Analog Eurorack Sound Synthesizer

I have over the years built several synthesizer modules and complete mini-synths. This one is the second rendition of a synth that I lost in a fire in 2022. Both are based on several ideas that I got a long time ago, and it is actually a complete “proofs-of-concepts”. Since this is the first and probably only version, I have “stuffed” some extra features and 4 more frontpanels into the design. The main purpose now, is to use it as a workbench for testing new module ideas, and this also is why many of the internal signals are also brought to the front-panel:

Here is a little example of the strange sounds it can produce without any patching, with only one CV/GATE-pair input from a Behringer 960 sequencer:

 

All the modules are permanently installed in a single 19″ Eurorack, powered by a Doepfer tripple analog power supply. The bus is a long one, also from Doepfer, but divided into two sections that can be turned on separately from the front panel. In addition the bus CV and GATE can also be separated and connected from the front. The part of the bus that can be separated is for testing new additional modules.

Everything is mounted on a shielding 4 mm aluminium plate that slides into the cabinet on top of the (noisy?) power supplies in the bottom. In the bottom I also have a fan, a fan-speed control and a small class-D stereo amp. An extra 12VDC supply was necessary for the amp, the fans and future “Arduino-things”. These are too power-hungry and noisy to put on the synth power bus.

Finally there are two Joy-It variable V/A regulators (switchmode) with two MeanWell fixed 24VDC supplies as power inputs. This setup is coupled as a split +/- 0-24V 0-4A “lab-supply”. This actually works, but of cause it is not as good as a low noise, analog supply. But it is much less heavy and it serves the purpose for testing less critical stuff.

I was very skeptical about putting two JoyIt regulators “on-top of each other” to create a split supply. Four switchmode regulators tied together sounds a little scary. I could not find anything about it on the web, but I found that the MeanWell DC outputs are galvanically very well isolated from “everything”, and since the only common connection is between +output on one JoyIt and -output on the other, the setup has no unwanted loops. The big question was then if the JoyIt regulators would work well together, if e.g. a load was connected directly between + on one, and – on the other. But this and various unbalanced loads works fine, also with current limits activated. I have for safety put a large diode in reverse across each output, just in case. I have not tested the setup at maximum output yet, but at around 1A everything seems fine.

Here below you can see the whole setup with the Eurorack pulled out:

All the space to the left in the rack is for future modules and some Arduino- and RasPi4-things.

More sound examples:

 

Here is a list of most of the features:

(Please note that functions/signals described with brown text is only available in the special version shown here).

VCO module

The VCO module consists of two circuit boards: The main VCO on one pcb, and some extra “odd stuff” (see 7 and 8) on an AUX pcb. The main VCO board can function separately as a normal eurorack module, but AUX is and extension of the VCO and also interfaces to the internal bus (see 9).

  1. In the main VCO all sound generating circuits are analog, DC-coupled and balanced with no capacitors in the signal paths. Low drift, low noise, high accuracy components are used all over, also resulting in a minimum amount of trimmer-potentiometers.
  2. The main voltage controlled oscillator (VCO) generates a very precise, symmetrical and amplitude-stable triangle over 10 octaves. The following signals are derived at the VCO frequency: Sine-wave, sawtooth, square-wave and pulses. Pulses can be pulse-width-modulated (PWM) from inputs.
  3. A function for STOP-VCO is available on the front panel as well as from the Envelope generator.
  4. The following signals are simultaneously generated at double VCO frequency: Triangle, sine and sawtooth.
  5. The sine-wave shapers are of the JFET-type. Unlike most circuits found elsewhere, the JFETs are coupled in a current mode, completely symmetrical, and they produce sine-waves with negligible upper harmonics.
  6. The VCO has several summed frequency control inputs, both linear and exponential 1V/octave. The exponential conversion is basically done via a super nice (but rather expensive) THAT VCA-chip. The generated exponential signal is also available as a buffered output. The idea is that this can be utilized by other VCO- and VCF-modules for precise frequency tracking.
  7. A very special “Odd-circuit” that I have invented produces upper frequency pulse-multiples, selectable as 1, 3, 5, 7, 9, or 11 (odd) times the VCO frequency. The selected frequency can then be divided by 2  from 1-to-12 times (selected with rotary switch). This divider is of cause not analog, but super strange sounds can be generated so it had to part of the synth. The pulses are alternating positive-negative-going with reference to 0V. In this special version a double frequency square-wave output is also available at the front.
  8. As a special feature in this version, the “Odd-circuit” can be selected to also produce 2, 4, 6, 8 or 10 (even) times the VCO frequency. The selection of both odd and even multiples can be modulated via an analog input. An extra up front analog factor 2 division can also be selected for more stable operation at higher frequencies.
  9. All inputs and outputs from both main VCO and AUX are present at both front panel and on an internal backplane-bus. The bus makes it possible to use most of the main features in the synth without any external patching.

