositories the latest PCB version, so you will find the most updated file there!

If you are interested in one of these modules, you can have PCBs and panels manufactured by uploading them at the manufacturer's of your choice site. I strongly suggest you JLCPCB - the sponsor of this instructable - but it's not mandatory of course: files I share are a universal standard accepted by any PCB manufacturer.

Please notice that JLCPCB only realize aluminum PCBs in white (cheapest) or black (more expensive). If you are interested in coloured aluminum panels you should take a look elsewhere.

My projects are free and for everybody. You are anyway welcome if you want to donate some change to help me cover components costs and encourage the development of new projects.

>>HERE<< is my paypal donation page, just in case... :)

A special "Thanks!" goes to those nice girls and guys at JLCPCB. They already supported most of the modules I developed and shared in the past, and this time gave me the opportunity to remedy the original sin of adopting non-standard 1/4" connectors in those projects. With this sponsorship they made possible for me to test the new Eurorack-compatible faceplates, thus mitigating the risk for any Eurorack fellow to adopt my (100% free!) files.

JLCPCB is a high-tech manufacturer specialized in the production of high-reliable and cost-effective PCBs. They offer a flexible PCB assembly service with a huge library of more than 350.000 components in stock. 3D printing has "recently" been added to their portfolio of services so one can create a full finished product all in one place!

Their customer service is responsive and helpful and PCBs a great value for your money.

By registering at JLCPCB site via THIS LINK (affiliated link) you will receive a series of coupons for your orders. Registering costs nothing, so it could be a nice opportunity to give their service a try ;)