Voltmeter

Voltmeter faceplate

Voltmeter faceplate

The circuitry of this module was developed by Ray Wilson of MFOS. It was part of his Multi-Function Module. I have changed some resistor values to use with low current LED.
The LED voltage level meter gives you a graphical idea of where the voltage level you like to use to modulate your filter or VCA is. This circuit lights the LED in real time giving you a rough idea of the voltage level you are looking for. This is very useful when you want to move a CV from bipolar to only positive values.

Specs and features
• Three voltage ranges (+/-1V, +/-5V, +/-10V).
• Realtime LED display
• Runs on +/-15V and +/-12V
• Power consumption below 15mA each rail

The documentation for download can be found in my website.

Voltmeter schematic

Voltmeter schematic

IC4A is used for input buffering. IC4B and IC4C and associated components build the range selection circuitry. The heart of the circuit is the group of comparators build with IC1x, IC2x, IC3A and IC3B. The left side is used to measure the positive voltage, the right side for the negative voltage. LED 11 is the “near ground” indicator. IC3C and IC3D build a window comparator. For a more detailed description please refer to the original source at Ray Wilson’s Site MFOS (Multi-Function Module).

Voltmeter populated PCB

Voltmeter populated PCB

Voltmeter side view

Voltmeter side view

VC LFO

VC LFO front

VC LFO front

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
• Voltage controlled pulse width
• Range switch
• Coarse and fine frequency setting
• Runs on +/-15V and +/-12V
• Power consumption below 10mA each rail

The documentation for download can be found in my website.

VC LFO schematic

VC LFO schematic

VC LFO waveforms screenshot

VC LFO waveforms screenshot

VC LFO waveforms screenshot

VC LFO waveforms screenshot

VC LFO populated PCB

VC LFO populated PCB

Basic PSU with 78xx voltage regulator

Basic PSU with 78xx voltage regulator

Basic PSU with 78xx voltage regulator

In many cases it is useful to have a easy to build fixed voltage PSU at hand. For this case I have made a PCB which works with the 78xx series voltage regulators in TO220 housing. The implementation follows closely a straight forward design as in the data sheet. I have tested this design with a current load of 1A for some hours. No problems whatsoever.

Specs and features
• Input: Select transformer according to your needs
• Output: Voltage depends on the regulator used / 1A DC

The documentation for download can be found in my website.

Basic PSU with 78xx voltage regulator schematic

Basic PSU with 78xx voltage regulator schematic

Nothing special here. Just refer to the data sheet of the 78xx. Calculate R1 to keep the current through the LED within the specs.

Basic PSU with 78xx voltage regulator

Basic PSU with 78xx voltage regulator

DC-VCA / flat Version

DC VCA front view

DC VCA front view

This is the flat Version of my NGF DC VCA. I wanted a decent DC coupled audio VCA to process audio as well as control voltages. And easy available parts (2018/07). I tried different architectures and then decided to separate audio and DC VCA. With specialized VCA it is easier to reach the goal. For control voltage processing I implemented a VCA with active control voltage feedthrough compensation. For lowest control voltage feedthrough you have to use low Iabc as well. So the audio performance is not as good as in my Sims VCA. THD is about 0,2% with selected LM13700. Still not bad and sufficient for DC usage. For DC processing you want to keep the feedthrough and bias of the control input as low as possible. There can be 500mV or more offset depending on the control voltage (I_abc) and it doesn’t vary linearly. There’s a big bump in offset right in the middle of CV range. First thing to do is to lower I_abc to around 250uA. This helps for DC performance but degrades AC performance. The main idea is to use the second OTA of the 13700 to compensate for the CV feedthrough. Both OTA in the 13700 are on the same chip and behaves the same. So I inverted the output of the second OTA and added it to the signal path. You still have to select the LM13700 for best performance.

Specs and features:
• DC coupled
• Active CV feedthrough compensation
• Linear response
• CV 0..+5V
• 5Vpp in- and output
• Positive and negative output
• Runs on +/-15V and +/-12V
• Power consumption below 20mA each rail

The documentation for download can be found in my website.

