Audio send and receive

Audio send and receive: Front view
Audio send and receive: Front view

This module is thought for sending and receiving audio signals. I use it to add some external effects to my modular. Albeit simple, it is very useful. For connecting pedals and external CV sources I recommend my Pedal/CV Send and Receive module.

Specs and features

  • Attenuates the out signal to line level
  • Amplifies the input signal to modular level
  • Amplitude control for in and output
  • Runs on +/-15V and +/-12V without changes
  • Power consumption around 10mA each rail

The documentation and the Gerber files for download can be found in my website.

Audio send and receive: Side view
Audio send and receive: Schematic

The outgoing signal is attenuated 1:4 with IC1B. P1 sets the output signal level. The incoming signal is amplified 1:7 with IC1D. P2 sets the input signal level.

Audio send and receive: Side view
Audio send and receive: Side view
Audio send and receive: Side view

Voltage controlled quadrature LFO

Voltage controlled quadrature LFO front view
Voltage controlled quadrature LFO

I want to rotate sound through four channels of my modular synthesizer. Or move successive through different CV or audio sources. This is easily achieved with a quadrature LFO and four VCA. The core of this voltage controlled quadrature LFO was published in Electronotes EN#122 pg13ff 1981 and designed by Thomas Henry. I took the core and added the voltage control and the sine shapers.

Specs and features

  • Four triangle quadrature outputs, 90° apart
  • Four sine quadrature outputs, 90° apart
  • Voltage controlled
  • Hi-Low range switch
  • Frequency from 30Hz down to some minutes
  • FM lin
  • FM log
  • Runs on +/-15V and +/-12V
  • Power consumption around 30mA each rail

The documentation and the Gerber files for download can be found in my website.

Voltage controlled quadrature LFO schematic 01
Voltage controlled quadrature LFO schematic 01
Voltage controlled quadrature LFO populated PCB
Voltage controlled quadrature LFO schematic 02

The voltage control part and the sine shaper are well known designs. The triangle core is commonly used as well. The interesting part is how the 90° triangle is derived. The Electronotes EN#122 gives a very detailed description what is going on.

Voltage controlled quadrature LFO populated PCB
Voltage controlled quadrature LFO triangle output screenshot
Voltage controlled quadrature LFO triangle output screenshot
Voltage controlled quadrature LFO sine output screenshot
Voltage controlled quadrature LFO sine output screenshot
Voltage controlled quadrature LFO back view
Voltage controlled quadrature LFO back view

Quad waveshaper for trapezoid quadrature thru zero VCO

Quad waveshaper for trapezoid quadrature VCO
Quad waveshaper for trapezoid quadrature VCO

This is the waveshaper for my Trapezoid quadrature through zero VCO. It gives the quadrature outputs for triangle, sine and the outputs for saw (ramp up, ramp down) and pulse. To use it you need my Trapezoid quadrature VCO. The waveshaper has no external input for waves, it is internally connected with the Trapezoid quadrature VCO

Specs and features

  • Four triangle quadrature outputs
  • Four sine quadrature outputs
  • Two saw (ramp up) outputs 90° apart
  • Two saw (ramp down) outputs 90° apart
  • Two pulse outputs 90° apart
  • Voltage controlled pulse width
  • Runs on +/-15V and +/-12V
  • Power consumption around 50mA each rail

The documentation and the Gerber files for download can be found in my website .

Schematic 01 quad waveshaper for quadrature thru zero VCO
Schematic 01 quad waveshaper for quadrature thru zero VCO
PCB quad waveshaper for quadrature thru zero VCO
Schematic 02 quad waveshaper for quadrature thru zero VCO

The triangle waves are created by algebraically averaging two trapezoid waves 90° apart. This is done here with IC3A for 270° and 0°. IC3C adds and averages 90° and 0°. You can use other combinations as well. IC3B and IC3D gives the inverse triangle waves. The sines are derived from the triangle with well known “old style” circuitry. The ramp outputs are build from two triangles 180° apart, level shifted and switched between them with the square wave. IC5A takes the 90° triangle, shift it up to 0..10V and lowers the amplitude to 0..5V. IC5B takes the 270° triangle shift it down to 0..-10V and lowers the amplitude to 0..-5V. IC6 (DG409) switches between this two triangles with means of the 270° square. Switching in the right moment put the needed parts of the triangle back together to the saw. The pulse outputs are done with the usual technique moving the switching point of a comparator around with the ramp wave.

