Trapezoid quadrature through zero VCO (Euro version) with waveshapers

Trapezoid quadrature through zero VCO with waveshapers: Front
Trapezoid quadrature through zero VCO with waveshapers: Front

This is my third take on the Trapezoid VCO core designed by Don Tillman. My first implementation for a 15V banana system with separate waveshaper can be found here. My second implementation for a 15V banana system with integrated waveshaper can be found here.This time I moved on to the 12V Eurorack format. The core is still based on the original design from Don (used with permission). 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. Another reference voltage device is used. A octave switch is added. And quadrature square outputs are implemented. As well as the additional waveforms triangle, sine, ramp up, ramp down and pulse.

Specs and features

  • Trapezoid quadrature output
  • Square quadrature output
  • Triangle quadrature output
  • Sine quadrature output
  • Pulse output, 0deg, 90deg
  • Ramp up output 0deg, 90deg
  • Ramp down output 0deg, 90deg
  • Octave switch
  • Through zero modulation
  • PWM input
  • V/Oct, FM log and trough zero CV input
  • Temperature compensated
  • Fine frequency setting
  • Runs on +/-15V and +/-12V
  • Power consumption around 110mA each rail

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

Trapezoid quadrature through zero VCO with waveshapers: Schematic control PCB
Trapezoid quadrature through zero VCO with waveshapers: Schematic control PCB
Trapezoid quadrature through zero VCO with waveshapers: Schematic main PCB
Trapezoid quadrature through zero VCO with waveshapers: Schematic main PCB
Trapezoid quadrature through zero VCO with waveshapers: Schematic main PCB
Trapezoid quadrature through zero VCO with waveshapers: Schematic main PCB

J. Donald Tillman did an excellent job describing the core of his Trapezoid VCO. Please refer to the original article as linked above. Don Tillman gave me the advice to use only two capacitors in the core. The exponentiator I use is a well known and a classical design. You can find many description of it out there. The rest is straight forward.

Trapezoid quadrature through zero VCO with waveshapers: Populated control PCB
Trapezoid quadrature through zero VCO with waveshapers: Populated control PCB
Trapezoid quadrature through zero VCO with waveshapers: Populated main PCB
Trapezoid quadrature through zero VCO with waveshapers: Populated main PCB
Trapezoid quadrature through zero VCO with waveshapers: Back view
Trapezoid quadrature through zero VCO with waveshapers: Back view
Trapezoid quadrature through zero VCO with waveshapers: Side view
Trapezoid quadrature through zero VCO with waveshapers: Side view
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot square wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot square wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot sine wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot sine wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot triangle wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot triangle wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot pulse wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot pulse wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot triangle through zero out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot triangle through zero out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid through zero out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid through zero out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid through zero out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid through zero out

Sequencer Nostalgia

This was my first build when I came back to SDIY. A three row/16 step sequencer. Completely build on stripboard. Still working after all this years. Only hand sketched schematics. Nothing to publish. Pictures only.

Sequencer three rows/16 steps: front
Sequencer three rows/16 steps: front
Sequencer: Clock close up
Sequencer: Clock close up front
Sequencer: Clock close up back
Sequencer: Clock close up back
Sequencer: Inside
Sequencer: Inside
Sequencer: Inside
Sequencer: Inside

6..36dB VCF Lowpass/Highpass

6..36dB VCF Lowpass/Highpass
6..36dB VCF Lowpass/Highpass

This is the Eurorack version of my NGF 36dB VCF. I have brought out the 6dB, 12dB, 18dB, 24dB, 30dB and 36dB poles. You have two audio inputs for easy mixing sound sources. And CV inputs for linear TM, log TM, envelope, V/Oct tracking and emphasis. The exponential circuit is temperature compensated with KTY81-110. If the 12dB output is patched back to input 2 the filter can serve as a sine oscillator.