VCA DC schematic back PCB

VCA DC schematic back PCB

VCA DC schematic front  PCB

VCA DC schematic front PCB

On the top is a plain forward implementation for an VCA. Next row is the same VCA with input set to ground and output inverted. The inverted output of the second VCA is added to the output of the first one to compensate for the CV feedthrough. The output voltage is adjusted with TR2/TR4 for different Gm off OTA’s.

VCA DC side view

VCA DC side view

VCA DC left view

VCA DC left view

VCA DC back view

VCA DC back view

VCA Sims / flat Version

VCA Sims - flat Version

VCA Sims – flat Version

This is the flat Version of my Sims VCA. The original Elektor Formant VCA used two 3080 in series. In most cases only one is used and the second one only adds to noise and distortion. This VCA is AC coupled only. The THD in the Elektor Formant VCA is about 1% and the noise to voltage ratio is not that good either. Time to look for a replacement. I wanted a decent DC coupled audio VCA to process audio as well as control voltages. And easy available parts (2014/08). I tried different architectures and then decided to separate audio and DC VCA. With specialized VCA it is easier to reach both goals. I made my own implementation of the Sims-VCA introduced by Mike Sims in the EDN Magazine January 1995. With this architecture it is possible to achieve THD of 0,02%. Unfortunately i can not confirm the statement from Mike Sims that trimming the circuit for minimum THD achieves minimum control voltage feedthrough. Trimming for minimum THD causes an constant DC bias at the output. I have had to add a output capacitor to avoid the bias at the output. And you need test equipment to measure the THD for correct trimming. If you can not measure THD better build my DC_VCA. You can achieve 0,2 % THD here. Still good.

Specs and features:
• AC coupled 0,02% THD
• lin and log response
• CV 0..+5V
• 5Vpp in- and output

The documentation for download can be found in my website.

VCA Sims schematic back PCB

VCA Sims schematic back PCB

VCA Sims schematic front PCB

VCA Sims schematic front PCB

Please refer to my first implementation (link) or the original article from Mike Sims

VCA Sims side view

VCA Sims side view

VCA Sims side view

VCA Sims side view

Output Module

Output Module: Front view

Output Module: Front view

This is my replacement of the original Elektor Formant COM module. I discarded the original circuitry because of the TL085 used with his unusual pinout and the availability of dedicated audio operational amplifiers. I used a more effective filter implementation for tone control. The tone control is derived from “Small Signal Audio Design” by Douglas Self Chapter 15. A optional level indicator makes it easier to find the right volume level for best SNR. The maximum output volume is adjustable to protect your PA. You can connect the output directly to active monitors.

Specs and features
• Bass, middle, treble tone control
• Adjustable maximum output volume
• Optional volume indicator
• Direct connection to active monitors
• Runs on +/-15V and +/-12V (with minor resistor changes)

The documentation for download can be found in my website.

Output Module: Schematic front PCB

Output Module: Schematic front PCB

Output Module: Schematic back PCB

Output Module: Schematic back PCB

A description can be found in “Small Signal Audio Design” by Douglas Self Chapter 15

Output Module: Stuffed PCB back view

Output Module: Stuffed PCB back view

Output Module: Stuffed PCB side view

Output Module: Stuffed PCB side view

Output Module: Side view

Output Module: Side view

NGF Project: Dual Sample and Hold

Dual Sample and Hold: front view

Dual Sample and Hold: front view

Storing analog signals is a often used function in analog synthesizers. This sample and hold implementation follows closely the original Elektor Formant version of Book 2 “Formant Erweiterungen” p84ff. It is build for my Next Generation Formant project. Because I use the LM13700 here as replacement for the CA3080 I have build a dual sample and hold version. The PCB size is reduced from 100x160mm for a single version to 50x70mm for the dual version.

