Schematics / Oakley Sound Systems Filter Box
Oakley Sound Systems Filter Box by Tony Allgood as shared kindly on http://www.emusic-diy.org/Schematics/Oakley/FilterBox ,follow the link for the full article containing further the envelope and lfo section of this module . From there the short description of this transistor ladder filter :
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.
for more info on Oakley Synthesizer modules visit : http://www.oakleysound.com/