|OUTBOARD FILTER COMBINATIONS USED by WQ6X|
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)|
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
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.
500k-pot range problems.
|FILTER COMBINATION - AUGUST 2018|
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?