Band Pass Resonant Filter and Amplifier Module for DIY Synthesizer

It's nice to have separate modules for separate functions in our modular synthesizers, but we always have to deal with strict space limits. This sometimes turns into an opportunity, like in this very case study, where two multimode filter boards in different configuration (low-pass and high-pass) where combined with a final audio stage voltage controlled amplifier to give rise to a 16HP combi module.

In this Instructable I will show you how to realize a full, sort of "filter amp" combined module for modular synthetizers. I will share with you both main boards PCB's Gerber files and those for the dedicated front panel so that you can have them directly manufactured by Professionals of the sector at the best price.

Let's go!

Instructables of this series:

4 Channels MIDI to CV Interface Module
3340 Voltage Controlled Oscillator Module
Arduino Wavetable Voltage Controlled Oscillator
4 Channels Mixer
Voltage Controlled Multimode Resonant Filter
Discrete Voltage Controlled Amplifier
Arduino ADSR Digital Envelope Generator
Variable Waveshape Low Frequency Oscillator
DIY Linear Regulated Eurorack Power Supply (and Power Bus Bar)
Fiberglass Panels for DIY Modular Systems

In a previous Instructables I showed you how to realize a multimode resonant filter based on the one Korg realized for the MS20 (MKII). An iconic filter, with a non neglible part of it's charm coming from the voltage controlled high-pass resonant circuit placed before the low-pass circuit in a sexy, resonant bandpass configuration.

I had the opportunity to make some minor modification to that circuit (in particular some components value has been changed since), but this work is mainly a PCB redesign in order to have two of these boards behing the same front panel.

Being that PCB manufacturers realize such PCB's in multiple of 5, having the possibility to reuse the same circuit for both high-pass and low-pass configurations makes this filter the best candidate.

The filter mode can be easily selected by grounding the low-pass input pin for a high-pass configuration, or the high-pass input pin for a low-pass configuration (see attached pictures). Simple as that!

Please, notice that the filters PCB is full featured, with pads to solder an input, an output and two control voltages (CV1's level being adjustable by the dedicated potentiometer). The PCB, in other words, is a full featured stand-alone high pass or low pass filter despite being part of a combined module.

BOM

Follows the list of components for one filter:

Capacitors

1x 4.7nF poly capacitor
2x 1nF poly capacitor
5x 100nF poly capacitor
1x 470nF poly capacitor
2x 10uF electrolitic capacitor

Amplifiers, Transistors, Diodes

1x NE5532 audio amplifier
1x LM13700 dual OTA
1x TL072 operational amplifier
2x BC557 PNP transistor
2x LED (3mm, yellow)
1X 1N4004 diode

Resistors, Trimmers and Potentiometers

3x 4.7K ohm resistor
4x 220 ohm ressitor
1x 220K ohm resistor
4x 10K ohm resistor
1x 1.5K ohm resistor
2x 1K ohm resistor
1x 470K ohm resistor
2x 100K ohm resistor

1x 10K ohm trimmer

1X 10K potentiometer
1X 10K potentiometer
2X 100K potentiometer

Others

1x IDC connector (2x5 pin)

All elements values are silkscreened on printed circuit boards to speed up assembly and reduce the risk of positioning errors.

PS: isn't that purple PCB damn nice looking? :)

A stand-alone voltage controlled oscillator have various uses inside a modular synthesizer. It's not simply a final stage amplifier being that, in it's most common use, it's in charge to open and close the audio path as a function of the modulation source governing it.

The VCA circuit here is a re-design of THIS previous project of mine, and is intended to assolve the very specific function of output's shaping at the end of the audio chain, immediately after the filters. Despite this, the PCB has been designed to host all the components of the previously designed, stand-alone VCA.

The circuit is built around a NPN transistor pair in a single SMD package (BC847BS), which has the advantage of an intrinsic good matching of the transistor pair and a less tendency to drift over time thanks to a far better junction temperature pairing being made of two transistors in the same package.

No improvement comes without a price, unfortunately and in this case the "cost" reside on the chip itself being in SOT363 package: a tiny, to say the least, 0.65 mm pitched integrated circuit. Nothing to be scared of, anyway: in the aforementioned Instructable I added some tips to have it soldered in place even with a cheap stylus soldering station.

Please notiche that, even if the combi module misses the CV level potentiometer in the amplifier stage, the PCB have the place to solder it. The PCB, as said, is a full featured stand-alone voltage controlled amplifier despite being part of a combined module.

BOM
Here is the bill of materials for the VCA:

Capacitors
1x 100nF non-polarized capacitor
2x 10uF electrolitic capacitor

IC's, Diodes, Transistors

1x 1N4004 diode
1x TL072P op-amp
1x BC547 NPN transistor
1x BC847BS NPN general purpose double transistor

Potentiometers, Trimmers and Resistors

2x 50K 16mm potentiometer (up to 100K will work here)

3x 100K ohm resistors
5x 10K ohm resistors
1x 47K ohm resistor
3x 220K ohm resistor
2x 1K ohm resistor
3x 22K ohm resistor

1x 100K ohm trimmer

Others

1x 10 pin IDC connector, male

All elements values are silkscreened on printed circuit boards for yours (and mine) convenience ;)

Grouping three modules into one has it's advantages in terms of HP occupied in the case. These three components are often used in series one after the other (or at least this is how I almost always end with), so grouping them was a logical step to save some room for other modules.

The module is made up of three printed circuit boards (two of which are unique) and a faceplate. The unique PCBs are the multimode filter board and the voltage controlled amplifier board.

The same filter PCB is used in two different configurations for the high-pass and low-pass resonant filter. The populated filter board will work as low-pass by shorting to groud the high-pass intput pin (HPIN), or high-pass by shorting to ground the low-pass input pin (LPIN).

Boards are intended to be mounted perpendicular to the aluminum front panel. Jack connectors are wired to the appropriate, pre-labelled, pads on the board.

Boards sequence is: hi-pass filter, followed by low-pass filter, followed by VC amplifier.

Inter-boards connections are:

- faceplate jack connector "INPUT" to high-pass filter "HPIN"

- high-pass filter "OUT" to low-pass filter "LPIN"

- low-pass filter "OUT" to VCA "IN" input

- VCA "OUT" pad to "OUTPUT" jack connector on the faceplate.

The VCA board has two control voltage inputs. CV1 is connected to a voltage divider ("CV_LEVEL" potentiometer) which is left out of the board in the combi configuration. This means that if you connect the CV jack connector on the faceplate to pad "CV1", it will not work. Use pad "CV2" instead, which is directly wired to the amplifier circuit.

Pre-drilled holes are dimensioned for 1/4" jack connectors, not the eurorack 1/8" standard connectors.

You can download all Gerber files at THIS link (Github). You can have all PCBs manufactured at the best price at THIS link (JLCPCB). Take care of the PCB material you select: aluminum boards are the best choice for the faceplate and cost the same as fiberglass boards!

My projects are free and for everybody. You are anyway welcome if you want to donate some change to help me cover components costs and push the development of new projects.

>>HERE<< is my paypal donation page, just in case you would like to contribute ;)

Filters and amplifier main PCBs, but also aluminum front plate pictured in this Instructables where sponsored by JLCPCB, a high-tech manufacturer specializing in the production of high-reliable and cost-effective PCBs.

Their customer service is very good and PCBs a great value for the money!

Without their contribution this module would likely not have been realized sooo... many thanks!