PCB quad waveshaper for quadrature thru zero VCO
Triangle screenshot from quad waveshaper for quadrature thru zero VCO
Triangle screenshot from quad waveshaper for quadrature thru zero VCO
Sine screenshot from quad waveshaper for auadrature thru zero VCO
Sine screenshot from quad waveshaper for auadrature thru zero VCO
Saw screenshot from quad waveshaper for auadrature thru zero VCO
Saw screenshot from quad waveshaper for auadrature thru zero VCO
Pulse screenshot from quad waveshaper for auadrature thru zero VCO
Pulse screenshot from quad waveshaper for auadrature thru zero VCO
Quad waveshaper for auadrature thru zero VCO back view
Quad waveshaper for auadrature thru zero VCO back view

Trapezoid quadrature VCO

Trapezoid quadrature VCO front view
Trapezoid VCO front view

I always wanted a VCO with through zero capacity. Why not combining this with a unusual wave form and quadrature outputs? Usable as LFO as well? I found the original article and schematic about the Trapezoid VCO on Don Tillman’s site (Link to original article from 19 July 2003). The article consists off three parts with the core implementation in part 2. I kept the basic idea and changed nearly everything else. I use an other exponentiator scheme and temperature stabilization. The addressing hardware for the DG409 is changed. A LFO option is added. Another reference voltage device is used. And quadrature square outputs are implemented. The additional wave forms triangle, sine, ramp up, ramp down and pulse are covered in an extra module

Specs and features

  • Trapezoid quadrature output
  • Square quadrature output
  • Through zero modulation
  • V/Oct, FM log and trough zero CV input
  • Temperature compensated
  • Voltage controlled pulse width
  • LFO Range switch
  • Coarse and fine frequency setting
  • Runs on +/-15V and +/-12V
  • Power consumption around 65mA each rail

The documentation and the Gerber files for download can be found in my website.

Trapezoid VCO schematic
Trapezoid VCO schematic

J. Donald Tillman did an excellent job describing the core of his Trapezoid VCO. Please refer to the original article as linked above. The exponentiator I use is a well known and classical design. You can find many description of it out there. The rest is straight forward. The LFO option is implemented with the DG202. Here are just four capacitors added switchable in parallel to the audio frequency capacitors. The connectors shown are for adding the waveform module to generate triangle, sine, ramp up, ramp down and pulse.

Trapezoid VCO populated PCB
Trapezoid VCO populated PCB
Trapezoid VCO quadrature output
Trapezoid VCO quadrature trapezoid output
Trapezoid VCO quadrature square output
Trapezoid VCO quadrature square output
Trapezoid VCO through zero modulation
Trapezoid VCO through zero modulation
Trapezoid VCO through zero modulation
Trapezoid VCO through zero modulation

Voltage controlled LFO: Flat Version

Voltage controlled LFO front view
Voltage controlled LFO

This is the flat version of my VC-LFO I’ve build this flat version to minimize the depth of the module and avoid the wiring for the potentiometers. A VC LFO with multiple synced output waveforms is a very useful and versatile module. You can’t have enough of them. They can add a lot to sounds making them more animated. This one provides triangle, ramp up, ramp down pulse. square and sine wave output (-5V to +5V). The frequency range is easily adjusted to your needs from some minute per cycle up to 700Hz. I started with the VC LFO design form Ray Wilson MFOS but changed the exponentiator and pulse adjust schematic completely. I have added a range switch and a linear FM input as well.

