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


Trigger Delay

Trigger Delay front

Trigger Delay front

This module can delay an incoming trigger signal up to 8 seconds. The trigger length is adjustable up to 8 seconds as well. The trigger pulse length is independent from the input signal length. Both parameters are independently adjusted with potentiometers. The output state is shown with a LED.

Specs and features
• Input: positive Trigger or any fast changing voltage.
• Output: 5V pulse with adjustable delay and length.
• PSU +15V/-15V or +12V/-12V

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

Trigger Delay schematic

Trigger Delay schematic

Trigger Delay populated PCB

Trigger Delay populated PCB

Trigger Delay back view

Trigger Delay back view

CV Mover

CV Mover faceplate

CV Mover faceplate

This utility module provides you with different functions. You can use it as attenuator and sign changer for any input signal. You can use it as CV Source. It gives you a DC offset between -2.5V ans + 2.5V with coarse and fine adjustment. This voltage range is easily adopted to your needs with simple resistor change. Most interesting application is using it as “CV Mover”. This means adding a DC offset to the input signal. Say you have a LFO signal between +/-5V and want to shift it in the positive range. Then you can divide the signal in half with the attenuator to 1/2 and add the +2.5 threshold and you get a 0..5V positive LFO signal. This comes in handy for steering filters VCA’s and other modules. The output signal is visualized with LED

Specs and features
• CV source -2.5..+2.5V with coarse and fine adjustment
• Attenuator
• Positive and inverted output signal
• Adjustable DC offset for the input signal
• Positive and negative CV output indicator with LED
• Runs on +/-15V and +/-12V

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

CV  Mover: schematic

CV Mover: schematic

IIC1B acts as a simple inverting voltage adder. The input signal and the offset voltages are added together. The direct output from IC1B is the negative of the input signal. IC1A converts the signal back to the original phase. IC1C is a simple buffer and in the feedback loop of IC1D are the indicator LED’s.

CV Mover populated PCB

CV Mover populated PCB

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