Dear Andy, LF Group,

Unfortunately, this doesn't quite work... If you think about it, at very low frequencies, any low-pass filter will just be a "straight through" connection, so in your example will just give whatever voltage the 50ohm source delivers to a 500ohm load with no filter present. The filter synthesis algorithm then makes a circuit that extends a more-or less flat response at this level up to somewhere near the cut-off frequency. You could, however, design a "peaked low-pass" circuit that would give voltage gain over a relatively narrow frequency range just below the cut-off frequency, falling to unity at DC - a pi matching network does this, for example.

Cheers, Jim Moritz
73 de M0BMU

----- Original Message ----- From: "Andy Talbot" <[email protected]>
To: <[email protected]>
Sent: Thursday, December 16, 2010 8:25 PM
Subject: Re: LF: Sound Card mic Impedance

If its a passive L/C filter, why not do a version with unequal Zin and Zout
and take advantage of the voltage gain in going from 50ohms to, say, 500
ohms?   That will give you an apparant 10dB without trying.