Specs and features

  • Synced triangle, ramp up, ramp down, pulse, square and sine wave output
  • Output -5V to +5V
  • log and lin CV input
  • Temperature compensated
  • Voltage controlled pulse width
  • Range switch
  • Coarse and fine frequency setting
  • Runs on +/-15V and +/-12V
  • Power consumption below 10mA each rail

The documentation and the Gerber files for downloadcan be found in my website.

Voltage controlled LFO schematic
Voltage controlled LFO schematic back PCB
Voltage controlled LFO schematic
Voltage controlled LFO front PCB

C1 and associated components comprise a linear voltage to log current converter. IC1A sums the control voltages. IC1B provides the temp compensation realized with KTY81-110. TR3 adjusts the V/Oct characteristic. Q1 and Q2 forms the log converter with IC1D as constant current source. IC1C scales the control voltage for the linear FM input. The transconductance of IC1OTA1 controls the frequency of the oscillator. IC2C, C1 and associated components comprise an integrator. When current flows into IC1OTA1 output the integrator ramps up, when current flows out of IC1OTA1 the integrator ramps down. When the integrators output goes above the threshold of comparator IC2D its output goes high. The output of IC2D is fed to the non-inverting input of IC1OTA1 OTA through D1, D2, R1, R2 and TR1. TR1 balances the current flowing during the high and low periods of IC2D. With TR1 you can adjust the symmetry of the triangle. While IC2Ds output is high current flows out of IC1OTA1 OTA and the integrator ramps down until the voltage at the input of IC2D goes low enough to overcome the hysteresis provided by R13 and its output goes low. When this happens the comparator starts to ramp up again and thus we have a triangle wave at the output of IC2C. The bias of the comparator IC2D is controlled by the current generated by the linear voltage to log current convertor. This controls the current that flows in and out of IC1OTA1 and thus the frequency of the oscillator.

The sawtooth is created by mixing portions of the original triangle wave and an inverted version of the triangle wave. N-FETs Q1 and Q1 are used as analog switches.

Voltage controlled LFO screenshot waveforms: ramp up, pulse, square
Voltage controlled LFO screenshot waveforms: ramp, pulse, square
Voltage controlled LFO screenshot waveforms: triangle ramp down, sine
Voltage controlled LFO screenshot waveforms: triangle ramp down, sine
Voltage controlled LFO back view
Voltage controlled LFO back view
Voltage controlled LFO side view
Voltage controlled LFO side view

Dual PSU with 3 pole external AC input

Dual voltage PSU with 3 pole external AC input
Dual voltage PSU with 3 pole external AC input populated PCB side view

For a standalone project with a small case I was in need of an small dual voltage PSU. I can’t find a dual voltage wall wart, but I had an external AC Power Adapter from Yamaha the PA-30. The PA-30 has a 2x18V AC output with a three pole connector. So I decided to build a small PSU with the LM317L and LM337L. The design is straight forward. It is easy to expand for more current and other voltages if you need to. If you don’t want to use a external AC Power Adapter you can use a center tapped transformer.

Specs and features

  • Input: Select 3 pole AC power adapter like Yamaha PA-30 or center tapped transformer according to your needs
  • Output: +/-xV; Voltage depends on the voltage divider around LM317 and LM337. Values given here are for +/-15V.

The documentation and the Gerber files for download can be found in my website.

Dual voltage PSU with 3 pole external AC input schematic
Dual voltage PSU with 3 pole external AC input schematic

Nothing special here. Just refer to the data sheet of the LM317L/LM337L. Calculate R1/R2 and R3/R4 to your needs. The given values here are for +/-15V. Input is from a center tapped transformer or external AC Power Adapter like the Yamaha PA-30. For more current replace the LM317L/LM337L Version with the TO220 version. You’ll need to redesign the PCB then

Dual voltage PSU with 3 pole external AC input populated PCB top view
Dual voltage PSU with 3 pole external AC input populated PCB top view

12V to 5V gate converter

12V to 5V gate converter schenatic
12V to 5V gate converter front panel

This utility module converts a 12V gate to a 5V gate. It is needed when you have a module with 12V gate output and your receiving module only accepts 5V gate voltage.

