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Message-ID: <777992D581AA4E399712DC695BA65A16@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <FDCC39D43D974DED978401025DB874DF@lindavideo><AANLkTikWex1pGTvAmhyPidAHh-LbQ_XrUxHtdV=Q0Tnn@mail.gmail.com><F97BEB3D03B443448AD64A730E9CB7AB@JimPC><AANLkTi=uaStRJGHXdAFz=eFBr-S4ommOhR+oTdT2R3Ot@mail.gmail.com> <AANLkTinBWmGAG3gSv2ZOjQEVG+M+EqD9RK+_ePVyLP=v@mail.gmail.com>
In-Reply-To: <AANLkTinBWmGAG3gSv2ZOjQEVG+M+EqD9RK+_ePVyLP=v@mail.gmail.com>
Date: Thu, 16 Dec 2010 23:51:17 -0000
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Subject: Re: LF: Sound Card mic Impedance
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Dear Andy, LF Group,

G4JNT wrote:
> Have just being playing with AADE, to try to see why it pretends to be 
> able
> to do low pass filters with unequal impedances....
>
> ...Which all seems totally and utterly pointless.   Might just as well 
> slug one
> of the unequal impedances with an auxilliary R to make them equal, then
> design an equally terminated filter.

Well you could do that, but the trouble is that adding more resistors is 
bound to require more signal power at the filter input for a given level at 
the filter output. Which in one way or another boils down to reduced gain, 
SNR, output intercept, or increased bias current, dissipation, component 
count etc. in the overall circuit.

Apparently one advantage of doubly terminated filters is that they are less 
sensitive to component tolerances than an equivalent singly-terminated 
filter - but in many cases the practical difference is not that significant.

Cheers, Jim Moritz
73 de M0BMU