Posted on Feb 6, 2014

This circuit is a resonant twin-t filter. In this circuit, its purpose (according to the creator), is to give a decent fake lowpass response , which it does fairly well. Tim`s schematic shows two variations, one with self-oscillation, and one without. I built the one with oscillation. Since I was breadboarding it with a dual op amp (an NE5532,

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to be exact), I tried adding an overdrive-able input buffer/preamp, which worked pretty well, so I went with it. I had a plastic box sitting around with no lid, so I`ve temporarily built it into that for use with Loid. It will eventually become part of the piece I mentioned in the last post. Pots from left to right are: gain, cutoff, resonance. Here`s an audio demo, with the Q&D filter being fed with an XOR pseudo-ring mod, which in turn is being fed with two squarewaves from Schmitt trigger oscillators- at 1:06, the input is removed, and you can hear just the filter`s self-oscillation (modulation via 4069 LFO): Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser. These circuits currently exist on Loid`s breadboard extension. As I`m out of space in the containers, and am also reaching the limit of the power supply I`m using, Loid will be getting another piece added soon. In addition to the modules discussed here, I also plan to add a 4017-based sequencer, a noise source, a couple of VCO`s, and most likely some other stuff. If all goes according to the current plan, the result will be a sort of utility/ brain-box , which will also function as a take-away sequencer and beat box. The simple VCA is in this series of pages, towards the back of the book . This is from a project Ray Wilson was involved...

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