Specs and features

  • Converts 12V gate to 5V gate
  • Runs on +/-12V

The documentation and the Gerber files for download can be found in my website.

12V 2 5V gate converter schematic
12V 2 5V gate converter schematic

The 12V input is divided down with the input voltage divider to 5V and buffered.

12V 2 5V gate converter populated PCB
12V 2 5V gate converter populated PCB
12V 2 5V gate converter side view
12V 2 5V gate converter side view

Send and Receive

Send and Receive front view
Send and Receive front view

As a wind player I need to keep my hands on the wind instrument. This module was first thought for connecting pedals to my modular for changing control voltages. But it is useful to include external signal changers like echo or delay or any other sound and CV source as well.

Specs and features

  • Connects a banana system to external sources.
  • Runs on +/-15V and +/-12V without changes

The documentation and the Gerber files for download can be found in my website.

Send and Receive schematic

I think you need no description here.

Send and Receive side view
Send and Receive back view

Connecting a Banana System with 3.5 Eurorack

Banana to 3.5 Eurorack front view
Banana to 3.5 Eurorack front view

This module is needed for easy connecting a banana system to a 3.5 eurorack system. It is a passive module. No signal changes are made.

Specs and features

  • Connects a banana system to 3.5 eurorack
  • Passive. No signal changes

The documentation and the Gerber files for download can be found in my website.

Banana to 3.5 Eurorack connection schematic
Banana to 3.5 Eurorack connection schematic

I think you need no description here.

Banana to 3.5 Eurorack connection populated PCB
Banana to 3.5 Eurorack connection populated PCB
Banana to 3.5 Eurorack connection side view
Banana to 3.5 Eurorack connection side view
Banana to 3.5 Eurorack connection back view
Banana to 3.5 Eurorack connection back view

24dB VCF LP/HP with gain loss compensation

24dB VCF LP/HP with gain loss compensation at high Q


VCF This is a 24dB lowpass / highpass with gain loss compensation for high Q. This one is basically derived from my 24dB VCF LP/HP which i build for my Next Generation Formant Elektor project. I just added the compensation circuitry from my Moog Ladder filter to compensate for the volume loss when Q is turned up. I have brought out all 4 filter stage outputs. Depending on your wiring you can use a switch to select between the outputs or/and bring all outputs out in parallel. The LP/HP switching is done with electronic switches on the PCB to avoid the problems (hum, noise…) of the wiring with a mechanical switch.

Specs and features

  • 24dB voltage controlled low pass and high pass filter
  • Switchable output 6dB, 12dB, 18dB, 24dB
  • Volume loss compensation with high Q
  • 10Vpp signal level
  • Voltage controllable Q
  • Voltage controlled lin and log timbre modulation
  • Positive and negative ENV control with sign changer
  • Runs on +/-15V and +/-12V (with minor resistor changes)
  • Power consumption below 60 mA each rail

The documentation and the Gerber files for download can be found in my website.

24dB VCF LP/HP with gain loss compensation at high Q: Schematic back PCB .
24dB VCF LP/HP with gain loss compensation at high Q: schematic front PCB

Straight forward design. Four state variable filter cells are connected together in series, The output of each filter cell is brought out. There are a lot descriptions of those state variable filters out there. I feel no need to add another one. The resonance (Q) is voltage controlled with means of the OTA IC2OTA1 in the upper right corner (page 1). To compensate the volume loss when the resonance (Q) is turned up a second OTA (IC2OTA2) is used. This two OTA shares the same Iabc source. The amplification of this second OTA is increased when Q is going high and add volume to the output signal.

24dB VCF LP/HP with gain loss compensation at high Q: back view
24dB VCF LP/HP with gain loss compensation at high Q: populated front PCB
24dB VCF LP/HP with gain loss compensation at high Q: populated back PCB
24dB VCF LP/HP with gain loss compensation at high Q:side view