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-E Project: 24dB LP / HP

NGF-E Project: 24dB LP / HP VCF

NGF-E Project: 24dB LP / HP VCF

This is my take on the 24dB LP / HP VCF. This filter type is widely used in many synthesizers. Because this one is for my Next Generation Formant project i started with the original Elektor Formant schematic and added my changes to the design. All parts are updated to today (2017/7) 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 basic functions as in the original Elektor Formant. The additional functionality is put on an add-on board. The CA3080 are replaced with LM13700. The LP/HP switch is replaced with DG419 to avoid wiring problems. The signal level is raised to 10Vpp for a better signal to noise ratio. The exponentiator for generating Iabc for the OTA’s is temperature compensated. The additional function on this PCB is the linear TM input and the sign changer for the ENV input for easier use when the filter is switched to high pass mode. All other additional functions are on the add-on board. The add-on board provides voltage control for Q and the volume indicator.

The documentation for download can be found in my website.

NGF-E Project: 24dB LP / HP VCF schematic

NGF-E Project: 24dB LP / HP VCF schematic

NGF-E Project: 24dB LP / HP VCF PCB

NGF-E Project: 24dB LP / HP VCF PCB

NGF-E Project: 24dB LP / HP VCF

NGF-E Project: 24dB LP / HP VCF rear view with AddO PCB

NGF Project: 12dB Multimode VCF

This is my take on the 12dB Multimode VCF. This filter type is widely used in many synthesizers. You can find it in Electronotes or in the SEM modules and in many others as well. Because this one is for my Nest Generation Formant project i started with the original Elektor Formant schematic and added my changes to the design. All parts are updated to today (2017/4) 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 basic functions as in the original Elektor Formant. The additional functionality is put on an add-on board. The The CA3080 are replaced with LM13700. The JFET transistors are replaced with OpAmps. The signal level is raised to 10Vpp for a better signal to noise ratio. The exponentiator for generating Iabc for the OTA’s is temperature compensated. The only additional function on this PCB is the linear TM input. All other additional functions are on the add-on board.

The add-on board provides voltage control for Q, a sign changer for the ENV input for easier use when the filter is switched to high pass mode and volume indicator. This will be covered in another post.

Specs and features

  • 12dB highpass filter, 12dB lowpass filter, 6dB bandpass filter, notch filter
  • Temperature compensated exponentiator
  • 10Vpp signal level
  • TM log input
  • TM lin input
  • Positive and negative ENV control (with AddOn PCB)
  • Volume display (with AddOn PCB)
  • Voltage controlled Q (with AddOn PCB)
  • Runs on +/-15V and +/-12V (with minor resistor value changes)

    NGF Project: NGF 12dB Multimode VCF stuffed PCB

    NGF Project: NGF 12dB Multimode VCF stuffed PCB

    NGF Project: 12dB Multimode VCF schematic

    NGF Project: 12dB Multimode VCF schematic

    NGF Project: 12dB Multimode VCF Front view

Vocoder: Synthesizer mechanic

I am in the process of completing the synthesizer part of my vocoder project. So far I have the faceplate, the mechanic, the VCA part and a few filters ready. Still a lot of work ahead. Stuffing all the filter PCB and trimming, doing the voiced/invoiced detection, input/output section, exciter part and housing. Here are some pictures.

 

Faceplate and the mechanical parts sawed and drilled.

Vocoder: synthesizer mechanic

Vocoder: synthesizer mechanic parts ready

Waiting for PCB.

Vocoder: Synthesizer mechanic parts mounted

Vocoder: Synthesizer mechanic parts mounted

Vocoder: Synthesizer mechanic back

Vocoder: Synthesizer mechanic back

Does the VCA PCB fit for the Vocoder fit?

Vocoder: Synthesizer VCA PCB mounted

Vocoder: Synthesizer VCA PCB mounted

Some Filters to the Vocoder added.

Vocoder: Synthesizer VCA with some filters added

Vocoder: Synthesizer VCA with some filters added

Moog Ladder Filter – Add on. Bringing out 6, 12 and 18dB

When designing my Moog Ladder PCB it was a goal to bring out the 6, 12 and 18dB filter poles. I have done it with an additional PCB of the same size. You can easily stack the PCB on each other. The connectors are lined up for easy connection. The schematic follows the same way as the output stage of the basic PCB. With the circuitry to compensate for the gain loss with increasing emphasis. I added a basic level indicator and a linear VCA as well.

