"I just like to say I absolutely love the Filtrex. It never stops amazing me with the sound quality especially on material with a rich harmonic content. The classic break beats just sound so punchy and well 'valvey'.
This filter is just such a good tool in the studio, what you get for your money is a fabulous preamp, a lovely moog filter, envelope followers of three types and a LFO. The warm loveliness of the Mr Moog's design in conjunction with Tony Allgood's implementation make this filter a wonderful addition to anyones rig. The extra harmonics and mild but pumping compression really bring drums to life like no other filter I've had the pleasure to use."
Paul Bennett, August 2002
The prototype Filtrex with custom front panel made by Scheaffer
The original and popular Oakley VCF-1 design was updated to create a new powerful filter processing unit. The Filtrex 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 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 types to be made when the two outputs are mixed together in various degrees. Another reversible attenuator, Thru, allows the audio input to be mixed in too, to create very unusual phasing 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 left hand side of the front panel showing the audio path controls
- An inbuilt low frequency oscillator with triangle and square waveforms. The frequency can be changed from 0.2Hz to over 40Hz. A LED gives visual indication of the speed of the LFO. Use this to create auto wah and phaser type effects.
- 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.
- 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.
- 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.
- The total cost of this unit will be around 110UKP although this very much depends on the quality of your chosen enclosure.
The issue 2 boards are slightly different to the original issue 1 boards. The newer boards have improved component layout and feature an extra grounding pad for easier wiring.
The right hand side of the controls. This shows the envelope processor and LFO sections
Sound Samples
These are five short mp3 samples of the unit in action. Its 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. Recorded at 32KHz sample rate.
fil-1.mp3 Synth
strings with manually swept frequency. 75.9kb
fil-2.mp3 Choir sound
processed with the internal envelope follower. 82.4kb
fil-3.mp3 Choir sound
with auto triggered envelope on AD setting.164kb
fil-4.mp3 Cheesy synth
bass. Filter swept manually and using the Fizz output. 122kb
fil-5.mp3 More cheesy
synth bass. Filter deliberately overdriven and swept using the
internal LFO. 188kb
Downloads
User Guide Containing parts lists, operating instructions, parts guides and a full description of how it works.
Use 'save as...' button to download and view the files. IE users need to right hand mouse click and then choose 'save as'. Links to schematics are available to purchasers of the PCB or module and will be sent via e-mail when the board(s) are shipped.
Schematics and PCB data
Schematics, gerber and drill data can be purchased for non commercial use. The schematics are 4UKP, and the gerber plots are an additional 5UKP. All data is provided as downloadable files only.
Schematics and any other data is provided for non commercial use only. You may build as many PCBs as you wish but the EULA forbids any PCB to be sold for commercial gain.
E-mail us here to purchase any of the schematics for any of the discontinued projects. See the prices page for ordering information and payment methods.
A view of part of the original issue 1 PCB. The filter itself is based around the traditional ladder arrangement with the four red capacitors in a row. Notice also all the pots, switches and LEDs fit direct to the PCB. The only wires to be connected go to the power supply and the jack sockets.
Front Panel database
A Schaeffer front panel can be made for a 1U high rack panel. The database can be found here filtrex.fpd To read this file you will need a copy of 'Frontplatten designer' from German company Schaeffer. The program will run in English mode, and the company representatives speak good English too.
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