BLAST from the Past: External Audio Filters I have Known

Over the last 50 years a gradual transition has taken place inside the transceiver end of things
as BOTH transmit and receive audio processing shifts from analog to digital, and even a hybrid combination of the two technologies.  Always on the lookout for the "ultimate" in signal-copying solutions without investing $10k to get there has prompted me to consider external audio filtering
as a viable solution.

It does not matter how much power
I run, or, how Super-Duper the antenna system is, if signals on the receive end are unintelligible, quality point-to-point communication is all but impossible.


During simple ragchewing, compromised signal intelligibility can be tolerated to some degree.  In radiosport, primo-intelligibility is why so much money is invested in transceivers with the latest Doop-de-Doop DSP circuits built-in. 


For those of us with a more limited budget, external audio filters are frequently almost as effective as the built-in IF-based filters found in the $5k+ to $7k+ radios.  Not only is my radio budget somewhat limited, on the other end of the spectrum I like to experiment with ad-Hoc equipment configurations;
if one configuration doesn't work as hoped, sometimes all that is needed is to swap the equipment into a different configuration to achieve a more effective result.

Additionally, because I absolutely love to knob-twiddle, external filters completely satisfy that
knob-twiddle need; and, I don't have to feel guilty for spending $5,200 or more to make it all happen. 

As you can see, the above graphic illustrates the variety of different filter combinations I have made use of over the last 15 years.  What to me is amazing is the fact that I still possess all of the above-listed filters, while the TS-50, TS-450, FT-900 and FT-920 have all moved on to other venues. 

I have yet to test-run the English Datong analog filters, or the highly coveted TimeWave DSP filters. 
If you are aware of any of these devices available for chump-change, lemme know.

In recent years I have written up the various filter combinations attempted during a myriad of different Cw, Ssb & RTTY contests.  To make things easier, I recently posted a Blog that made a reference to these write ups.  ([CLICK HERE] to read that.)

The first external audio filter I ever encountered was the [then] JPS NIR-12 External DSP unit. 

It's not surprising that JPS biggest customers were/is the military; the NIR-12 weighs several pounds and is all but bullet proof.  My only issue with the NIR-12 is that those 2 large DSP chips generate a LoT of heat.  I have currently pulled the NIR-12 from active service to add a dual-fan cooling system under the hood, made with the fans rescued from a repurposed laptop cooler.

The NIR-12 makes an Excellent DSP add-on for the FT-1000mp sub-RX.  The eDSP unit in the Yaesu radio works only with the Main-RX; the sub-RX is relatively wide-open.  The NIR-12 adds adjustable selectivity, dynamic signal peaking, digital noise reduction (DNR) and an Auto-notch filter (DNF) to
the sub-RX, all on par with the main-RX's built-in eDSP.

Sometime later a pair of MFJ-752 (a Model B and a Model C) "Signal Enhancer II" filters were found on eBay for chump change.  Descended from the original Signal Enhancer - the MFJ-751, the 752 units provide reasonable results, altho the peak filtering was never quite what I hoped it would be.  Then again, used in manual notch filter mode, the MFJ-752 performs rather admirably.

Out of frustration, an experiment was undertaken to cascade the MFJ-752C into the JPS NIR-12.  While that configuration worked alright, the major benefit came from the NIR-12, not the MFJ-752.  What was needed was the CW filtering capabilities of the NIR-12 without having to cascade the

entire JPS unit to get it.

Remembering that an old CWF-2 filter was languishing in storage, a project was undertaken

to move the circuit card from the CWF-2 into the MFJ-752B (leaving the 752C intact for further experimentation).  While this was certainly an improvement, it wasn't long before a pair of Autek

QF-1A's made their way to the operating position, one for each of the receivers in the FT-1000mp, bumping the 752's to the back of the storage cabinet.

If you have been following the WQ6X contest Blog over the years, you know that I've become
quite enamored with the Autek QF-1A filter; so much so that I now own 4 QF-1A units: 2 for remote access to Fallbrook from Alameda and 2 in permanent operation @W7AYT.  Evolved from the original QF-1 filter, the 1A provides a significant improvement in performance. 

While the MFJ-752 and QF-1A units are functionally similar, the QF-1A peak and notch filter circuits are far superior.  The 40+ year old Autek QF-1A audio filter is more functional than most built-in APF filters found in transceivers from that design period.  I personally appreciate the built-in power supply in the QF-1A; no need to search for a 12-volt source.  One of the 4 QF-1A's has been modified to provide power for the MFJ-752's and an MFJ-751 that I stumbled onto last year.

The next progression came from removing the 752-C and replacing it with the NIR-12 cascaded
after the QF1-A in the FT-1000mp's sub-RX audio line.  Variations of this combination have been in use for nearly a year until recently when an MFJ-784 DSP was brought in, temporarily replacing the NIR-12.  The MFJ-784 has been languishing in storage for years, motivating me to search for a .Pdf operation manual.  The MFJ-784 is nearly twice the size of the NIR-12 however with an aluminum chassis (instead of steel) it weighs about 1/2 as much.

Unlike the MFJ-752 or even the Autek QF-1A (both analog filters), the MFJ-784 is TRULY a fully

functioned DSP unit.  To get the most out of the MFJ-784 several read-thru's of the instruction manual was required.  The concept of "storing" DSP settings is new to me and required a bit of imagination to wrap my brain around it.  Compared to the NIR-12's noise reduction circuits, the 784's noise reduction capability is Truly AWEsome; even surpassing (in my experience) the main-RX's eDSP DNR circuit in the FT-1000mp.

I am not used to seeing an audio-AGC circuit; however compared to the NIR-12's DYN-Peaking, the AGC is incredible; almost too good.  It is easy to overload the MFJ-784 causing distortion; especially with the AGC enabled.  The solution of course is to back off the transceiver receive audio.  Because receive audio is currently routed through a ROCKVILLE 4-channel stereo mixer @W7AYT, audio to/from the 784 is easily adjusted for the most effective DSP action.

During the German DRCG RTTY contest, work commitments kept me in Alameda the entire weekend.  This offered up the opportunity to test-run the MFJ-751 with the Autek QF-1A filters evaluating unit compatibility.  You may remember the center-channel concept in stereo audio, configuring a 3rd speaker between the Left and Right channels.  I am experimenting with a similar idea using a 3rd audio filter between the outer two.  When the concept has been properly and thoroughly tested, I will write it up as part of this series.

As you can see, I have a love of external audio filters and the signal-recovery rescue they can provide.  In addition to giving me more knobs to twiddle, the ability to pick signals out of the oblivion
is truly AMAZING.

Do YOU make use of external audio filters, Analog or DSP?

What combinations do YOU use?