Friday, August 24, 2018

Q-Filtering for Fun and Profit - Part 2: What I've Learned So Far

In recent months, I have been experimenting w/various combinations of outboard audio filters for the FT-1000mp I operate from W7AYT's QTH in Concord.  In March of this year I wrote the original BLOG on this subject, not knowing what kind of changes I would be considering, each and every month.  ([CLICK HERE] to read the original BLOG entry.)   Recently, an original (ca. 1972) Autek QF-1 has been added to the mix of outboard filter equipment I use in contest operations; whether operated remotely from Alameda or portable @ W7AYT.  The original testing and evaluation of the QF-1
was made during my remote operation of NX6T in the recent NAQP Cw contest.  The QF-1
offered enough promise that it was transplanted to W7AYT joining the other Autek QF-1A unit.

QF-1 (Top) and QF-1A (Bottom)
Comparing the QF-1 & QF-1A filters,
it would seem that while the PEAK, NOTCH & LOWPASS capabilities are functionally similar, a close-up look at
the equipment schematic diagrams
show the circuits to be implanted
quite differently.

The original QF-1 shares a design flaw with the MFJ-752 series of filters.  Both filter designs utilize a pair of dual-500k pots (for selectivity and frequency) to control the feedback circuits for Peaking and Bandpass filtering.

Both designs suffer from a "cramped" tuning range caused by the selection
of a 500k pot value.  I am proposing a bridging resistor across each dual pot. 

Doing this will "spread" the actions of each knob more evenly "across" the dial, making the adjustment less "touchy".  How this experiment turns out I will document in Part 3 of this BLOG Series.

An interesting conundrum is that the QF-1A filters ALSO make use of dual-500k pots and yet there is no "dial-cramping" with the newer (1977) design.  Go Figure.  Operationally, the two filters are quite similar.  The 1A simply adds a dedicated notch filter allowing Peaking and Notching at the same time.

Component-wise, the QF-1 filter unit uses an LM-741 IC along with LM-747 IC's (which are dual-741 OP-amp IC's) in the filtering circuits.  The QF-1A uses LM-348 IC's which are QUAD-741 OP-amps inside one DIP package.  Comparing the two filter units, operationally I find the QF-1A does a superior job of filtering compared to the original QF-1; and of course, the QF-1A does not have
500k-pot range problems.

A not-externally noticeable advantage of the QF-1A is a pair of trim-pots inside of the unit.  Page 7 of the Autek instruction manual details a relatively simple alignment procedure to peak the performance of the QF-1A unit.  The manual ALSO says [re-]alignment should not be necessary.  In fact however, 3 QF-1A units I have refurbished ALL were somewhat out of alignment.  Investing a mere 20 minutes per unit yielded increased output gain on all units.


In recent years I have made use of various combinations of external filters.
While the Autek QF-1A and MFJ 752-C are functionally similar, I find the Autek
unit to produce sharper and/or more-pronounced audio peaks than the MFJ units.

For the current configuration, I brought back the JPS NIR-12, reinstating it in the Sub-RX
audio path of the Yaesu FT-1000mp.  It is NoT well understood that the Sub-RX has nothing
in the way of IF-Shift, NOTCH or DSP.  The NIR-12 effectively provides this functionality. 
The NIR-12's DNF (Digital Notch Filter) is quite effective.

While the NIR-12 has a sharply tunable bandwidth, outputting the signal to the MFJ-752C allows taking advantage of the MFJ CWF-2 custom-installed "under the hood" replacing the MFJ's cheesy NL (noise limiter) diodes.

Bottom-line: No matter how highly-rated your receiving equipment, they can all benefit from external audio filtering.  Additionally, if you are a "knob-twiddler" like me, external filtering keeps me busy and provides the illusion that I can eliminate ANY QRM that seems to be problematic.

What about YOU?
Do YOU utilize external filters?
If NoT, WHY NoT?

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