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Message-ID: <F97BEB3D03B443448AD64A730E9CB7AB@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <FDCC39D43D974DED978401025DB874DF@lindavideo> <AANLkTikWex1pGTvAmhyPidAHh-LbQ_XrUxHtdV=Q0Tnn@mail.gmail.com>
In-Reply-To: <AANLkTikWex1pGTvAmhyPidAHh-LbQ_XrUxHtdV=Q0Tnn@mail.gmail.com>
Date: Thu, 16 Dec 2010 20:49:24 -0000
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Subject: Re: LF: Sound Card mic Impedance
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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" <andy.g4jnt@gmail.com>
To: <rsgb_lf_group@blacksheep.org>
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.
>