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

24dB Ladder filter with gain loss compensation

24dB Ladder filter with gain loss compensation

This is the Eurorack version of my NGF Moog Ladder Filter. I have brought out the 6dB, 12dB, 18dB and 24dB poles. Gain loss is corrected when turning up the emphasis/feedback. As a side effect off the gain corrections I got the emphasis voltage controlled as well. 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

  • Two inputs for easy mixing
  • 6dB, 12dB, 18dB, 24dB output
  • Gain loss compensation when emphasis is turned up
  • Temperature compensation with KTY81-110
  • Voltage controlled emphasis
  • Usable as sine oscillator
  • Runs on +/-12V and +/-15V (with minor resistor value changes for best performance)
  • Power consumption around 40mA negative rail, 50mA positive rail

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

24dB Ladder filter with gain loss compensation.
24dB Ladder filter with gain loss compensation.
24dB Ladder filter with gain loss compensation.
24dB Ladder filter with gain loss compensation.

This is a classical Moog Ladder design implementation, nothing special to say about. All poles are brought out with instrumentation amplifiers. The voltage controlled emphasis is realized with means of one half LM13700 OTA (IC1OTA1). The control voltage for the emphasis is used to control the gain of four OTA (IC6OTA2, IC6OTA1, IC9OTA2, IC9OTA1) as well, one for each pole. The outputs of this OTA’s are added to the instrumentation amplifiers for each pole to compensate for the gain loss with high emphasis. The added amount is adjustable by changing one resistor (R28, R47, R66, R69). Higher values means higher gain. The temperature compensation for the expo-converter is realized with KTY81-110. This is not realy needed for a filter but a nice additional feature if you use the filter as sine VCO. If not wanted it is easily replaced with a 1k resistor.

Screenshot High Q1
24dB Ladder filter: Screenshot High Q1
Screenshot High Q2
24dB Ladder filter: Screenshot High Q2
24dB Ladder filter screenshot sine out
24dB Ladder filter: Screenshot sine out
24dB Ladder filter: Populated control PCB
24dB Ladder filter: Populated control PCB
24dB Ladder filter: Populated main PCB front
24dB Ladder filter: Populated main PCB front
24dB Ladder filter: Populated main PCB back
24dB Ladder filter: Populated main PCB back
24dB Ladder filter: Module back
24dB Ladder filter: Module back
24dB Ladder filter: Module side
24dB Ladder filter: Module side

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


36dB VCF LP/HP

36dB VCF LP/HP populated PCB

36dB VCF LP/HP populated PCB

I just wanted to know how a 36dB VCF sounds like. So i build one. This one is basically derived from my 24dB VCF LP/HP which i build for my Next Generation Formant project. I just added two more filter stages and the compensation circuitry from my Moog Ladder filter to compensate for the volume loss when Q is turned up. I have brought out all 6 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
36dB voltage controlled low pass and high pass filter
Switchable output 6dB, 12dB, 18dB, 24dB, 30dB, 36dB
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 mA each rail
Volume indicator (optional)

The documentation for download can be found in my website.

36dB VCF LP/HP schematic page one

36dB VCF LP/HP schematic page one

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. The resonance (Q) is voltage controlled with means of the OTA IC16OTA2 in the upper right corner (page 1). To compensate the volume loss when the resonance (Q) is turned up a second OTA (IC16OTA1) 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 (See page 2 of the schematic).

36dB VCF LP/HP schematic page two

36dB VCF LP/HP schematic page two

36dB VCF LP/HP module back view

36dB VCF LP/HP module back view

36dB VCF LP/HP module front

36dB VCF LP/HP module front

NGF Vocoder Project: Voiced – unvoiced detection

NGF Vocoder : Voiced - unvoiced detection PCB

NGF Vocoder : Voiced – unvoiced detection PCB

Here is the voiced – unvoiced detection module for my Vocoder project. It is made off a high pass and a low pass filter. The outputs of the filters are compared. The comparator switches the output signal between carrier and noise.

The documentation for download can be found in my website.

NGF Vocoder project: Voiced - unvoiced detection schematic 01

NGF Vocoder project: Voiced – unvoiced detection schematic 01

The speech input signal derived from the speech input module initially reaches the buffer IC1A and is then split into two signals, each passing a filter. The high pass filter is constructed around IC3C and IC3D and the low pass around IC3A and IC3B. Their peak values are at 2500Hz and 600Hz respectively. The high pass filter is a Sallen Key filter, highly damped, gain around 5.6 dB. The low pass filter is constructed accordingly. Both filter sections have a slope of 24dB per octave to obtain a good separation. They are each followed by a precision full wave rectifier (IC4C, IC4D and IC4A, IC4B) and by a 12dB per octave smoothing filter. The rectified and calibrated filter output signals are now feed to a comparator IC1B. The voiced – unvoiced decision is taken by the comparator according to the signal levels. The comparator output steers the switch IC5 which selects if the carrier or the noise signal is send to the output.

NGF Vocoder project: Voiced - unvoiced detection schematic 02

NGF Vocoder project: Voiced – unvoiced detection schematic 02

The carrier and the noise input are both fed to OTA’s. The carrier input is buffered with IC9A, the noise input with IC9B. The noise input is switchable between internal and external source. The OTA’s are turned on or off according to the signal from switch IC5 (schematic page 01).

NGF-E Project: RFM

NGF-E Project: RFM stuffed PCB

NGF-E Project: RFM stuffed PCB

This is my replacement of the original Elektor Formant RFM module. It follows closely the original. Because this one is for my Next Generation Formant project i started with the original Elektor Formant RFM schematic and added my changes to the design. All parts are updated to today (2017/10) available parts. The connections are the same as in the original to keep the possibility for internal wiring. If you don’t need those features just leave them out. This PCB provides all functions as in the original Elektor Formant RFM. The only change I have made is adding a wet/dry potentiometer. So you can adjust the balance between the input signal and the filtered signal

The documentation for download can be found in my website.

NGF-E Project: RFM schematic

NGF-E Project: RFM schematic

The schematic follows closely the original Elektor Formant RFM. For details refer to the original documentation.

NGF-E Project: RFM faceplate

NGF-E Project: RFM faceplate

NGF-E Project: RFM back view

NGF-E Project: RFM back view