Moog Ladder Filter Add On PCB

Moog Ladder Filter Add On PCB

Moog Ladder Filter add on schematic:

Moog Ladder Add On schematic

Moog Ladder Add On schematic

Connecting the PCB’s:

Moog Ladder Filter connecting the PCB's

Moog Ladder Filter connecting the PCB’s

Moog Ladder Filter basic PCB mounted:

Moog Ladder Filter basic PCB mounted

Moog Ladder Filter basic PCB mounted

Moog Ladder Filter add on PCB mounted:

Moog Ladder Filter Add on PCB mounted

Moog Ladder Filter Add on PCB mounted

Moog Ladder Filter Faceplate:

Moog Ladder Filter Faceplate

Moog Ladder Filter Faceplate

Moog Ladder Filter – operating at 12V

I was frequently questioned if my design of the Moog Ladder runs at 12V. The answer is yes. You can run it at 12V out of the box. However the performance at 12V can be optimized by changing a few resistors.

– R47 and R13 from 13k to 11k (Diode bias current for LM13700).

– R56 and R2 from 13k to 10k (I_abc for LM13700).

Moog Ladder Filter – gain loss correction

Here is a detail of my Moog Ladder Filter implementation. The Moog Ladder Filter suffers from gain loss when the emphasis / feedback is turned up, as many other filters do. Here is my take on correcting this. With this method the gain loss can be compensated to 100% (or more) if wanted. I made the feedback voltage controllable. So I have already a control voltage proportional to the feedback. This voltage can be used to control a second OTA in the output stage of the filter. The output of the OTA is then added to the regular filter output. No feedback CV means no additional output. Increasing the feedback CV causes the OTA to add to the regular output. The amount is determined by the load resistor (R18 on the schematic) of the OTA. You can adjust it to your needs. It is possible to use a potentiometer here as well.

Moog Ladder Filter. Gain loss correction. Schematic detail.

Moog Ladder Filter. Gain loss correction. Schematic detail.

 

Here are some screenshots from the output with different amounts of gain correction starting with 0% up to 100%

Moog Ladder Filter, output with no gain loss correction. As you can see, the output (blue line) drops significantly.

Moog Ladder Filter, no gain loss correction

Moog Ladder Filter, no gain loss correction

 

Moog Ladder Filter, output with gain loss correction. R18 30k:

From here on the peek of the output signal is above the input signal. Watch the headroom of your system!

Moog Ladder Filter with gain loss correction R18=30k

Moog Ladder Filter with gain loss correction R18=30k

 

Moog Ladder Filter, output with gain loss correction. R18 50k:

Moog Ladder Filter with gain loss correction R18=50k

Moog Ladder Filter with gain loss correction R18=50k

 

Moog Ladder Filter, output with gain loss correction. R18 100k:

Moog Ladder Filter with gain loss correction R18=100k

Moog Ladder Filter with gain loss correction R18=100k

 

Moog Ladder Filter, output with gain loss correction. R18 200k:

As you can see the gain loss is completely corrected here. If you look at the last picture you can see that the peeks of the signals are about 10V. If you use 100% gain loss correction you should have a look at the headroom of your system.

Moog Ladder Filter with gain loss correction R18=200k, Scale 2V

Moog Ladder Filter with gain loss correction R18=200k, Scale 2V

Moog Ladder Filter with gain loss correction R18=200k, Scale 5V

Moog Ladder Filter with gain loss correction R18=200k, Scale 5V

Moog Ladder Filter with emphasis correction and multiple outputs

Why another Moog Ladder Filter? There are many decent implementations out there in the SDIY domain. Any reasons beside the joy of making something yourself? Well, I wanted to have some features that are not found in this combination elsewhere. First of all I wanted to get rid of loosing gain when the emphasis / feedback is turned up. And I wanted to bring all filter poles out. This means 6dB, 12dB, 18dB and 24dB outputs (Discussed in my next post). As a side effect off the gain corrections I got the emphasis voltage controlled as well. The feedback loop can be opened so you can bring in some filtering or what ever you want in the feedback loop. The exponential circuit can be stuffed with discrete transistors or LM394 as well. And this circuit is temperature compensated with KTY81-110.

Moog Ladder Filter with LM394

Moog Ladder Filter with LM394

The PCB is made for use with ordinary (matched) transistors and with matched pairs like LM394.

Moog Ladder Filter with discrete Transistors

Moog Ladder Filter with discrete Transistors

The schematic:

Moog Ladder Filter schematic

Moog Ladder Filter schematic

Some closeups:

Moog Ladder Filter LM394 closeup

Moog Ladder Filter LM394 closeup

Moog Ladder Filter with discrete transistors closeup

Moog Ladder Filter with discrete transistors closeup

Moog Ladder Filter temperature compensation with KTY81-110 and discrete transistors

Moog Ladder Filter temperature compensation with KTY81-110 and discrete transistors

Moog Ladder Filter temperature compensation with KTY81-110 and LM39

Moog Ladder Filter temperature compensation with KTY81-110 and LM39