DIY Synth Modules - a "Modular" Approach (Ep.1)

When one starts a modular synthesizer journey, tends to focus on main audio generation (oscillators), manipulation (filters, VCAs) and modulation sources (LFOs, enelope generators and so on).

When modules are in a number sufficient to actually patch something rewarding, one starts to evaluate less "appealing", but useful, modules, like attenuators, voltage processors and so on.

This at least is the path I found myself following after developing main modules and faceplates for my modular synthesizer in the last few years.

While thinking at the features I wanted for two voltage processing modules and some tinkering with my favourite CAD software, I ended with an idea. Most of those aforementioned "subdued" modules (go easy on me please) offer simple functions that are the sum of small circuits in series. Most of these circuits are the same for different modules, like the power supply filters section or voltage attenuation section, just to name a few.

So, here is the idea: modular modules!

Modular Modules? What's the fact?

Take your modular synthesizer: it's "modular" because made of your favourite modules, in your preferred order. Modules are the building blocks of your synth.

What if any of those modules are, themself, made of smaller components you can stack toghether at your will? What if we split a whole module in smaller circuits and use them as building blocks?

In this Instructables I will show you how I managed to realize some Eurorack modules made of six main building blocks:

• Front block
• Power filtering block
• Passive Attenuation block
• Inverting amplifier block
• DC bias block
• Phase shift block

I will share with you Gerber files to have all the PCBs manufactured and dramatically speed up the building process, if you are interested (lazy butted brigade to the rescue!).

I will also share with you links to Falstad's circuit JS1 main block circuits simulations: a very nice piece of software to toy with, if you ask me!

Here is the bill of materials for the modules building blocks:

Front Block

• 1x 50K ohm single turn potentiometer (WH148, linear or logaritmic)
• 2x 1/8" vertical mount female connector (PJ301M)
• 1x 6 pins male pinheader

Passive Attenuator

• 1x 1K ohm resistor
• 1x 6 pins female pinheader
• 1x 5 pins male pinheader

Power Filter

• 2x 100nF non polarized capacitor
• 2x 10uF electrolitic capacitor
• 1x 1N4004 diode
• 1x 5X2 IDC connector
• 2x 3mm led (optional)
• 2X 2K ohm resistor (optional)
• 2x 5 pins female pinheaders

Inverting Amplifier

• 2x 47K ohm resistors
• 2x 1K ohm resistor
• 1x 100K ohm potentiometer (for 2X gain)
• 1x TL072 op-amp
• 1x 100nF capacitor
• 1x 6 pins female pinheader
• 1x 5 pins male pinheader

DC Offsetter

• 7x 47K ohm resistors
• 1x 1K ohm resistor
• 1x 500K ohm potentiometer (linear or log)
• 1x TL072 op-amp
• 1x 100nF capacitor
• 1x 6 pins female pinheader
• 1x 5 pins male pinheader

Phase Shifter

• 2x 47K ohm resistors
• 1x 1K ohm resistor
• 1x 5K6 ohm resistor
• 1x 47nF capacitor
• 1x 10K ohm potentiometer (linear or log)
• 1x TL072 op-amp
• 1x 100nF capacitor
• 1x 6 pins female pinheader
• 1x 5 pins male pinheader

A full module will ask for at least four of the previous circuits and a faceplate.

If you are planning to use the shared feceplate, notice that the potentiometer I built it around is a common 90° mount potentiometer with some single conductor to gain electrical connection with the PCB.

Only easy to source and cheap components have been used in this project.

As said in the introduction, the idea is very simple: use small circuits as building blocks for eurorack modules. Not clear enought? Take a look at the video I realized for you then!

Power Filter

The power filtering circuit is very simple. Four capacitors in total, two electrolitic and two non polarized, are in charge to filter high and low frequency noise from the PSU +/-12V lines.

A diode with the cathode pointing the +12V and anode to -12V assures module's inversion polarity protection.

Two LEDs are there to monitor voltages from the PSU. These (and their current limiting resistors) are optional if you already have a power indication somewhere in your synthesizer and want to save some mA.

