DIY Synth Modules - a "Modular" Approach (Ep.2)

In the first episode of this series of Instructables I showed you the concept behind the "modular modularity" project and introduced the first lot of building blocks to create your own modules.

All the blocks already shared in first episode have the common feature of being compatible with panel blocks made of two jacks and a potentiometer. In this Instructable I will introduce a brand new front block with 4 jacks which opens the way to a new set of circuits to be added to the previous list:

1. Buffered multiple (1 in, 3 out)
2. Non-attenuated fixed-gain mixer (3 in, 1 out)
3. Dual inverter
4. Track and hold
5. Voltage controlled attenuator
6. White noise generator
7. Dual-LED voltage monitor

Two new front plates have been designed: one with four jacks per half module and a mixed one with four jacks in the upper side and two jacks plus a potentiometer in the lower side. This makes possible mixing new blocks and blocks from the previous iteration opening a whole lot of possibilities.

In this Instructables I will show you the circuits I adopted, I will share Gerber files to have all the PCBs manufactured and dramatically speed up the building process. I will also share Falstad's circuit JS1 main block circuits simulations to toy with...

Here is the bill of materials for the new blocks:

4 Jacks Front Block

1. 4x 1/8" vertical mount female connector (PJ301M)
2. 1x 5 pins male pinheader

2 Jacks, 2 LEDs Front Block

1. 2x 1/8" vertical mount female connector (PJ301M)
2. 1x 4 pins male pinheader
3. 1x 3 pins male pinheader
4. 2x LED

Buffered Multiple

1. 3x 1000 ohm resistors
2. 1x 2.2 M ohm resistor
3. 1x TL074 op-amp
4. 1x 100nF capacitor
5. 1x 5 pins male pinheader
6. 1x 5 pins female pinheader

Dual Signal Inverter

1. 2x 2.2 M ohm resistors
2. 4x 1000 ohm resistors
3. 4x 47K ohm resistors
4. 1x TL074 op-amp
5. 1x 100nF capacitor
6. 1x 5 pins male pinheader
7. 1x 5 pins female pinheader

Track & Hold

1. 1x 2.2 M ohm resistor
2. 3x 1K ohm resistor
3. 1x TL072 op-amp
4. 1x 100nF capacitor
5. 1x 1nF capacitor (non ceramic)
6. 1x 2N5484 JFET
7. 1x 5 pins male pinheader
8. 1x 5 pins female pinheader

Voltage Controlled Attenuator

1. 7x 10K resistor
2. 2x 1K ohm resistor
3. 1x TL072 op-amp
4. 1x 100nF capacitor
5. 2x 2N5484 JFETs
6. 1x 5 pins male pinheader
7. 1x 5 pins female pinheader

White Noise Generator

1. 2x 10K resistor
2. 1x 47K ohm resistor
3. 1x 220K ohm resistor
4. 1x TL072 op-amp
5. 3x 100nF capacitor
6. 1x 1uF capacitor
7. 2x 2N3904 transistor
8. 1x 5 pins male pinheader
9. 1x 6 pins female pinheader

Polarity LED Indicator

1. 1x 2K resistor
2. 1x 4K7 ohm resistor
3. 2x 1K ohm resistor
4. 1x TL072 op-amp
5. 1x 100nF capacitor
6. 1x 5 pins male pinheader
7. 1x 3 pins female pinheader
8. 1x 4 pins female pinheader

Non-Attenuated Fixed-Gain Mixer

1. 6x 47K resistor
2. 1x 1K resistor
3. 1x TL072 op-amp
4. 1x 100nF capacitor
5. 1x 5 pins male pinheader
6. 1x 5 pins female pinheader

Power Filter

1. 2x 100nF non polarized capacitor
2. 2x 10uF electrolitic capacitor
3. 1x 1N4004 diode
4. 1x 5X2 IDC connector
5. 2x 3mm led (optional)
6. 2X 2K ohm resistor (optional)
7. 2x 5 pins female pinheaders

A full module will ask for at least two front blocks, two circuit blocks, a power/filter block and a faceplate.

