Microrack Modular Synth: Ring Modulation With LogicBoard From MH-EDU-Electronics

Microrack is a series of modular analogue synthesizer modules in a very small format designed to plug into a standard 830 point breadboard. The modules use Eurorack-style control voltages with connectivity provided by single conductor jumper wires and a common ground.

This article shows how to make a patch (module configuration) using ring modulation. Ring modulation can be performed with the mixer module which provides three voltage-controlled amplifiers (VCA). This article uses a different approach, an XOR logic gate on an external MH-EDU-Electronics LogicBoard. This may be useful if you own a LogicBoard or some logic gate chips and don't own a mixer module!

1. Microrack (check the shipping time)
2. Synth Starter Kit: Microrack
3. 2x oscillator modules: Microrack
4. 1x envelope module: Microrack
5. A ring modulator
6. MH-EDU-Electronics LogicBoard 2: Tindie (this article uses the version 1 board)
7. or CD4070BE chip (XOR gates) direct onto the breadboard
8. or mixer (vca) module: Microrack
9. An extra full length (830 point) breadboard with continuous power rails: Microrack | Mouser (BusBoard)
10. 2x male-female 20cm (8in) jumper wires, one red, one black, for connecting power to LogicBoard (this must be done with care)
11. A good quality power supply and USB-C power leads are useful if you are powering many modules.
12. USB-C tablet/smartphone/laptop charger which features Power Delivery. This may be marked with PD, or is sold as "fast charger" or have a wattage of 15-20W or more.
13. A 12V wallwart power supply rated at minimum of about 2.5A.
14. A Raspberry Pi power supply with USB-C connector.

Ring modulation is a form of signal mixing where the two signals, often a carrier and a signal, are multiplied together as can be seen in the plot above of a sine wave and a variable frequency square wave.

The Microrack (vca) mixer module which can perform multiplication in Attenuverter mode to act as a ring modulator. This module is in the Explorer Kit but not the Starter Kit.

An approximation** of one channel of the mixer is shown above in the Falstad Circuit Simulator for both Attenuverter (BI LED) and Attenuator (UNI LED) modes. The y scales on the four plots on each screenshot are not identical making it a bit visually misleading. The P-P (peak to peak) voltages give an idea of the relative magnitudes. The simulation of the Attenuator mode shows some clipping on the troughs of the output at certain gain pot settings which may not occur on the real module.

** The opamps are modelled as 741s, the power to the opamps doesn't include the drop from schottky protection diodes, the potentiometer value is a guess, the diode characteristics won't match and there are probably many more small differences.

For the specific case of a unipolar, full amplitude (0V to 5V) square wave an XNOR logic gate is equivalent to multiplication of a bipolar equivalent signal. This can be seen from the plots above comparing multiplication of a bipolar signal with logic gates with unipolar input.

An XOR gate (on an external board) is used in this patch as the inversion of the output isn't important.

Powering the LogicBoard

The MH-EDU-Electronics LogicBoard can be powered from the Microrack's power rails. This is convenient and it ensures the LogicBoard is always powered when the Microrack synth is in use. The jumper wires for power should be connected with the power off.

1. +5V breadboard rail to 1 on LogicBoard - red wire.
2. GND breadboard rail to 0 on LogicBoard - black wire.

The LogicBoard may be damaged if these are the wrong way around or the wrong power rails are used!

The GND- and VIN+ pins below the word EXTEND on the module look tempting on the power module but this is the voltage going into the module. For USB there can be a drop below 5V particularly with the high currents required for many Microrack modules and for barrel jack power it can range from 5V to 12V making these pins unsuitable.

Powering the Power Module

A small but important detail is the way the power module works. It takes the input voltage between 5V and 12V and

1. increases it as necessary to produce the +12V and -12V rails using a boost converter and
2. decreases it as necessary to produce the +5V rail using a buck converter.

One issue with this approach is if the input voltage is below 5V. The USB voltage has a permissible range of 4.75V to 5.25V but the voltage can drop considerably when high currents are used due to the effective output impedance (often from current limiting) and the resistive losses in the cable. This can be seen from some measurements of typical power supplies in order from good to poor where input is the voltage measured at the GND- and VIN+ pins of the power module and rail is measured between the +5V and GND rails on the breadboard.

