The band-pass itself is constructed from a high pass and a low pass filter. [Ryan] used simple ... would quickly attenuate [Ryan’s] input signal, so each stage uses two op-amps.

A simple two-transistor circuit generates the pulses at the zero-crossing point, a low-pass filter cleans up the signal, and a cheap audio amplifier reproduces the original audio. It’s a crude ...

While the 2045 filter has just a single low-pass mode, the R-8 can be run in low-pass ... Trigger options include gate a signal, an audio signal threshold and a manual trigger button, while there’s ...