Rack Mountable Analogue Filter: Filtrex II

The PCB for this project is no longer available.

The suggested front panel design for the Filtrex II rack unit

The original and popular Oakley Filtrex design has been updated to create a new powerful filter processing unit. The Filtrex II contains:

A four pole true analogue filter. This is based on the famous transistor ladder filter developed by Dr Bob Moog in the 1960s. A front panel pot controls the filter cut-off frequency, another controls the resonance. The resonance control is configured not to cause volume drop when turned up. The filter can be made to oscillate if required.

Two filter outputs are available. One is the Fizz output. This is a type of -6dB/octave filter and it gives a unique filtering effect with a real bite. The other is the Smooth output. This is a classic -24dB/octave filter, and gives a smooth warm analogue filter sound. Each filter output goes via its own reversible attenuator. This means not only can you adjust the output level, but also the phase of the output signal. This allows complex filter shapes to be made when the two outputs are mixed together in various degrees. This includes a very nice sounding band pass filter.

A wet/dry control allows the filtered audio to be mixed with the original signal. Not only is this useful in controlling the depth of the effect, but it can also create unusual phasing and vibrato type effects.

A wide range pre-amplifier is included. You can use the Filtrex with virtually any input from guitars to synthesisers. A peak indicator will allow you to monitor overloads, although the Filtrex is designed to overdrive beautifully.

The Filtrex II populated PCB awaiting its case

An inbuilt low frequency oscillator with triangle and square waveforms. The frequency can be changed from 0.05Hz (one cycle in 20 seconds) to over 50Hz. A bi-colour LED gives visual indication of the speed of the LFO. Use the LFO to create auto wah and phaser type effects. The LFO can also be selected to trigger the envelope generator.

An ingenious dynamic envelope section that can move the filter cut off point up or down. It can be put into one of three modes at a flick of a switch:

1) It can function as a useful envelope follower, with controllable attack and decay times. Perfect for simulating many classic dynamic follower filter boxes. Play louder and hear the filter open up.

2) It can be a standard attack-release envelope generator. This is triggered either by the LFO, external gate signal or by an automatic audio trigger circuit with a variable threshold level. Powerful for creating rising crescendos.

3) It can be a standard attack-decay envelope generator. This is a type of one shot envelope, that rises and falls at the presence of an initial trigger signal. Useful for percussive type sounds.

The final output volume can be controlled with a level control. The output circuitry is configured to drive long length cables with ease and without ground loops.

A side chain input is available. This is a second audio input that can control the main input, but is not heard directly. Use this to trigger the envelope generator, or drive the envelope follower. A common use of this is to make the main audio program respond to a drum machine's output for syncopated effects. Use a deep rich string patch as your main input, and listen to the drum machine automatically pulse the output to the beat. Awesome!

CV and gate inputs are available for connection to a modular synth or midi-CV convertor. The CV input will control the filter frequency, and the gate control will trigger the envelope generator if selected.

With a second Filtrex PCB a stereo unit may be built. The two channels can either be run independantly or linked together for true stereo processing.

Power is derived from a standard 15V - 18V AC wallwart or internal mains transformer. The latter must only be fitted by those who are competent at mains wiring.

A MOTM/Oakley four way header is also provided so you can either power a small number of other modules with the Filtrex, or have the Filtrex powered with your MOTM or Oakley power supply.

The circuit board size is 398 mm x 76 mm. The board is double sided with through plated holes. Solder mask is provided on both sides in the most glorious blue. As with all Oakley boards the copper tracks and pads are very generously sized which makes them very rugged even for beginner solderers. The pots are 16mm Alps or similar.

The total cost of this unit will be around 150GBP although this very much depends on the quality of your chosen enclosure.

The left hand side of the PCB showing the pre-amplifier and ladder filter circuitry. Note the THAT300P NPN array for low CV breakthrough.

All components, with the exception of the input and output sockets, are board mounted for ease of construction and reliability. This shows the LFO and envelope follower parts of the circuitry.

Sound Samples of Filtrex

These are five short mp3 samples of the original Filtrex in action. It's impossible to give an example of everything Filtrex can do, but these will be able to give you a flavour of what can be done.

All these use the Filtrex to process the output of a general midi (GM) sound module. No other effects are used and it is the Filtrex's output you are hearing only.

Digital synth bass. Filter deliberately overdriven and swept using the internal LFO.

Synth strings with manually swept frequency.

Choir sound processed with the internal envelope follower.

Choir sound with auto triggered envelope on AD setting.

Digital synth bass. Filter swept manually and using the Fizz output.

Project Downloads

Filtrex II User Manual

Filtrex II issue 2 Builder's Guide

Construction Guide Our handy guide to building Oakley DIY projects

Parts Guide Our handy guide to buying parts for Oakley DIY projects

Schematics are only available to purchasers of the PCB and will be sent via e-mail when the board(s) are shipped.

Front Panel database

A Schaeffer front panel can be made for a 1U high rack panel. The databases can be found by downloading the following links:

Traditional black panel design with white legending

Natural finish panel with multicoloured legending

To read these files you will need a copy of 'Frontplatten designer' from Schaeffer. The program also features on-line ordering, so its now even easier to buy your panels from Scheaffer.

The company are based in Berlin in Germany and will send out panels to anywhere in the world. Delivery to the UK normally takes around ten days. For North American users you can also order your Schaeffer panels from Front Panel Express.

The completed Filtrex II PCB. All pots use pot brackets for securing the board to the front panel. Two additional M3 mounting holes are available for extra strength if needed.

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Copyright: Tony Allgood.
Last revised: December 17, 2018.