1. Variable wallwart 2.5A PSU set to max 12V: input 11.93V, rail 4.86V
2. Smartphone USB-C charger + good quality USB-C lead: input 8.64V, rail 4.86V
3. Raspberry Pi 15W PSU: input 4.73V, rail 4.46V
4. 5000mAh power bank (charged recently) with very short lead: input 4.45V, rail 4.19V
5. 5000mAh power bank with long lead: input 4.20V, rail 3.95V
6. Power strip USB type A with long lead: input 4.14V, rail 3.89V
7. Desktop computer USB3 (blue) type A with long lead: input 3.93V, rail 3.67V
8. Desktop computer USB2 type A with long lead: input 3.86V, rail 3.61V

There is a potential problem here with powering the logic chips at a lower voltage than their input voltages. The data sheet for the CD4070B logic gate indicates the permissible input voltage ranges are between -0.5V and Vdd+0.5V where Vdd is the logic gate supply voltage. For the last power supply the rail voltage is only 3.89V meaning 0.0V to 5.0V from a unipolar square wave will exceed the 3.89+0.5=4.39V limit.

Only the first two power supplies are truly safe to use when powering the LogicBoard. The third is likely to be fine for non critical use.

This patch is inspired by preset 46 on the Elka Synthex.

This patch uses two oscillator modules (osc2 and osc3) at VCO rate with the unipolar square wave output going to an XOR logic gate on an external board. The output from the XOR gate is then mixed with osc2 output at a filter module with the CUTOFF being controlled by an envelope module. A second envelope provides a one-off downward ramp which is used to slide the osc2 note via frequency modulation (FM) like the Elka Synthex's unique glide function**. An oscillator in LFO mode, osc1, drives pulse-width modulation (PWM) on the osc2 using a bipolar triangle wave.

The module settings for the buttons and potentiometers can be see in the high resolution photograph above. The jumper cables are listed below.

1. osc1 triangle out to osc2 SHAPE in (red).
2. osc2 square out to LogicBoard XOR top input (yellow***).
3. osc3 square out to LogicBoard XOR bottom input (yellow***).
4. envelope1 out to osc2 FM (blue).
5. envelope2 out to filter CUTOFF (blue).
6. filter out to left input on audio 3.5 (green).
7. stylo
8. 5V to stylus lead (black).
9. CV to osc2 PITCH (white***).
10. CV to osc3 PITCH (white***).
11. GATE to envelope1 GATE (blue).
12. GATE to envelope2 GATE (blue).
13. LogicBoard XOR output to filter IN (green).
14. LogicBoard XOR top input to filler IN (green). This is really the output from osc2 - this allows a short cable to be used.
15. 5V power rail (red) to LogicBoard 1 (red****).
16. GND power rail (red) to LogicBoard 0 (black****).

Modules: oscillator (osc) 1 oscillator (osc) 2 oscillator (osc) 3 power envelope 1 envelope 2 output 3.5 stylo

USB measured current: 1.09A.

** The envelope's output curve differs to the Synthex's glide making it only an approximation.

*** The yellow and white cables are longer.

**** These are additional 20cm (8in) jumper cables not included in the Starter Kit.

The modules need to be plugged in to make the patch and then left powered on for about 20-40 minutes to reach a stable temperature before tuning is attempted.

The frequencies for the oscillators are shown below. These should be set with the stylus held on the low C key or clipped/attached to the adjacent pin.

1. oscillator 1 (left) - this is an LFO and should be set to whatever sounds good, probably about 1/4Hz.
2. oscillator 2 (middle) - tune this to C2 (65.41Hz).
3. oscillator 3 (right) - tune this to six times the value of oscillator 1, i.e. 392.46Hz which is close to (equal temperament) G4 (392.00Hz). This is difficult to do because the oscillator is in FM mode causing the fine tuning feature to be unavailable.

If oscillator 3 is not tuned well then the patch will sound gritty.

