Calculation of Rin,out of audio filter

Dan Mills said:

Ok, at high frequency the lowest impedance possibly seen at the filter input is trivially R1||R5||R2, so say somewhere about 12K or so at a guess, it rises towards DC as the reactance of the input coupling cap rises and the various filter capacitors become more significant reaching infinity at DC due to the input blocking cap.

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Dan Mills said:

Not any kind of academic, I never got around to finishing my degree, never mind a masters or phd. I have however been around this stuff for 30 years which helps.

AF = Audio frequency.

RF uses matched impedance to opimise power transfer, which way back in the day was how audio was also done (They used to use 600 ohm rather then 50, but that is a detail), modern audio usually does voltage transfer which is where the load being > 10 times the source comes in.

Now you could terminate the mixer into a 50 ohm resistor to ground, then use a filter having an input impedance > 500 ohms wired in parallel, the effect will be that the mixer sees a ~50 ohm termination, but it will totally kill your noise performance because the filter will be seeing a tiny input voltage.
Better would be to design an audio filter having a 50 ohm input impedance and a stiff voltage output, hence my suggestion of a common base buffer stage (low input Z).

Note that the output from the mixer is NOT audio, it has components way up into the radio frequency spectrum, it becomes audio once you strip off all the ultrasonic and rf energy in the filter.

Seriously hit the books.

73 Dan.

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