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Carl Becker
FUSION CORE #99: Build log

FUSION CORE #99: Build log

·1860 words·9 mins
Analog Circuit Design PCB Manufacturing
Table of Contents

Hi!

Here, I post updates on my synthesizer development every once in a while. Have a look! I don’t do direct technical descriptions over here, that’s for separate blog-posts in the future.

2026-05: Acquire BNC cable & adapters to make use of the frequency generator
#

Try amplitude modulation on the function generator
Figure 1: Try amplitude modulation on the function generator
Try out BNC adapters
Figure 2: Try out BNC adapters

2026-04: Abusing my step sequencer as a Midi-Controlled voltage reference
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  1. A Python Script on my computer:
    1. Reads a Midi file
    2. Extracts one voice
    3. Sends Midi Commands to the Beatstep Pro connected over USB
  2. The Beatstep Pro
    1. Receives the Midi Commands
    2. Directly converts the Midi Commands over to an analog 1V/Oct voltage reference
    3. The voltage reference is put into my synth VCO
  3. My synth VCO
    1. receives the voltage reference
    2. Generates a sound signal with an according frequency

Plus some extra effects!

Video 1: Do you recognize the melody?

2026-04: Acquire my own oscilloscope
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It's 4 channel, 70 MHz oscilloscope: Siglent SDS804X HD
Figure 3: It's 4 channel, 70 MHz oscilloscope: Siglent SDS804X HD
Current setup of lab devices
Figure 4: Current setup of lab devices

2026-02: Setting up an automated SPICE simulation using KiCad and PySpice to understand the VCF schematic
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Filter schematic in KiCad
Figure 5: Filter schematic in KiCad
Using NGSPICE, PySpice and Matplotlib for analysis
Figure 6: Using NGSPICE, PySpice and Matplotlib for analysis
Getting weird, funny graphs
Figure 7: Getting weird, funny graphs

2026-01: Visit Look Mum No Computer Concert, again!
#

It was nice!
Figure 8: It was nice!

2025-11: Acquired new lab devices from employer
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  • Fluke 81 Function Generator
  • TTi EL302RT Triple Power Supply

2025-08: First proper jam on the modular synth!
#

Video 2: An unpolished, but complete piece!

I extended my physical, analog synth with some VCV Rack modules to add the beat and some reverb effects.

2025-08: Buy VCF PCBs from Look Mum No Computer as a reference for my filter design
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Look Mum No Computers Filter, Front
Figure 9: Look Mum No Computers Filter, Front
Look Mum No Computers Filter, Back
Figure 10: Look Mum No Computers Filter, Back

I needed to cut of the top of the yellow PCB so that the module fits inside my case! I also therefore cut some traces, which I reconnected afterwards using wires. It’s pretty tight though!

2025-08: Design some utility modules
#

Simple mixer (without voltage control of the volume)

  • 4 sound inputs with potentiometers volume control
  • 1 sound output
  • not yet manufactured & built
Render of mixer module PCB
Figure 11: Render of mixer module PCB

Simple math module

  • Functions
  • Invert sound
  • remove offset voltage
  • add offset
  • attenuate / amplify (using potentiometer as control)
  • not yet manufactured
Render of math module PCB
Figure 12: Render of math module PCB

2025-05: Try to understand VCF (state variable)
#

  • First design on breadboard works, based on LM13700 Operational Transconductance Amplifier, but I want to make sure it works in every situation and the output is in voltage bounds
  • current challenge: Linear to exponential converter
    • if understood, the VCA module is also unlocked for design!
  • I automated the SPICE simulation with python to find variation of cutoff frequency due to input voltage changes
State variable filter schematic from the Music from outer space website
Figure 13: State variable filter schematic from the Music from outer space website
  • trying to understand state variable filter from music from outer space:
Breadboard test of another filter schematic
Figure 14: Breadboard test of another filter schematic
Signal looks promising
Figure 15: Signal looks promising

2025-05: Trying some Class-AB amplifier architectures from The Art of Electronics book
#

Circuit doesn't work as expected
Figure 16: Circuit doesn't work as expected
Class AB amplifier notes
Figure 17: Class AB amplifier notes

2025-01: Design audio output module
#

  • It buffers synth audio signals and attenuates them significantly to line level and use NE5532 OpAmps for headphone output
  • can switch between mono & stereo!
Finished audio output module
Figure 18: Finished audio output module

(I don’t have videos testing the output module individually, but some related here)