The photograph above was taken by Karen Zhao.

In the video above the oscilloscope shows the audio output in cyan (CH2) together with an FFT in red showing the audio spectrum. CH1 in yellow (with yellow wire) is used to show the output of the low frequency oscillator (left) which is modulating the duty cycle of the square wave of the first audio-rate oscillator (middle).

1. 00:00 Microlaser Harp patch.
2. 00:03 Tapping C2 key with stylus.
3. 00:08 Chromatic scale.
4. 00:24 Rendez-rack 2...
5. 00:45 Ran out of keys :(

This is a very small one octave keyboard which must be played with a wire acting as a stylus.

The CV (control voltage) output is between 0.0V and 1.0V depending on which key is pressed with a wire connected to the +5V pin on the module. The voltage can also be applied to the adjacent pins. The GATE output is high while the key is pressed. The module is designed to hold the last control voltage.

Measurements

1. USB measured current: 0.10A (stylus on key).
2. Unloaded note voltages (voltage in brackets is 5s after release):
3. c: 13.2mV (19.9mV)
4. c#: 97.0mV (102.4mV)
5. d: 180.7 (189.7mV)
6. d#: 264.8 (273.8mV)
7. e: 348.6mV (355.5mV)
8. f: 432.9mV (438.7mV)
9. f#: 516.4mV (522.4mV)
10. g: 600.4mV (605.9mV)
11. g#: 684.4mV (688.3mV)
12. a: 768.9mV (774.0mV)
13. a#: 853.1mV (857.7mV)
14. b: 936.9mV (941.4mV)
15. c: 1021.1mV (1025.4mV)

Observations

1. The CV voltage stays at the value for the last note played although this does immediately rise a bit after the stylus is lifted and then it very slowly drifts towards 205mV - the drift rate is so low this would never affect music.

Tuning

The voltages are shown below. The Actual values are from the measurements with the stylus held on the key, the Expected values are calculated using 1/12V for a semitone. The Tuned values are those with the assumption the patch was tuned to the low C. The Error is the difference between the tuned and expected value in cents (1/100th of a semitone). The standard deviation of the error is surprisingly low.

stylus keyboard (stylo) module: documentation | shop | forum thread | schematic (v1.1)

Other articles/videos on the Microrack:

1. DIY Electronic Music Blog: Microrack – First Impressions
2. Parks and Rex: Microrack! - Demo and First Thoughts (YouTube) - more modules on display than just the Synth Starter Kit, at 03:53 the single breadboard is replaced by the rack chassis.
3. Cinematic Laboratory: MicroRack - Modular For The Masses? | Starterkits Ep. 14 (YouTube)
4. Nosnibor Senoj (YouTube)
5. an early Microrack patch
6. two oscillators & two oscillators (again)
7. An (unofficial) Microrack patchbook app.

Similar systems and ones for learning about electronics for audio synthesis:

1. Eurorack
2. AE Modular
3. Korg littleBits synth kit (sadly discontinued)
4. Bread Modular
5. Moritz Klein Instruments
6. Synthux Academy
7. Clacktronics: Build Your Own Modular

Microrack performances:

1. Ray State and Radio Revolution: Coreglitch — Live on Microrack at Ray State Wave (YouTube)
2. Many Small Functions: Quick jam with MicroRack and Beat Friend (YouTube)
3. Fedir Tkachov Music: Microrack modular ambient (YouTube)

Ring modulation:

1. Magnus Gladen: Jean Michel Jarre Laser Harp sound by Elka Synthex (YouTube)
2. Sweetwater: Ring Modulation – Synth Clips 21 – Daniel Fisher (YouTube)
3. Alex Ball: Dalek Voice Tutorial (YouTube)
4. Analog Devices: Activity: Diode Ring Modulator - ADALM2000

Logic gates:

1. Adafruit Learn (1-7 series)
2. Digital Circuits 1: Binary, Boolean, and Logic
3. Digital Circuits 2: Combinatorial Circuits
4. Digital Circuits 4: Sequential Circuits
5. Instructables Collection: Logic Gate Boards