Specs and features

  • 36dB voltage controlled low pass and high pass filter
  • Two inputs for easy mixing
  • 6dB, 12dB, 18dB, 24dB 30dB, 36dB output
  • HP/LP switch
  • Positive and negative ENV control with sign changer
  • Temperature compensation with KTY81-110
  • CV inputs for linear TM, log TM, envelope, V/Oct tracking and emphasis
  • Usable as sine oscillator
  • Runs on +/-12V and +/-15V (with minor resistor value changes for best performance)
  • Power consumption around 60mA negative rail, 65mA positive rail

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

6..36db VCF Highpass/Lowpass: Main board schematic
6..36db VCF Highpass/Lowpass: Main board schematic
6..36db VCF Highpass/Lowpass: Control board schematic
6..36db VCF Highpass/Lowpass: Control board schematic

Straight forward design. Six 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.

6..36db VCF Highpass/Lowpass: Side view
6..36db VCF Highpass/Lowpass: Side view
6..36db VCF Highpass/Lowpass: Populated control PCB
6..36db VCF Highpass/Lowpass: Populated control PCB
6..36db VCF Highpass/Lowpass: Populated main PCB
6..36db VCF Highpass/Lowpass: Populated main PCB

Utility Mixer I

Utility Mixer I
Utility Mixer I

This is an often used utility module. The mixer comes in handy for mixing CV sources. The mixer is DC coupled, so you can use it for DC and AC mixing. The input impedance is constant 1MOhm. The input signals are amplified by a maximum factor of two. It is possible to offset every input with +/- 5V. The offset is signaled with LED’s for every channel and for the summing output. The outputs are normalized, so you can remove selected channels from the output mix. The summed output has a -6dB switchable attenuator. There is an inverted summed output added as well. The added volume indicator us useful for finding the appropriate signal level.

Specs and features

  • Three inputs, three outputs
  • Inverted and non- inverted summed output
  • 2x amplification
  • +/- 5V offset for every channel
  • -6dB switch
  • Volume indicator
  • Normalized outputs
  • Runs on +/-12V and +/-15V
  • Power consumption below 20mA each rail

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

Utility Mixer I: Main board
Utility Mixer I: Main board
Utility Mixer I: Control board
Utility Mixer I: Control board

Control board: Straight forward design. The mixer is completely DC coupled. So you can use it for CV mixing as well as audio mixing. IC1B,C,D buffers the three inputs and and keeps the input impedance constant. P2, P4, P6 sets the amplification from zero to 2X. P1, P3, P5 sets the offset voltage. The LED indicates the offset and signal level.

Main board: IC3B, IC4B, IC6B adds the offset voltage and the signal. IC1C, IC1D, IC1B, IC1B drive the low current LED. IC2A sums the signals and drives the negative output. IC2B drives the positive output.

Utility Mixer I: Populated control board
Utility Mixer I: Populated control board
Utility Mixer I: Populated main board
Utility Mixer I: Populated main board
Utility Mixer I: Side view
Utility Mixer I: Side view
Utility Mixer I: Back view
Utility Mixer I: Back view

ADSR Euro-rack

ADSR: Front view
ADSR: Front view

This is another derivation off the ADSR for my NGF-E project, adapted to 12V Euro-rack format. Because this one is a stand alone module I have removed all additional features from the Next Generation Formant project. Nonetheless it is still based on original Elektor Formant ADSR schematic. I made some error corrections and added my changes to the design. All parts are updated to today (2021/01) available parts. I have made a few changes to fix some shortcomings of the original. A triple range switch was added for finer adjustment of the ADSR CV-output signal. The attack rise time is shorter now as in the original. The gate input is buffered. The fixes a fault in the original when working with analog sequencers. The output voltage is slightly raised to reach really 5V. Due to the design of the original Elektor Formant ADSR the output of the original ADSR keeps a residual voltage of about 0,5V. I have put an compensation in my design to correct this. The driver circuitry for the output indicator LED is changed for better linearity.