Video 3: More testing!
Video 4: Putting reverb effect on power on and off sounds

2024-12: Build a CLI to search in my part inventory
#

  • I ordered too many parts and unfortunately cannot remember all the component IDs! TL072 for my OpAmps is easy, but MCP3008 for an 8 channel ADC is not!
  • You find it here: https://github.com/NEOZEO64/InventoryTool
  • basically, it is only a json parser with some array filtering mechanics
InventoryTool Command Line Interface
Figure 19: InventoryTool Command Line Interface

2024-09: Soldering finished VCO PCBs
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Unfinished VCO module
Figure 20: Unfinished VCO module
Soldered VCO PCB
Figure 21: Soldered VCO PCB
Connected the VCO PCB to the front panel
Figure 22: Connected the VCO PCB to the front panel
Finished VCO modules
Figure 23: Finished VCO modules
Video 5: Testing!
Video 6: Finished VCOs! (Some CV inputs are missing)

2024-08: Design proper working, functional VCO
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Triangle to Sine converter
Figure 24: Triangle to Sine converter
Adding offset functionality with OpAmps
Figure 25: Adding offset functionality with OpAmps
Triangle to sine breadboard setup using transistors (not used)
Figure 26: Triangle to sine breadboard setup using transistors (not used)
Triangle to sine breadboard setup using diodes (this is used instead)
Figure 27: Triangle to sine breadboard setup using diodes (this is used instead)

2024-08: Hack AAAPPPCCC from Look Mum No Computer into my synth
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(easy one-day project)

  • measured dimensions
  • designed some laser cutter guide
  • laser cut at the student research center
  • sprayed black and drew fancy UI
  • soldered on adapter connectors (fortunately the AAAPPPCCC module needs 12VDC for power that can be easily provided from the eurorack plugs, also the audio signal levels are fine)
  • done
AAAPPPCCC Module, Front
Figure 28: AAAPPPCCC Module, Front
AAAPPPCCC Module, Back
Figure 29: AAAPPPCCC Module, Back
Video 7: The AAAPPPCCC sounds weird.

2024-08: Build decoration module
#

  • It was necessary to put my name tag on the synth xD
  • But with RGB leds!
  • Used a Raspberry Pi Pico, powered by the 5V line on the eurorack plug
  • Used WS2812B RGB leds
  • Soldered a simple PCB and wrote a small MicroPython firmware for the Pico to control the LEDs
Lasercutting the front panel
Figure 30: Lasercutting the front panel
Spraying the front panel black
Figure 31: Spraying the front panel black
Thinking about light behind the cutouts, diffused by wax panels
Figure 32: Thinking about light behind the cutouts, diffused by wax panels
Testing the complete circuit before putting it inside the box
Figure 33: Testing the complete circuit before putting it inside the box
Video 8: Complete setup inside test rail, applying reverb effect on the sound

2024-08: PSU PSBs arrived! Solder and test…
#

PSBs yee
Figure 34: PSBs yee
soldering the PSU PCB
Figure 35: soldering the PSU PCB
Testing the PSU with a baking plate below it since the electrolytic capacitors might explode
Figure 36: Testing the PSU with a baking plate below it since the electrolytic capacitors might explode
The PSU fits inside the case!
Figure 37: The PSU fits inside the case!

2024-07: Meet with friend to build the case
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  • Wooden box sprayed black
  • 1 m wide from the inside, 20 cm deep, 20 cm height from the inside
  • materials bought from Bauhaus Germany, for around €55
  • used 2x Adam Hall 6161 as cheap rails
The case inside the workshop of my friend's father
Figure 38: The case inside the workshop of my friend's father

2024-07: Visit the Thomann music store in Bavaria, Germany
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Try out their local modular synthesizers!
Figure 39: Try out their local modular synthesizers!

2024-07: Design proper PSU PCB without the anxiety of shorts because of bad soldering
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  • similar circuitry like the prototype power supply
  • it has a lot of proper 16 pin eurorack connectors
  • it has fuses, a prototype area and debug headers
  • contains very ugly mistake: The main power PCB traces are a little thin
PCB render, top
Figure 40: PCB render, top
PCB render, bottom. Of course with a nice, very important artwork
Figure 41: PCB render, bottom. Of course with a nice, very important artwork

Next challenges

  • Make PCB trace width bigger
  • Connect all Gate / CV Eurorack ports

2024-06: Design first revision of VCO (and do first PCB designing experience)
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  1. Build and validate the circuitry found on the internet