Specs and features
• Dual sample and hold
• 10Vpp input and output
• Runs on +/-15V and +/-12V
• Power consumption below 25mA each rail

The documentation for download can be found in my website.

Dual Sample and Hold: schematic

Dual Sample and Hold: schematic

This implementation follows closely the original Elektor Formant implementation. Refer to the original documentation if needed. You can find it on the net. My changes are the input buffers, using the LM13700 instead of the CA3080 and the adaption to my 10Vpp signal level.

Dual Sample and Hold: populated PCB

Dual Sample and Hold: populated PCB

Dual Sample and Hold: back view

Dual Sample and Hold: back view

NGF Project: 440CPS

NGF-E Project: 440CPS front view

NGF-E Project: 440CPS front view

Not much to say. A 440CPS module. Quite useful for tuning in a bigger system. OK, one more sentence. It is the replacement for the 440CPS module from the Elektor Formant in my Next Generation Formant project Project.

Specs and features
• On/Off Switch to keep the 440Hz out of the system when not needed
• Runs on +/-15V and +/-12V
• Power consumption below 25mA +rail / 5mA -rail

The documentation for download can be found in my website.

NGF-E Project: 440CPS schematic

NGF-E Project: 440CPS schematic

Everything is done in software. Output is a 440Hz Square wave. That’s it.

NGF-E Project: 440CPS populated PCB

NGF-E Project: 440CPS populated PCB

NGF-E Project: 440CPS back view

NGF-E Project: 440CPS back view

Limiter / Compresssor

Limiter / Compressor front view

Limiter / Compressor front view

To handle the great dynamic range of the Shakuhachi I needed a compressor for my Shakuhachi 2 Synth project. Because a limiter is not that different I added this feature as well. This comes in handy with my Vocoder project also. The structure used here is derived from “Small Signal Audio Design” by Douglas Self p682ff. The audio signal did not flow through a VCA as in many other implementations. Instead the compression or limitation is done by subtracting the audio signal at the output summing node according to the control voltage derived from the audio signal.

Specs and features
• Switch compress or limit
• Switch Compression/Limit rate 50% or 90%
• Compression/Limit rate adjustable 0–max
• Runs on +/-15V and +/-12V (with minor resistor changes)
• Power consumption below 15mA each rail

The documentation for download can be found in my website.

Limiter / Compressor schematic 01

Limiter / Compressor schematic 01

The audio signal flows unaffected through IC1A/B. When the compressor – limiter kicks in the inverted signal is added (=subtracted) at the summing node of IC1A. The signal level to subtract is regulated through a Sims VCA. The CV generation for the VCA is pretty standard. Linear for the compressor and exponential for the limiter.

Limiter / Compressor schematic 02

Limiter / Compressor schematic 02

Precision full wave rectifier with filter to generate the control voltage for the VCA from the audio signal.

Limiter / Compressor populated PCB

Limiter / Compressor populated PCB

Limiter / Compressor back view

Limiter / Compressor back view

Clock Divider with prime numbers

Clock Divider with prime numbers

Clock Divider with prime numbers

This clock divider divides the incoming clock signal down to the prime numbers /11, /13, /17, …. /31. The output is a 5V positive pulse. The length of the incoming pulse is kept. The trigger is on the rising edge of the incoming signal. The reset input can be used for syncing with other clocks. All outputs are buffered and brought out parallel with LED signaling the pulse.
Specs and features
• Regular input clock/square wave +5V
• Input signal divided by prime numbers
• Output +5V pulse with the length of the input signal (pulse)
• Runs with +15V/-15V or +12V/-12V (with minor changes)

The documentation for download can be found in my website.

Clock Divider with prime numbers, schematic

Clock Divider with prime numbers, schematic

Most work is done by the microprocessor. The micro takes care of the input and output timing. All outputs are independently buffered. The clock is made visible with LED.

Clock Divider with prime numbers, populated PCB

Clock Divider with prime numbers, populated PCB

Clock Divider with prime numbers, rear view

Clock Divider with prime numbers, rear view