20Hz High-Pass Rumble Filter, Subsonic Blocker

This is a schematic diagram of 20-Hz high-pass rumble filter circuit.  Rumble noise could come from many sources, such as wind blow around an outdoor microphone, mechanical disturbance on phonograph, or many other possibilities. We can remove this noise by filtering the signal before the amplifier. The This circuit is designed for 20-Hz cutoff frequency and phonograph amplifier with gain […]

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Bandpass Filter for Bell103A Originator Modem

Bell103A modem standard specify that the originator (the caller) uses 1070Hz and 1270Hz frequency shift keying for transmission, and listen for 2025/2225Hz FSK transmitted by the answering modem. This bandpass circuit filter out any frequency beyond 2025-2250 Hz to improve originator’s decoding of the received FSK signal. Here is the schematic diagram of the circuit:

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