In today's whirrrrl'd, most transceivers use micro-minature SLIC's (Surface Level Integrated Circuits) and are not easily serviceable by the average home-brew amateur. Linear amplifiers and antennas
are large enough that we can still have a hand in their design and upkeep. We can also add external audio and I-F filtering to most radios allowing us to improve their performance somewhat.
I've written extensively on the subject of various approaches to audio filtering for survival in the 21st century. Many of those Blogs have links to earlier Blog entries written on variations of this subject. Several Blogs that come to mind include:
- [X] - (Apr. 2021) - WQ6X Wanders thru an Experimental Contest Weekend
- [X] - (Nov 2020) - More Lessons Learned From Listening to the DR.
- [X] - (Aug 2020) - Learning the Art of Leveraging Dual Receive:
Yet Another Look (w/Links to the Past) - [X] - (Dec 2019) - Analog or Digital - Which shall we choose?
To combat interference issues, receivers became increasingly complex. NoT only were they more difficult to build, accurate alignment required test equipment that was usually way out of the reach of most amateurs. Companies like Heathkit, KnightKit, Eico and Viking (among others) bridged the gap by offering kits with units requiring alignment already pre-built and aligned. By the 1960's, 80% of the equipment was commercially made, turning us into [so-called] "appliance operators".
Over the years, I have amassed quite a collection of external audio filters and switch boxes, including:
- 4 Autek QF-1A's (2 in Alameda and 2 in Concord) - one for each ear, creating a "Stereo" effect.
- 2 MFJ-752's - a MFJ-752c (L-Ear) and a modified MFJ-752b (R-Ear) with a CWF-1 filter in place of the kludgy noise-limiting diodes.
- An MFJ-751 used in Alameda as a balancing-unit between the two Autek QF-1A filters.
- An MFJ-784 Super DSP for Left-channel audio processing in Concord.
- A JPS NIR-12 DSP unit for Right-channel audio processing in Concord.
- A Radio Shaft DSP-40 audio DSP unit (under evaluation in Alameda).
- A Radio Shaft 15-band per channel Equalizer (32-2059) for use in audio
experiments at the Alameda location. - A pair of Radio Shaft 4-IN/1-OUT COMPOSITE AV SELECTOR SWITCH
boxes (15-313) - one in Alameda and one in Concord.
ROCKVILLE 4-Device (8-Channel) Audio Mixer
Because my WQ6X portable operation at W7AYT utilizes 3 different transceivers and several audio filters, a ROCKVILLE audio mixer has been installed making it easy to blend the audio from each radio into a series of different filter configurations. The downside of this is very low output levels from the mixer. This problem was remedied by installing a 2-watt stereo amplifier board inside the mixer and routing the amplifier output to an 1/8" stereo plug matching the Radio Shaft 4-device AV-selector box.
Each of these devices have separate grounding considerations, making it quite a challenge to keep ground loops out of the audio. Having a multi-band equalizer between the mixer and the AV-switch box would be one way to accomplish this. Prices for another Radio Shaft 32-2059 15-channel equalizer unit are only around $40 on eBay. This may well become an addition to the Concord location in the future.
RADIO SHAFT 32-2059 15-CHANNEL EQUALIZER
Audio equalizers are an excellent way to tailor receive audio in most amateur stations.
The above equalizer has also been used to tailor the transmit audio of the old Kenwood
TS-450 transceiver and the current ICOM-7000. Because the Yaesu FT-1000mp and the
K3/0 Mini transceivers have their own menu-adjustable transmit audio equalization circuits,
I use the Radio Shaft equalizer to process only stereo receive audio.
With the 32-2059 equalizer 60-cycle hum and high frequency "hiss" can be rolled off.
Additionally, specific audio frequencies can be enhanced for improved Ssb and/or Cw reception. However it should be pointed out that like the ROCKVILLE mixer, equalizers often lack sufficient amplification output to drive speakers, altho headphones usually work fine.
STEREO-Cw - Center-channel Audio
In addition to Stereo-Cw, another (even older) idea is what has been called the stereo
"Center Channel". This is created by running the (-) leads from each speaker into a
THIRD speaker, positioning it electrically "between" the Left and Right channels.
STEREO-Cw - Center-channel Filtering
For some time, I have been experimenting with a variation on the center-channel approach, substituting an MFJ-751filter in place of the center channel speaker. To make this work properly,
it was necessary to electrically isolate the two QF-1A units. The MFJ-751 input/output is inserted between the shield conductor of the Left QF-1A and the shield conductor of the Right QF-1A. It just so happened that the units are sitting on a wooden platform; wood being a good indoor insulator.
The WORLD of MFJ-752 Signal Enhancer II analog filters
Having completely forgotten about the Autek filter line, I began making use of external audio filters by way of the MFJ-752 (models B & C). When the FT-1000mp made the scene, the one noticeable lack was audio processing for the sub receiver - the MP's eDSP works ONLY with the Main RX.
Remembering an MFJ CWF-2 was languishing in the junk box, the circuit board was removed and retrofitted to fit inside the MFJ-752b unit. If you look real close at the photo you will see that the NL positions of the On/Off/NL switch were sacrificed to provide 180-hz and 80hz Cw filtering. Today, the 752C is used to process SSB audio for the Main RX (Left Channel), leaving the 752B to process SSB/Cw audio for the Sub RX (Right Channel).
setup in Concord, I happened onto a $9.99 MFJ-751 Signal Enhancer; predecessor to 752 Signal Enhancer II.
While technically, they are descendants the filtering capabilities of the 752 relegated the 751 to the back of the used equipment shelf in Alameda.
As described above, a legitimate
(albeit unusual) use for the MFJ-751
was found as an "arbitrator" unit between a pair of Autek QF-1A filters.
The WORLD of AUTEK QF-1A Analog Filters
Coexistent with happening onto the idea of Stereo-Cw, I also happened onto a pair of Autek QF-1A filters (I now have 4; 2 in Concord and 2 in Alameda). I was about to order the individual components to replicate the circuit in the original 1977 article on the subject when it occurred to me that the individual QF-1A filters would offer an even better idea: Variable Frequency Stereo-Cw.
Having variable frequency filters allows the shifting to left/right channel frequency cutoff points.
That way, if a particular set of filter parameters become too monotonous, they are immediately/easily shifted into something different. A variety in listening-experience contributes considerably to fatigue-reduction.
Occasionally, one encounters a QF-1A with an after-market pilot lamp added as you can see in the above pair. The power transformer in those Autek units is so hefty, that I have been contemplating removing the power cord on the 2nd QF-1A and running a power pole connection to the unit with
the modification. Another possibility would be to power each of the MFJ-752 units from a power
pole connection to the two Autek QF-1A's.
FRONT-ENDing and REAR-ending the AUDIO PATH
Having an array of filters is nice, but ONLY if that array is properly in-Fed and out-Fed.
Switch boxes do that nicely. I found the small box (on the right) on e-Bay a couple of years ago.
I shoulda bought 3 or 4 because when I put in an order to the Korean vendor last year I received the information that for now they are not shipping to the USA. As a replacement, I settled on a Radio Shaft AV Selector box (15-313). The AV box has the advantage of switching an additional YELLOW wire at each switch position.
WRAPPING IT ALL UP
As you can see, I love to play with external switches and filters. For the most part, even the best of amateur transceivers can benefit from the addition of external filters. Many operators sold off their old filters when they bought the new radios, giving me the opportunity to snap them up for an average of $35 per unit.
Do YOU make use of external filtering units.
What has been YOUR Experience?
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