    Testing the VCO on breadboard
    Figure 42: Testing the VCO on breadboard

  2. Design first revision & meet with friend to solder it

    Test first revision of VCO PCB
    Figure 43: Test first revision of VCO PCB
    • had some faults and wrong footprints, I wasn’t happy
    • also, the holes at the corners were a lot too small to mount it anywhere

  3. Design & 3D-print the front panel (with a height of 16 cm)

    3D-Print the front panel, put UI elements in and connect PCB to it
    Figure 44: 3D-Print the front panel, put UI elements in and connect PCB to it

  4. Set up prototype test rail from toy metal kit

    Video 9: Complete setup inside test rail, applying reverb effect on the sound

2024-05: Gather more lab devices!
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(frequency generator & oscilloscope are borrowed from student research center)

A stack of multiple lab power supplies, a frequency generator, an oscilloscope, and a soldering iron power supply.
Figure 45: A stack of multiple lab power supplies, a frequency generator, an oscilloscope, and a soldering iron power supply.

2024-04: Go to Look Mum No Computer Concert in Hamburg!
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Look Mum No Computer Concert in Hamburg
Figure 46: Look Mum No Computer Concert in Hamburg

2024-03: Plan the case
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Draw some fancy ideas!

Maybe a test rail would be cool, so I can easily swap out modules?
Figure 47: Maybe a test rail would be cool, so I can easily swap out modules?
Case plan, page 1
Figure 48: Case plan, page 1
Case plan, page 2
Figure 49: Case plan, page 2

Buy more parts!

More parts
Figure 50: More parts

2024-02: Playing with circuits (voltage controlled oscillators and filters)
#

  • I built some simple VCOs (voltage controlled oscillators)
  • I built a simple VCF (voltage controlled filter)
  • Made a lot of plans
    • I chose the Cosmo standard from Look Mum No Computer for dimensions and electronic properties of signals etc.
    • could not decide on whether the module height of 16 cm or 20 cm is better. 16 cm seemed more handy, but 20 cm are definitely a good decision! You have so much space and the synth takes in the room it deserves!
Video 10: Putting a reverb effect on a single VCO
Big setup, where a Beatstep Pro Sequencer controls the VCO and the computer applies some reverb effect to the VCO's output
Figure 51: Big setup, where a Beatstep Pro Sequencer controls the VCO and the computer applies some reverb effect to the VCO's output
A lot of circuitry chaos
Figure 52: A lot of circuitry chaos
Testing a filter circuit. The resonance looks cool!
Figure 53: Testing a filter circuit. The resonance looks cool!
Video 11: Testing multiple VCOs at once

I also tested some counter IC

Video 12: Testing a counter logic chip
Video 13: Testing a counter logic chip, now in the dark
Putting the counter logic chip on a PCB
Figure 54: Putting the counter logic chip on a PCB

2024-02: Build a prototype of a bipolar ±12V power supply
#

First power supply experiments on breadboard
Figure 55: First power supply experiments on breadboard
Prototype powersupply on stripboard for experimenting
Figure 56: Prototype powersupply on stripboard for experimenting

I didn’t want to use dual 9V block batteries, for a ±9V power supply, so built a wall wart power supply: How does it work?

  1. A 14V AC voltage is fed into this power supply from an external transformer (from Roland, bought from Thomann)
  2. A half bridge rectifier + 6x 4700uF capacitors are used to generate a bipolar, but unstable voltages (If I remember around 20V if unloaded)
  3. For +12V and +5V, step down converters are used (LM2596S) to avoid linear voltage regulators that heat up
  4. For -12V, the L7912 linear voltage regulator is used, as I did not see another easy way

2024-02: Buy, buy, buy!
#

To get started, I needed a lot of materials and books:

  • E-Books from Make
  • The Art of Electronics

And a lot of different electronic parts:

  • Mouser, Reichelt
  • Aliexpress
  • Thomann
  • ElectricDruid (for old analog chips)

The first order cost me around €700 in sum, I felt reallly excited, but also under pressure, whether my plan goes well.

A lot of Aliexpress packages on my desk
Figure 57: A lot of Aliexpress packages on my desk

2023-11: Trying out modular synths in real life
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Hamburg’s Elbphilharmonie offers a modular synth course every once in a while. I took part, it was fun to turn the knobs in real life and play together with other people!

Trying a modular synthesizer from doepfer brand
Figure 58: Trying a modular synthesizer from doepfer brand