Specs and features

  • AD/ADSR switch
  • Gate input 5V
  • CV output ..5V
  • CV output indicator
  • Range switch: fast, middle, long
  • Attack (A) 0,5ms…16s
  • Decay (D) 4ms…40s
  • Sustain (S) 0..5V
  • Release (R) 4ms..40s
  • Power consumption below 15mA each rail

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

ADSR: Schenmatic main PCB
ADSR: Schematic main PCB
ADSR: Schematic control PCB
ADSR: Schematic control PCB

This is a close clone of the Elektor Formant ADSR. Here i only describe the changes i have made. The description of the other parts of the circuitry can be found in the original Elektor Formant documentation. The gate signal input resistance is raised from 33kOhm to 1megOhm with the input buffer IC1A. This protects against double triggering with the falling edge of the gate signal when using sequencers. R30 is used to fix the input to a defined potential when no signal is attached to the input. C1 was lowered to 6n8 from 10nF. In combination with C2 and the raised charging voltage through IC1B/R9 this makes for faster attack time. The load capacitor of 10u was replaced with three selectable capacitors of 2,2uF 4,7uF and 10uF. This makes for a finer adjustment of the response times of the ADSR. The voltage divider R19/R21 was adjusted to ensure that the output level of 5V is reached. If this feature is not used R25 should be lowered to 5k1. Construction conditioned the output at IC1D only reaches a minimal voltage of about 0,5V. To compensate for this i added IC2A. If the ADSR is not used the output voltage is now at -0,5V. The current consumption was lowered with using the TL064 and a low current LED.

ADSR: back view
ADSR: Back view
ADSR: Populated main PCB
ADSR: Populated main PCB
ADSR: Populated control PCB
ADSR: Populated control PCB
ADSR: Side view
ADSR: Side view

Pitch to voltage converter

Pitch to voltage converter: Front view
Pitch to voltage converter: Front view

This is the software driven replacement for my all hardware pitch to voltage converter from my Shakuhachi to Synth project. The software driven approach has the advantage of easily adaption for different frequency ranges. In my case it is the range of the Shakuhachi. To change the range just adapt the software. It is completely temperature independent. The needed input is a pulse train derived from your original signal. You can use my Signal to Trigger converter to provide the pulse train. An offset voltage is added to the V/Oct output to fit the needs of your VCO (Synthesizer).

Specs and features

  • Software driven pitch to voltage converter
  • 12bit resolution
  • V/Oct output
  • Offset CV Fine and coarse adjustment
  • Runs on +/-15V and +/-12V
  • Power consumption around 45mA positive rail, 15mA negative rail

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

Pitch to voltage converter: Microprocessor board
Pitch to voltage converter: Microprocessor board
Pitch to voltage converter: Control board
Pitch to voltage converter: Control board

The incoming pulse train is feed to the microprocessor. IC1 (301-F) prevents the microprocessor from negative inputs. Zener D2 prevents from overvoltage. The trigger starts an internal timer of the microprocessor in input capture interrupt mode. The ticks are counted and the count is then looked up in a table. The lookup table provides the values for the V/Oct conversion. The read value is the send to the DAC MCP4921 which is follwed by a low pass (IC1A, 301-B)). IC2A (301-F) adds the offset voltage and IC2B (301-F) corrects the phase.

Pitch to Voltage converter: Populated PCB's
Pitch to Voltage converter: Populated PCB’s
Pitch to Voltage converter: Side view
Pitch to Voltage converter: Side view

Compressor with optional pedal steering

Compressor: Font view
Compressor: Font view

This is the revised version of my Limiter/Compressor. First built for my Shakuhachi to Synth project to handle the great dynamic range of the Shakuhachi. Here I left out the limiter and added a make up amplifier. The structure used is derived from “Small Signal Audio Design”, second edition by Douglas Self p682ff. The audio signal did not flow through a VCA as in many other implementations. Instead the compression is done by subtracting the audio signal at the output summing node according to the control voltage derived from the audio signal. The compression rate and the make up gain is adjusted by hand or/and optionally with foot pedals. The foot pedals are an additional option particularly made for wind players. It works without this option in your setup as well.

Specs and features

  • Compression rate and gain adjustable by hand or/and foot pedals
  • Audio path not affected when no compression is used
  • Runs on +/-12V and +/-15V (with minor resistor value changes for best performance)
  • Power consumption below 20mA each rail

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

Compressor: Schematic
Compressor: Schematic

When the ratio is set to zero and the gain to one the input signal passes through the circuitry unaffected (IC2C, IC2A IC6OTA1, IC6OTA2, IC2D). When the compression rate is turned up a DC voltage is derived from the input signal wit a precision full wave rectifier and some filtering (IC1A, IC1B, IC1C, IC1D). This voltage is used to open the VCA in the side chain (IC3OTA1, IC3OTA2, IC2B). The signal from the side chain is then subtracted from the main signal (R13, IC2A). The now compressed signal is then potentially amplified (IC6OTA1, IC6OTA2)