In my eurorack modules I most often use a flag-like geometry, with the PCB mounted perpendicular to the faceplate. This is the safest way to lay down a prototype circuit, with the drawback of a forced, minimum 6HP wide panel dimension.

In the very special case of this project, 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 commercial producers like so much.

Blocks can be divided into 3 main categories:

1. Front blocks
2. Main circuits blocks
3. Auxiliary circuits blocks

Front blocks are intended to be locked beneath the face plate and host the user interface components (potentiometers, jacks, LEDs, etc. etc.). This new iteration of blocks adds two front blocks out of a total of three.

The face plate hosts two front blocks.

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

Auxiliary circuit blocks are circuits that perform auxiliary functions, like filtering the power supply, giveing visual feedbacks (i.e. displays), hosting additional connectors, etc. etc.

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 series modulation is required.

Components used are the most common possible. The 1/8" signal jacks are everywhere online and cheap, so it is not difficult to source them. Signal jacks I used are commonly labelled PJ301M.

Potentiometers from the first blocks iteration are common linear (or logaritmic) 47K ohm pots I bent and "elongated" with some single pole conductor in order to reach the PCB holes. These are commonly labelled WH148.

Amplifiers, transistors, connectors and every single component here used are super cheap, easy to source and common.

Buffered Multiple

Most of the times a passive multiple will work, but what if you want carbon-copies of your voltage sweep?

The buffered multiple block is built around a TL074 quad op-amp with unity gain. The signal at the input jack is sent 1:1 at the three outputs.

By usign two of these blocks in a single module and connecting the output of one semi-module to the input of the other you have a 1 input -> 5 outputs buffered multiple. Cool :)

Dual Signal Inverter

Inverters are devices used to invert (!!) the polaity of an incoming signal. This block is built around a TL074 quad op-amp in inverting configuration (I should stop stating the obvious, isn't it?), with a voltage follower in the final stage for protection.

By usign two of these blocks in a single module you obtain a quad inverter. This block is similar to the "Inverting Amplifier" block of EP1, but dubled and with fixed gain.

Track & Hold

Track and holds perform a very special function: they hold an incoming, variable voltage for a brief time when triggered, then let the voltage pass as-is. They are different from Sample & Holds because in these the voltage is hold for higher times and the incoming wave sort of "digitalized". >>THIS<< picture will help you figure out the difference in more detail.

The circuit adopted is possibly the simplest you can find online (or very close to it) and it's built around a n-channel JFET (2N5484) in series switch configuration. Two voltage followers increase the block's input capacitance and increase the output current capability of the whole circuit (it also makes a good reverse connection protection).

Some very nice writing about JFET's sample and hold (but track and hold, actually...) could be found >>HERE<< (eeeguide.com).

Do you want to toy with the circuit? >>HERE<< is a CircuitJS simulation for you!

Voltage Controlled Attenuator

The very first and simplest module I designed for "Modular Modularity" series was a passive attenuator. It made use of one single resistor in conjuction with the front block's potentiometer and, despite it's simplicity, it is a very common circuit in modules.

In this second run of blocks the attenuator gains a voltage control thus further increasing it's interest status :)

The circuit is simple, with a JFET driven in it's ohmic (or linear) region to work as variable resistor.

>>HERE<< is the page where I got the circuit from. An additional input buffer stage in inverting configuration makes forward voltage instead of inverse voltage operation possible.

The Falstad's Circuit JS1 simulation I layed down for you is >>HERE<<.

White Noise Generator

A white noise generator is mandatory to synthesize special effects (wind), give rattle to a snare or breath to a pad.

The white noise generator is based on a simple design takeing advantage from the natural occurring PNP semiconductor junction thermal noise. The "barebone" noise is amplified by a dual inverting amplification stage. The first amp stage is a straight 5X amplfication to give the small noise signal (100-300 mV depending on the transistor) a boost. The second amplification stage has variable, user definable gain through a potentiometer.

