Brickwall audio filter

For my 40mDC receiver, i was looking for a good audio filter that I might reuse for multiple receivers. While browsing in old magazines I came across a filter designed by Robert Penfold. It was featured in an article in Ham Radio Today March 1986 (see download section).

The brickwall audio filter

The audio filter is build around four almost identical low 24dB per octave low pass filter sections. This is followed by four almost identical 24dB per octave high pass filter sections. The filter chain is preceded by a buffer stage with a gain of 0dB (1x).

Brickwall audio filter – block diagram

The input buffer amplifier was changed a little bit. The section around R6/C4 were added. This guarantees a better power supply decoupling then when the non inverting input of the buffer amplifier was connected straight to the junction of R4 and R5.

Brickwall audio filter – input stage

Finally, the passband was measured with the audio USB interface and the excellent audioTester V 3.0.

Brickwall audio filter – passband curve

The result looks really good! Steep, almost vertical, walls. Almost flat passband and good attenuation of over 70dB. Really happy with the results. Not much care was taken to separate input and output cables, so the actual attenuation might be even better!

Ham Radio Today, March 1986 page 38-42
– Gerbers for the PCB

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