The PCB hosts a 5x2 pin IDC connector with Eurorack pinout for power supply.

Passive Attenuator

By Learning Modular definition, "an attenuator is a device that reduces (or "attenuates") the level of a signal. A fader on a mixer is an attenuator. Attenuators are particularly useful for adjusting how much "depth" of modulation you send from one module to another module's parameter – for example, whether an LFO makes the pitch of a VCO go up and down a lot (the result with no attenuation) or a little (the result of attenuating the LFO's signal)".

The simplest circuit we can adopt for the task is a so called "voltage divider".

By Sparkfun definition, "A voltage divider is a simple circuit which turns a large voltage into a smaller one. Using just two series resistors and an input voltage, we can create an output voltage that is a fraction of the input. Voltage dividers are one of the most fundamental circuits in electronics".

A voltage divider in it's simplest form is made up of two resistors, but this way you have a fixed voltage division. We want a continuous variable voltage out of the circuit, so we use a potentiometer.

Voltage dividers are often already takeing care of the input voltage level in most modules. When this is not the case, an attenuator module comes to the rescue.

Please notice: a voltage divider is NOT a circuit capable of delivering high currents. It is perfect for control voltages, but it cannot be used as a step-down voltage source.

This block is the simplest of the lot and rely heavily on the front block main component: the potentiometer.

>>HERE<< is a simulation of this very simple circuit.

Inverting Amplifier

An inverting amplifier is a circuit that outputs a sigal opposite in sign to the one at it's input (the two are 180° out of phase).

This block is built around a TL072 op-amp (as well as all the other active modules here presented).

The first op-amp stage is set as inverting amp with variable gain (up to 2X) set by the front panel potentiometer. The second op-amp stage is in voltage follower configuration.

Why an inverting amplifier instead of a "straight" one??

Well, because inverting amps are more entertaining (see later on this Instructable).

If you want to toy with a simulation of the circuit, click >>HERE<<.

DC Offsetter

Even if the Eurorack standard calls for signals in the +/-5V range (unbiased), some very important circuit work adeguately ony with biased voltages.

In example, CEM3340 PWM input works in the 0-8V range, meaning that an eurorack-compatible LFO would drive it to inaudible levels for half of it's cycle.

How could we solve? Well, by adding a DC offset voltage to our modulating signal.

The circuit adopted for this module pushes up or down 4V any input voltage when powering @12V.

At knob's half position the offset is zero. Turn left for negative offset. Turn right for positive offset.

This kind of circuit could be most often be seen in voltage processor modules.

The circuit is merely the most basic (inverting) mixer, where an input signal and DC offset voltage are added toghether. The signal is then inverted (again) in order to restore it's original polarity.

Want to try a simulation of the circuit with an additional attenuator at the input? Go >>HERE<<.

Phase Shifter

A phase shifter is used to, you can bet it, shift in phase a waveform.

In my modular synth I have four CEM3340 oscillators. Each of them has it's own FM/PWM input. I could send the exact same CV to all of them, but what if I send two (or more) different CVs, phase shifted one to the other?

Another application could be the modulation of the two sections of a band pass filter in order to open and close it in a less-predictable way.

Please notice that this is not a full featured Phaser effect in it's most common "audio" sense: harmonic components of the original signal and phase shifted signal do not interact. This is a CV shifter, for modulation voltage processing, not a main audio wave effect like those produced by pedals and multiFX.

If you want to toy with a simulation of the circuit, click >>HERE<<.

On my previous modules I used a flag-like geometry, with the PCB mounted perpendicular to the faceplate. This is the simplest and less prone to errors way to lay down a circuit and I prefer it over the parallel approach of most commercial modules.

The issue with flag-mounting is that it calls for modules at least 6HP wide, or the thickness of the populated PCB will be impossible to host whithin the panel lateral dimensions.

In this very special project case, blocks circuits are simple, and would not justify the use of a panel wider than 4HP. In addition, the building block appoach of the project calls for stacked PCBs, so I ended adopting the parallel mounting approach.

