Microrack Modular Synth: Exploring the Starter Kit Patches, Oscillator Stability and Noise Output Spectrum

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 uses control voltages with connectivity provided by single conductor jumper wires and a common ground.

This article follows on from a previous article on the basics and explores the three patches (module configurations) shown on a card which ships with the Synth Starter Kit.

1. Arp Lead.
2. Jacuzzi + The Drone.
3. Bass N drum.

There's also a look at the frequency stability of the oscillator module (v1.3) with temperature and spectral analysis of the output from the noise s&h module.

The black potentiometer knobs on the modules have been marked with white paint to make the settings more obvious in the videos.

Microrack was launched as a Kickstarter project in 2024 with an estimated delivery date of December 2024. The founders demonstrated prototypes at SuperBooth 2024. The project started delivering products in March 2026.

1. Microrack Synth Starter Kit: Microrack

This plays three notes continuously based on the last key touched with the stylus. The first note is at a different pitch. This is like a simple arpeggio function on a synthesizer.

The envelope here acts like an oscillator due its RST output being connected to its GATE input. Its maximum rate is at the lower end of the audio range but here it will typically be used around 2Hz to 5Hz. It is clocking the counter which resets when it reaches the fourth step creating a three step counter. The step 1 output from the counter is connected to the PITCH input on the oscillator to raise the pitch of the first note by frequency modulation (FM), the increase is controlled by the FM/FINE setting. The triangle wave output from the oscillator goes to the output 3.5. The envelope's output is also connected to the SHAPE input on the oscillator producing a more interesting timbre.

Modules: power envelope counter oscillator (osc) output 3.5 stylo

USB measured current: 0.49A.

Video Description

The oscilloscope shows the audio output in cyan (CH2) together with an FFT in red showing the audio spectrum. Channel 1 is used later in the video to show the output of the envelope module - both the wire and the scope trace are yellow.

1. 00:00 arp lead
2. 00:08 shortening notes
3. 00:33 envelope output on scope
4. 00:52 change note length

This makes a gurgling noise with a subtle drone in the background.

The gurgling noise (jacuzzi) is coming from a complex interaction between the pink noise controlling the sample and hold of the oscillator's sawtooth output and this value frequency modulating the oscillator's VCO rate. The envelope acts like an oscillator again. The low audio frequency of the envelope output goes to the delay and also modulates the TIME to create an interesting sounding drone noise. The two sounds go to the left and right inputs of the output 3.5 but are mixed together due to the MONO setting.

Modules: power oscillator (osc) noise s&h output 3.5 envelope delay

USB measured current: 0.72A

Video Description

The oscilloscope shows the audio output in cyan (CH2) together with an FFT in red showing the audio spectrum plus the noise/sample and hold module output in yellow (CH1).

1. 00:00 jacuzzi + the drone
2. 00:14 osc fine pitch change
3. 00:27 osc coarse pitch change
4. 00:41 envelope rate change
5. 01:06 touch pad track experimentation

This produces noisy clicks with a zapping noise which can have a bass drum character every 4 clicks.

The oscillator's square wave output in LFO mode is clocking the counter and making the noise s&h track its blue noise input. The envelope is triggered on step 1 of the 4 step counter. The envelope is controlling the output of the filter via CUTOFF together with the noise s&h output - the latter is effectively modulating the sound's envelope. The filter is self-oscillating and depending on the settings this can create a bass drum like sound. The clicks from the noise s&h and the bass drum/zapping noise from the filter are mixed together at the output 3.5.

If the counter isn't counting it's probably because the SHAPE setting is too low and the square wave is output has a 0% duty cycle, i.e. the output is always low.

Judging by what happens, this patch does not appear to be designed for the track touchpad to be used on the noise s&h module.

Modules: power oscillator (osc) counter noise s&h envelope filter output 3.5

USB measured current: 0.73A

Video Description

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 filter output and then later in the video the noise/sample and hold track input.

1. 00:00 bass n drum
2. 00:10 filter resonance tweaks
3. 00:32 increase LFO rate
4. 00:41 touch pad track experimentation
5. 00:55 envelope release tweaks
6. 01:18 LFO rate tweaks
7. 01:46 track input on CH2
8. 01:52 osc increase duty cycle
9. 02:00 scope timebase changes
10. 02:13 osc reduce square wave width

The Bass N Drum patch perplexingly uses the filter module with no input to produce sound. The high level of the resonance setting causes the filter to self-oscillate at a frequency determined by the CUTOFF setting. The output will be a sine wave but this may end up being clipped due to the high gain. The Moog Foundation explain and demonstrate this with and without keyboard tracking in The Filter- Minimoog Filter Self-Oscillation on YouTube - the latter is often referred to as "playing the filter".

The Microrack filter's output needs to be used carefully when it's self-oscillating as the output voltage can be large, almost covering the -12V to +12V range.

The Microrack oscillator is temperature sensitive and this is most obvious as the components warm up from self-heating when the modules are turned on. The audio frequency of the output in VCO mode is shown in the plot above for the first 47 minutes after not being used for an hour. In this case the oscillator module was placed between a power module and filter module with a small gap. The power module gets particularly hot and may contribute to the heating due to the proximity.

This means it's best to tune oscillators after the temperature of the modules has stabilised which could be 30-40 minutes if they've not been on recently. After this, significant changes in ambient temperature and draughts (the modules aren't cased) may affect them a little. Unrelated to temperature, oscillators may be affected by the patch, therefore (fine) tuning is best performed when the patch is fully setup.

The design of the oscillator and the causes of the oscillator frequency changing with temperature for the version 1.3 module are discussed in Microrack Forum: Oscillator: VCO/LFO.

The noise s&h module produces two types of noise: blue and pink. White noise aims to have the same volume (more precisely, energy or power) across the desired frequency range, in this case the standard audible range, 20-20000Hz. Coloured noise has a different distribution, the descriptions from the Microrack documentation are reproduced below.

Pink noise — has equal energy per octave, resulting in a warmer, more natural sound. Commonly used for audio testing, ambient textures, and as a modulation source.

Blue noise — has increasing energy at higher frequencies, producing a brighter, more hissing character. Useful for adding high-frequency content or as a different flavor of random modulation.

The spectral analysis of the output from the module is shown above in Audacity from a 12 second sample recorded at 48k sample rate. The blue noise rises as it should up to about 3500Hz (A7) and then the slope reverses. The pink noise only falls from 300Hz (D4). Both of the slopes are approximations of the ideal 3dB change per octave.

The image above is a flipped analogue photograph of a tintype using the wet plate collodion process of a Microrack patch made under the tutelage of Guy Paterson.

After exploring and understanding these three patches it's time for your modular journey into sound design to start in earnest.

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)

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)