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A-weighting filter

Here is a simple A-wighting filter from an old Ampex service manual; it's close to exact A-weighting.

  input --600---+-0.33uF----+-0.33uF---+--10K---+-0.047uF--+- output
                |           |          |        |          |
               600        1.8K       1.8K    2200pF      100K
                |           |          |        |          |
   gnd  --------+-----------+----------+--------+----------+-- gnd

This filter circuit must be wriven with low impedance output and the putput of this circuit must be connected to very high impedance input.

Second issue for the circuit is that, properly loaded, the circuit describe has about a 12.4 dB insertion loss normalized at 1 kHz (A weighting, in fact, requires a "gain" of about 1.3 dB at 2.7 kHz or so). This insertion loss must be accounted for when used as the original poster describes.

The gain at 1 kHz should be 1, or 0 dB, thus the adjustment should be precisely +12.4 dB.