Compressor: Populated PCB
Compressor: Populated PCB
Compressor: Back view
Compressor: Back view
Compressor: Front with pedal connector
Compressor: Front with pedal connector

Foot switch connector

Foot swithc connector: Front view
Foot swithc connector: Front view

As a Shakuhachi player I need my hands on the flute. So I use me feet to manipulate parameters and switches on the synthesizer. This module was originally build for my Shakuhachi to Synth project to provide the possibility to connect foot switches with the synthesizer and keep the patch intact when they are removed. The signal is not routed through the foot switch. Instead CMOS switches are used, turned on and off with the foot switch. So the signal stays within the synthesizer and the connection to the foot switch carries only DC. Removing the foot switch does not interrupt the signal flow in the synthesizer.

Specs and features

  • Four independent switches
  • Signal flow stays intact when foot switch removed
  • Runs on +/-15V and +/-12V
  • Power consumption below 10mA each rail

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

Foot switch connector;: Schematic
Foot switch connector;: Schematic

The switch in the DG202 is hold in on position with a 100k resistor against the positive rail. With a foot switch attached you can pull down the hold voltage when you close the foot switch.

Foot switch connector: side view
Foot switch connector: side view
Foot switch connector: Back view
Foot switch connector: Back view

Signal to Trigger Converter

Signal to Trigger Converter: Front view
Signal to Trigger Converter: Front view

This module was originally build for my Shakuhachi to Synth project to provide the start/stop pulse for the Pitch to voltage converter. But it turned out to be much more useful. When you have the basics for your synthesizer like VCO, VCF, VCA, ADSR, LFO,… and some controllers and you want more, then using your keyboard to steer the synthesizer it is time for some modules to produce trigger signals out of different sources. Here is one of them. A signal to trigger converter. You can feed in a changing signal and every time the signal went through zero a trigger is generated dependent on the direction from where the zero point is crossed. You can add a threshold manually or CV controlled to move the zero point up or down as well. You can feed the signal in through input one ore two. When both inputs are used the signals are added together. When the signal crosses zero from positive to negative a trigger of about 0.1msec is generated at output -Trig. When the signal crosses zero from negative to positive a trigger of about 0.1msec is generated at output +Trig. Output +/-Trig provides both triggers. This output can be used to generate interesting rhythmic patterns when the threshold is set by a slowly moving CV or some DC offset is applied to the signal.

Specs and features

  • Two added inputs
  • Threshold manually and with CV
  • Output for +Trig, -Trig and +/-Trig: 0.1msec
  • Runs on +/-15V and +/-12V with minor resistor changes
  • Power consumption below 25mA each rail

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

Signal to Trigger Converter: Schematic control board
Signal to Trigger Converter: Schematic control board
Signal to Trigger Converter: Schematic main board
Signal to Trigger Converter: Schematic main board
Summed signal to trigger
Summed signal to trigger

The incoming signals are summed up. Every time when the summed signal changes polarity (moving through zero) a trigger is generated. Moving from plus to minus generates a trigger at the negative trigger output, moving from minus to plus generates a trigger at the positive trigger output. Trigger length is about 0.1msec.

Screenshot sine to trigger
Screenshot sine to trigger

The uppermost line (Yellow) shows the input signal. The second line (Blue) shows the trigger when the input signal moves to the positive site. The third line (Purple) shows the trigger when the input signal moves to the negative site. On the fourth line (Green) you can see both triggers added. This picture is taken without any threshold.

Signal to Trigger converter: Back view
Signal to Trigger converter: Back view
Signal to Trigger converter: Side view
Signal to Trigger converter: Side view

Passive Multiple

Passive Multiple: Front view
Passive Multiple: Front view

Just another often needed utility module. A passive multiple.

Specs and features

  • Seven passive parallel in/outs
  • No power connection

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

Passive Multiple: schematic
Passive Multiple: schematic
Passive Multiple: back view
Passive Multiple: back view
Passive Multiple: Side view
Passive Multiple: Side view