BaseVCO_AUX_XAUX_functional_diagram

Bass module

  1. The output of the “Odd-circuit” is also used to generate bass frequencies following the VCO. The output can be selected as triangle, sine or sawtooth. Very strange sounds can be produced.
  2. A low-frequency-generator (LFO) produces triangle- and square-wave outputs. The frequency can be modulated and the triangle signal can be routed to various modulating inputs.
  3. A very-low-frequency-generator (VLFO) produces a ramp output and an internal pulse signal. The slow ramp can be routed to various modulating inputs.
  4. A white-noise generator is used internally, and also sent to an output.
  5. A random-signal-generator (RND) is based on triggered sampling from the white-noise generator. The trigger can be selected to come from Bass, LFO or VLFO.
  6. A variable DC-signal is controlled from an internal reference chip. Maximum output range can be selected as +5V, +10V or -10V.

BaseBAS_XTRA_functional_diagram

Mix module

  1. A mixer is provided that can pan between input-jack/sine AND double-sine. The mixed signal goes to a voltage-controlled-amplifier (VCA1) that can be selected to work as an amplitude- or ring-modulator (AM or DSB). Modulation can be selectable to come from LFO-triangle, VLFO-ramp, input-jack OR none. Modulation is always summed with potentiometer input and signal from Envelope generator.
  2. A mixer is provided that can pan between input-jack/pulse(PWM) AND ramp/double-ramp/triangle. The mixed signal goes to VCA2. Modulation can be selectable to come from LFO-triangle, VLFO-ramp, input-jack OR none. Modulation is always summed with potentiometer input and signal from Envelope generator.
  3. Input-jack/odd-signal goes to VCA3. Modulation can be selectable to come from LFO-triangle, VLFO-ramp, input-jack OR none. Modulation is always summed with potentiometer input and signal from Envelope generator
  4. Input-jack/bass-signal goes to VCA4. Modulation can be selectable to come from LFO-triangle, VLFO-ramp, input-jack OR none. Modulation is always summed with potentiometer input and signal from Envelope generator.
  5. Outputs from VCA3 and VCA4 are added and sent to output at front-panel.
  6. Outputs from all four VCAs are added together with a signal from input-jack/variable-DC, and sent to output at front-panel. This is considered as the final resulting signal from the synth.

BaseMIX_XTRA_functional_diagram

Envelope module

  1. This is an Arduino Nano based envelope generator, not exactly analog, but as an excuse 🙂 most envelope generators must rely on some sort of logic circuitry.
  2. Six envelopes are generated, each with six steps, where each step can be set individually for duration and final amplitude. The settings can be changed “on-the-fly”.
  3. All six envelope outputs are passed through 3.order filters.
  4. Four envelopes are unipolar, and intended as amplitude envelopes. They are sent to VCA1 (sine-waves), VCA2 (ramps and pulses), VCA3 (Odd) and VCA4 (Bass) or to individual output-jacks.
  5. Two envelopes are bipolar, and intended as frequency and pulse-width modulators. They are sent to VCO frequency control and PW-control or to individual output-jacks. PW-control can in addition be selected to send signal to LFO-frequency modulation.
  6. Triggering the start of the envelopes can be selected to come from GATE, CV or input-jack. If analog input is used the trigger level can be set from the controls. In addition the start-trigger can be delayed individually for the six envelopes.
  7. Individual STOP signals can be sent to VCO and RND/LFO.
  8. A digital AUX output can be generated.
  9. A very useful timed NEXT-GATE signal is generated, and the output can e.g. be used to restart. Both the timing and the total number of NEXT-GATEs can be set from the controls.
  10. As user interface an I2C display, a mode switch and two rotary encoders are provided. In addition a USB-port for programming is also on the front panel.

BaseENV_XTRA_functional_diagram

 

 

Envelope Examples:

VCA-envelopes for 4 different waveforms. Note the exponential attack and decay.

 

Two unipolar VCA envelopes and two bipolar waveforms for frequency and pulse-width