The AM Forum

Hi All,

Is there any virtue in external audio filters such as those made by Datong, Autek,NIR, etc.

I understand that they are usefull for C.W. reception but how about A.M.?

Thanks, Roland

My casual observation is that they are great for narrow bandwidth modes but they turn modes like AM to mud. When I listened more seriously to the VLF beacons using an older Autek audio filter it was possible to easily hear weak ones that there was no trace of without the filter. Not exactly a scientific answer to your question but it's the best I've got!

:O)

Rob W1AEX

I really like the Autek QF-1A for CW.  I have several and the one connected to my SX-88 gets used for hetrodyne removal on AM (no notch filter in the SX-88) and the low pass function is somewhat useful also on AM. But I agree with Rob that it really shines on narrow band modes.  My Drake R-4C and Autek QF-1A are my favorite combo for pulling out weak CW on 160 and 80.

Thanks for the info.

I noticed the Autek QF-1A epay price went up four fold since last month.

Seasonal thing?

Ebay pricing often defies simple logic.  A few years ago SX-100 receiver were breaking 4 figures and recently I saw one of the Heathkit novice transmitters sell for just under $300 (no, it wasn't still in kit form).  The Autek filters seem to maintain a fairly high price (at least to me) on ebay but both the QF-1A (has an auxiliary notch filter) and the original QF-1 are both common hamfest items.

I have several QF-1A filters and they work well but there is nothing really special about them, they are built using op amps and there are several others that are similar in structure/performance like the the MFJ-752.  You can find plenty of construction articles if you want to build your own.  The later 70s through the 80s era QST magazine and ARRL handbooks of the era have some construction ideas.