Oakley Sound Systems Filter Box

This filter is designed to be used as rack mounted effects unit. It would process a keyboard's or sound module's output and alter its sound. Filter sweeps could then be applied to drum loops or harsh sounding digital synths. The unit is made to be fully integrated, all it needs is a sound source. Commercial devices are available, the Mutronics Mutator and the Sherman Filter Bank are examples, but these are very costly.

The filter box has been split into three GIF files:

At the heart of the unit is a four pole Moog ladder low pass filter. The cut off frequency of this filter can be controlled by several sources. There is a manual frequency control, a low frequency oscillator, an AD/R envelope generator, an envelope follower and an external CV input. The envelope follower can either track the main audio in or use an auxiliary audio input. The AD/R generator can be triggered by several means. The main audio input, the auxiliary input, the LFO, and an external gate of at least 1 Volt.

The audio input is pre-amplified by a variable gain stage, the first stage uses a low noise op-amp NE5534, while the second uses a quarter of a TL074. If the level reaching the filter is too big then a red LED will light indicating distortion will be heard. Of course, this may be what you want.. I have reduced the inevitable CV breakthrough to minimum by using super-matched pairs at the top and bottom of the ladder. This is very important, since the filter will normally be in operation at the end of the sound generation stage.

The filter has a neat way of reducing the usual gain drop common with the Moog design. The resonance pot is dual ganged. This allows the input signal to the filter to be attenuated at low values of Q. The output of the filter is fed to an output amplifier after an output level pot.

The exponential converter is a simple design, remember that filter tracking is not as important as for a VCO. The scale is roughly 1V/oct. It would be a good idea to stick the two transistors together with thermal gunge and hold them together with a small cable tie. Matching is superfluous in this application.

The envelope generator is a simple full rectifier followed by a 4-pole 33Hz filter. The latter effectively eliminates the output from growling when a chord is played.

The envelopes can be driven either by the main audio in or an auxiliary input (side chain). This is where the fun starts. Gate the filter with drum sounds and you are in heaven when you use the filter to process thick string sounds. The LFO can control filter cut-off and gate the AD/R envelope generator. The LFO can be retriggered by an incoming gate if desired.

The filter is built on a single PCB that will fit into a suitably drilled 19“ rack mounted panel of 1U high. The controls on the front panel, from left to right, are: Input Sensitivity, filter cut-off frequency, resonance (no drop in audio level at high Q), output level,attack, decay/release, AD or AR switch, AD/R amount (+ and -), Trigger channel source (input or side chain), envelope follower amount (+ and -), LFO rate, LFO depth, LFO shape (tri/sq), LFO trigger, AD/R trigger from LFO. There are LEDs provided for overload (red), envelope gate (orange), and power on (green).

The unit was designed to run from a 15V ac, 500mA wall-wart, but you could put a decent transformer on the front.

All the parts are easily available. The quad op-amps are TL074 and all the transistors are BC549 or equivalent. Others can be used without too much bother. The 5534's are a luxury but I had a few spare so I used them. I guess a TL071 would do the job as well, but the 5534 can drive long leads very well. R29 is a link, but can be 220R if you don't want the filter to oscillate at high values of Q.

It is possible that if there is enough interest, circuit boards for this project will be manufactured.

Tony Allgood - Oakley Sound Systems

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Email the author: Tony Allgood