Voltage Controlled Multimode Resonant Filter

After getting my hands dirty with a linear Power Supply for my modular, it was time to develop a PCB for what I consider the most importan part of a synthesizer: the filter!

In this instructable I will show you the filter I have choosen for my synth and describe the PCB I have designed.

I will also share a link to have the PCB manufactured by professionals at a good price.

There we go!

During the glory days of analog synthetizers manufacturers fought their war developing original (and sometimes not-that-original) circuits for their products. What is left are incredibly expensive synthetizers, but also precious schematics which constitute a great source to get inspiration from, or even (respectfully) clone circuits.

One of my favourite filters is the one Korg developed for the MS20. In particular, I must admit that I have a soft spot for the so called "Mark II" filter because less aggressive than the Korg35 module and more suited to my musical tastes (shame on me).

There are various projects developed around that filter out there, and I had to notice the work of René Schmitz and especially Kassutronics on this regard. In this project, I wanted to keep the filter as similar to the Korg MS20 MKII as possible, using components most resembling those used by Korg and givin up others improvements to it, where possible. I am aware that a subtractive approach is easier than the opposite, so credits goes to Renè and Kassu for their shared work.

The filter here presented clones the original Korg MS20 MK II multimode resonant filter. As per Korg schematics (see the attached extrapolation and edit of the high pass filter part), the active components are LM13600 Dual Operational Transconductance Amplifiers (OTA), some generic amplifiers in buffer configuration at the LM13600 output and a JRC4558 dual op-amp in the feedback circuit. A dual PNP transistor in single package (A798F) serves the main circuit takeing care of the CV tracking.

These components are mostly obsolete, but there are very good replacements availabe. The LM13700 is a direct replacement for 13600 and high performane amps, specifically designed for audio applications like the NE5532 are the obvious choice as buffers in the audio path.

Feedback and especially CV track circuit are less prone to give audio interference, so it is possible to use a more generic (noisy) op amp like TL072. I made some testing and, honestly, could not hear any difference by using NE5532 in place of the TL072.

The A798F in the exponential converter circuit is replaced by two BC557.

I used yellow LEDs to emulate the presence of three diodes and get a voltage drop in the range of 2.1-2.2V. You can use a different color (i.e. green or even blue) if you want a more "wild" behaviour from the feedback loop.

There are two CV inputs, but no control over CV level. To gain control over CV level just connect a 50-100K linear potentiometer in voltage divider: one potentiometer external terminal to the jack socket, the other to ground and potentiometer wiper to PCB's CV in.

Warning: the filter goes wild (very very loud and eventually highly pitched) when the resonance is set "too high" and the filter starts self-oscillating. Keep the resonance pot at moderate levels or damage to your ear and/or equipment could occur.

A welcome characteristic of this filter design is the possibility to switch from a high pass mode to a low pass mode simply by changing the input of the incoming audio signal. The Korg MS20 was equipped with a high pass resonant filter followed by a low pass resonant filter (see previous step).

The PCB I have developed takes this into consideration, and the fact that PCB manufacturers give you multiple of 5 PCBs. So, instead of developing a bigger, double LP+HP filter, you can easily populate two PCBs, and set the two to different modes.

High Pass mode can be set by short circuiting to ground the low pass input (LP_IN) and feeding your audio source to high pass input (HP_IN).

Low Pass mode can be set by shorting the high pass input (HP_IN) to ground and input audio to LP_IN.

I placed ground pads all over the PCB: it's your choice to use one pad per jack connector or daisy chain grounds directly on connectors.

You can have your FR-20 PCB manufactured at this link (PCBWay):

https://www.pcbway.com/project/shareproject/Voltag...

Select your favourite color, place the order, have it shipped to you, source components, assemble it and have fun in the process!

Notice that the filter pictured in this instructable is my prototype and doesn't have an inverse polarity protection diode. The version shared actually is voltage protected against +12V and -12V invertion. It's also slightly rearranged in some relative distance between components.

The board is intended to be mounted perpendicular to the front panel. If your case is not deep enought, you can lock the board on the case bottom and use wires to gain electrical connection with elements (potentiometers, buttons, jacks and so on) on the front panel.

Follows the list of components for the filter:

Capacitors

- 1x 4.7nF poly capacitor

- 2x 1nF poly capacitor

- 3x 100nF poly capacitor

- 1x 470nF poly capacitor

- 2x 100u electrolitic capacitor

Amplifiers and Transistors

- 1x NE5532 audio amplifier

- 1x LM13700 dual OTA

- 1x TL072 operational amplifier

- 2x BC557 PNP transistor

Resistors

- 3x 4.7K ohm

- 4x 220 ohm

- 1x 220K ohm

- 6x 10K ohm

- 1x 1.5K ohm

- 2x 1K ohm

- 1x 470K ohm

- 2x 100K ohm

Others

- 1x IDC connector (2x5 pin)

- 2x LED (3mm, yellow)

- 1X 10K potentiometer (linear)

- 1X 100K potentiometer (linear)

- 1X 1N4004 diode

The PCB of this instructable was kindly sponsored by those nice girls and guys at PCBWay - PCB Prototype the Easy Way: their Customer Service is excellent and PCBs of the best quality.

Thanks!