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Date: Wed, 17 Apr 2002 14:55:21 -0400 (EDT)
From: Brien Pepperdine <pepperb at gov.on.ca>
To: qrp-l at lehigh.edu
Subject: [124974] Circuit for an audio filter - in old QST review of T-T by Demaw
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Often someone wants to know about making an audio filter. I figured I'd
give some info I have often sent privately.

Some time back Doug DeMaw reviewed the Ten-Tec Triton IV (model 544) and
commented on the audio filter. Internally mounted, with the switches T-T
usually provided (same thing for all intent is in the Argosy, OMNI C,
Argonauts) for its rigs only 2 widths were available via the switch used
on the rigs,
though the filter actually has 4 widths with the 2 opamps used, so no
problem adapting it via a complete switching arrangement and getting a
couple more widths than 'standard'.

You can see the whole circuit schematic as DeMaw provides it in QST:
July 1979, p. 46     

I believe this is also in QRP Classics (ARRL) as describes as an add-on
filter for a fellow's Yaesu (he needed a cw filter but did not want to
cough up the money for the Yaesu xtal filter). So he copied the T-T filter
in his other rig!

I have sent this info often off to persons lamenting they bought an
Argonaut/Argosy/Triton etc. without a filter inside of it. Not to fret,
here it is! Build it and the signals will come!
(by the way, this filter provided as standard and/or optional in things
like the OMNI C and Argosy
is SO CLOSE to the performance of an xtal filter it almost makes the xtal
filter redundant! Honestly, to MY ears I cannot/hardly discern a
detectable difference in use when heard against/compared to the xtal
filter.)
A lot better than no filter and its 'mobile'!

I have yet to dissect my MFJ CWF-2 filter
to prove this, but
for all intent this T-T dual opamp filter design is same as/close enough
to the MFJ/Vectronics/Ramsay audio filters sold commercially.

Brien Pepperdine Amateur Radio
Station VE3VAW
Toronto, Ontario Canada ARCI QRP #8773 - NORCAL QRP "ZOMBIE"


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Date: Wed, 17 Apr 2002 23:08:06 +1000
From: "Graeme Zimmer" <gzimmer at bigpond.com>
To: <Jimmy.Lee at bartow.k12.ga.us>,
        "Low Power Amateur Radio Discussion" <qrp-l at lehigh.edu>
Subject: [124995] Re: CW Audio Filter?

Hello Jim,

> I would like to build an inexpensive cw audio filter 

Have a look at  http://www.users.bigpond.com/gzimmer/

regards ................. Zim

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