Blocks can be divided into 3 main categories:

• Front blocks
• Main circuits blocks
• Auxiliary blocks

Front blocks are intended to be locked beneath the face plate and host the user interface components (potentiometers, jacks, LEDs, etc. etc.). At the moment there 's only one type of front block, but more are in the line.

The 4HP faceplate can host two front blocks in a row.

Main circuits blocks host the active (or passive) signal modification circuit. They are intended to be directly connected to front blocks.

Auxiliary blocks are circuits that perform an auxiliary function, like filtering the power supply, hosting additional connectors, etc. etc. The power filter block is the only aux block available at the moment.

A full 4HP "modular module" is made of two block stacks in a 4HP space.

One stack output must be connected to the other stack input if a series modulation is required.

Components used are the most common available.

The 1/8" signal jacks are everywhere online and cheap, so it is not difficult to source them - I hate when a project realization has to be put on-hold by components shortage or "for manufacturers-only" minimum orders. Signal jacks I used are commonly labelled PJ301M.

Even the potentiometers are common linear (or logaritmic) pots I bent and "elongated" with some single pole conductor in order to reach the PCB holes. These are commonly labelled WH148.

Here are some Eurorack modules you can assemble by using Blocks modules.

Dual Passive Attenuator

The simples, but probably most useful module. It's made of two front blocks and two passive attenuator blocks.

Attenuated DC Offsetter

I was laying down a module with these very features when the "blocks" idea came to my mind. I was in the need for a module to offset my LFO voltage in a positive region to gain full control over AS3340 VCO's PWM duty cycle. I also needed some attenuation to tune the effect amplitude.

Non Inverting Amplifier

This is actually a dual-inverting amplifier module, in series. It gives you the option to amplify and invert a signal in the first or second stage, and then make it non inverted at the other stage. This is why I preferred to include an inverting amp block instead of a non inverting one. Two non-inverting blocks in series are still not capable to invert a signal.

Two amps in series means that you can amplify your signal up to 4X.

Both inverted and non-inverted signals could be made available at the same time with the use of passive multiples.

Other possible combinations

Here is a brief list of modules other than those already described you could realize starting from the building blocks available at today:

• Dual DC offsetter
• Attenuated Phase Shifter
• Attenuverter (attenuated inverter)
• Phase Shifted DC offsetter
• Dual Phase Shifter
• ... and more.

Think at how fast this list could grow with the addition of new blocks: Voltage clampers? Multiples? Control voltage attenuators? What about logic blocks?

The assembly of different modules starting from building blocks has its own advantages:

• it gives you the possibility to build-up ad-hoc modules
• block-sharing makes modules cheaper than dedicated full module PCBs
• if a module is no more needed, you can disassemble it and reuse populated circuits
• it's funny!

There are also disadvantages:

• it calls for more i/o connectors
• only simple circuits are possible
• faceplates are not "verbose"

All the modules here presented are 3U tall, 4HP wide. PCBs are staked in parallel, which makes designing them a little bit more complicated than plain, flag-mount circuits like those I made before.

About cons #2, the adoption of shared faceplates calls for very basic functions. More complex circuits would ask for more specialized faceplates, even if it's a fact that sometimes it is possible to adapt one to different applications. This makes the approach best suitable for "secondary" modules such as attenuators, voltage processors, inverters, logic operators, simple effects.

About cons #3, I have seen people write straight on the faceplate, but this is probably not the best option for "function flexible" modules. A possibly better option could be the use of a cardboard overlay with the needed graphics on it. Replace the cardboard as per module functions, and there you are ;)

Many thanks to those nice girls and guys at JLCPCB for sponsoring the manufacturing of blocks main PCBs and the aluminum face-plate pictured in this Instructable.

The sponsorship made possible to turn "modular modularity" from the status of "idea" into reality.

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 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 the right opportunity to give their service a due try ;)

The very latest versions of all Gerber files I layed down for this project are available in >>THIS<< Github repository of mine.

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... :)