The white noise block is compatible with the front block from the first lot of circuits.

LED Voltage Monitor (polarity indicator)

A nice addition to modules are LED-based indicators. An example? Do you remember the DC offsetter block from the previous blocks set? We can now realize a DC offsetter module with polarity indicator. What about attenuators? You can now have a visual indication of how much attenuation you are applying to your signal.

This block main components are a couple of operational amplifiers and two LEDs. Nothing more.

The polarity indicator main circuit block was not compatible with existing front blocks, so I layed down a dedicated one. I made it compatible with "4 jacks" front panels, with LEDs aligned to the two middle holes.

Falstad's CircuitJS1 simulation of the circuit >>HERE<<.

Non-Attenuated, Fixed Gain Mixer

A 3-inputs mixer in a very limited space is possible if you left out the per-channel attenuation stage and set a fixed gain. This block adds up to three signals to a single output with fixed gain. A circuit simulation for you is >>HERE<<.

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

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

There are also disadvantages:

1. it calls for more i/o connectors
2. only simple circuits are possible
3. 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.

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, logic operators, auxiliary modules, etc.

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. Another is the application of Dymo stickers... it's up to what works best with you ;)

Now that all blocks have been revealed, we can start building our new set of full eurorack modules.

Voltage Controlled Multiple

Multiplying a signal n-times is very useful, but it could be even more useful to automate it's output level. Use your LFO or ADSR to vary the input voltage level and fed this modulator to your filter, VCA (vibrato anyone?) and/or other destinations at the same time for some cool effects.

To realize this module we are in the need for:

1. 2x "4 jacks" front blocks
2. 1x voltage controlled attenuator block
3. 1x buffered multiple block
4. 1x power filter block
5. 1x BLK2 faceplate

Connect the voltage controlled attenuator half block to the buffered multiple block input through a patch cable.

Voltage Controlled Noise Generator

A voltage controlled noise generator for special modulation pourpouses :)

To realize this module we are in the need for:

1. 2x "4 jacks" front blocks
2. 1x voltage controlled attenuator block
3. 1x noise source block
4. 1x power filter block
5. 1x BLK2 faceplate

Connect the voltage attenuator half block to the noise source half block input through a patch cable.

DC Offsetter with Polarity Indicator

The DC offsetter is something needed to finely offset a voltage to be sent, e.g., to oscillator PWM inputs or filters CV inputs. With a polarity indicator we can monitor how far from the middle line we are (how much offset we are sending) and see how the modulating incoming voltage (e.g. a sine wave) varies with time.

The module calls for:

1. 1x "pot + 2 jacks" front block (see ep.1)
2. 1x DC offsetter block (see ep.1)
3. 1x "LEDs" front block
4. 1x polarity indicator block
5. 1x power filter block
6. 1x BLK3 faceplate

Connect the DC offsetter half block to the LED indicator block input through a patch cable.

Single Stage Audio Phaser

A simple single-stage phaser is possible through the use of a phaser module from ep.1 and non attenuated fixed gain mixer from the new lot. Sort of MXR Phase 45 with single phasing stage and manual modulation.

To realize this module we are in the need for:

1. 1x "pot + 2 jacks" front block (see ep.1)
2. 1x phaser block (see ep.1)
3. 1x "4 jacks" front block
4. 1x mixer block
5. 1x power filter block (see ep.1)
6. 1x BLK3 faceplate

These are only some possible combinations of blocks to realize new modules that adds to those described in Episode 1. More than these are anyway possible, the limit being your creativity :)

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

The sponsorship made possible to turn "modular modularity" from the status of "idea" into reality in first place, then make it even more interesting with the addition of new circuit blocks.

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 is part of their portfolio of services so one could create a full finished product, all in one place!

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 ;)

All Gerber files, sketches and utilities I realized for this project are stored >>HERE<< (Github). I always upload the most recent file's versions, so it could be the case that some PCB looks a little different from those published in this instructable.

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 push the development of new projects.

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