Re: LF: Sound Card mic Impedance

[Andy Talbot <andy.g4jnt@gmail.com>]

Have just being playing with AADE, to try to see why it pretends to be able to do low pass filters with unequal impedances.

 

All it appears to do is choose a set of values that maintains the specified passband response when terminated with the specified Rin/Rout values - it does not actually attempt to match them so there is an actual resistive power loss in such unequal terminated filters.  

 

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.

 

Andy

www.g4jnt.com

 

 

On 16 December 2010 20:58, Andy Talbot <andy.g4jnt@gmail.com> wrote:

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.