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	<AANLkTikWex1pGTvAmhyPidAHh-LbQ_XrUxHtdV=Q0Tnn@mail.gmail.com>
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Date: Thu, 16 Dec 2010 20:58:00 +0000
Message-ID: <AANLkTi=uaStRJGHXdAFz=eFBr-S4ommOhR+oTdT2R3Ot@mail.gmail.com>
From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Sound Card mic Impedance
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Very good point...
of course it can't...
 - just saw the option in the design S/W and didn't actually try it out.

OOPs !!!!!!!

I've done this quite often with bandpass filters, where it does work very
well indeed, and can sometimes be used to improve the shape over an equally
terminated design.   Particularly for top coupled designs

Andy




On 16 December 2010 20:49, James Moritz <james.moritz@btopenworld.com>wrote:

> 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.
>>
>>
>
>

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<div>Very good point...</div>
<div>of course it can&#39;t...</div>
<div>=A0- just saw the option in the design S/W and didn&#39;t actually tr=
y it out.</div>
<div>=A0</div>
<div>OOPs !!!!!!!</div>
<div>=A0</div>
<div>I&#39;ve done this quite often with bandpass filters, where it does=
 work very well indeed, and can sometimes be used to improve the shape ove=
r an equally terminated design.=A0=A0 Particularly for top coupled designs=
</div>

<div>=A0</div>
<div>Andy</div>
<div>=A0</div>
<div><br><br>=A0</div>
<div class=3D"gmail_quote">On 16 December 2010 20:49, James Moritz <span=
 dir=3D"ltr">&lt;<a href=3D"mailto:james.moritz@btopenworld.com">james.mor=
itz@btopenworld.com</a>&gt;</span> wrote:<br>
<blockquote style=3D"BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8e=
x; PADDING-LEFT: 1ex" class=3D"gmail_quote">Dear Andy, LF Group,<br><br>Un=
fortunately, this doesn&#39;t quite work... If you think about it, at very=
 low frequencies, any low-pass filter will just be a &quot;straight throug=
h&quot; 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 co=
uld, however, design a &quot;peaked low-pass&quot; 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.<br>
<br>Cheers, Jim Moritz<br>73 de M0BMU<br><br>----- Original Message -----=
 From: &quot;Andy Talbot&quot; &lt;<a href=3D"mailto:andy.g4jnt@gmail.com"=
 target=3D"_blank">andy.g4jnt@gmail.com</a>&gt;<br>To: &lt;<a href=3D"mail=
to:rsgb_lf_group@blacksheep.org" target=3D"_blank">rsgb_lf_group@blackshee=
p.org</a>&gt;<br>
Sent: Thursday, December 16, 2010 8:25 PM<br>Subject: Re: LF: Sound Card=
 mic Impedance=20
<div>
<div></div>
<div class=3D"h5"><br><br><br>
<blockquote style=3D"BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8e=
x; PADDING-LEFT: 1ex" class=3D"gmail_quote">If its a passive L/C filter,=
 why not do a version with unequal Zin and Zout<br>and take advantage of=
 the voltage gain in going from 50ohms to, say, 500<br>
ohms? =A0 That will give you an apparant 10dB without trying.<br><br></blo=
ckquote><br><br></div></div></blockquote></div><br>

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