For WQ6X it's: Filters, Filters and MORE Filters - Part 2

When I looked at the publication data of Part 1 (Sept. 08, 2023), the most noticeable thing in the
lead photo is how much the same external filter configuration units have found new locations on
the operating table, and in some cases are positioned differently in the WQ6X Stereo Audio lines.

Then again, some units found an immediate home in the audio line 5+ years ago and have remained 
in that position - case in point being the Radio Shaft stereo equalizer.  Because we only need to hear
a "small" frequency range (~600hz to ~3000hz), the equalizer deemphasizes all frequencies outside of that range, BEFORE sending the resultant audio to the splitter for redirection to the various filter
audio lines.

One of the secrets to all of the above filter combinations are the Input mixer and the Output mixer units.  The 4-channel input mixer receives its power via a basic USB connection (for the power only). The Rockville output mixer blends the different types of filter outputs into a common stereo output
for the stereo amplifier block, receiving its power from a common SAMLEX 20a power supply. 
An A-B-C-D switch directs the output to wireless headphones, or one of 3 different speaker sets,
most importantly a pair of LEKATO JA-o2 mini guitar amp speakers (8).

The other units play different roles in the assistance of recovering sound intelligibility, altho individual units can occasionally contribute artifact to the output, at which time the mixer output for that device can be reduced.  A description of these individual units makes for better understanding of the system as a whole:

1. It ALL begins feed the output of the input mixer to a classic Radio Shaft 15-channel stereo equalizer (Model 32-2059).  While virtually any equalizer device can be utilized, I like the
   RS 2059 unit because it can either be rack mounted or used as a place to put other units,
   as in this configuration with the individual QF-1A filter and LEKATO speaker combinations.
   The Audio Passband for amateur radio is ideally set from ~400hz to more-or-less ~1600hz. 
   Setting the equalizer sliders to accomplish this also offers an immediate visual indication
   of the initial passband.
   
   
2. The original Stereo Cw article implemented a pair of fixed bandpass filters (800hz and 1200hz)
   to create the "stereo" effect.  Experimenting with different filters over the years, the vintage
   Autek QF-1A analog OP-amp filters allow using ANY pair of frequencies for the stereo effect.  Should fatigue set in from a particular setting, the individual settings can be altered to create
   a similar but different effect.  This is even more useful for peaking the individual tones during RTTY operation.
   
   
3. My first real exposure to external audio filters came by way of a pair of MFJ 752 filter units purchased at different times - a "C" model and a "B" model now setup for the Left and Right channels respectively.  The "B" model was modified to include a 2 setting (180hz and 80hz) repurposed MFJ Cw filter, which makes up for the the Sub receiver's lack of Cw filters in the FT-1000mp and FT-2000 Yaesu transceivers.  Currently, the MFJ filters with wider bandwidths have more application during Ssb contests.
   
   
4. At different times, I acquired a JPS NIR-12, and later a JPS NIR-10 external DSP filters.
   Currently, these filters run in cascade mode on the Right channel of the audio, providing DSP
   filtering for the Sub-receiver of the Yaesu transceivers which lack DSP for those receive lines.  The downside of these filters is that that their processing speed actually introduces about 120
   ms of audio delay, creating a sort-of echo when using stereo reception.  This was resolved by adding an external audio-delay device to the left channel, allowing the two outputs to properly synchronize.
   
   
5. I picked up an MFJ-784 "programmable" DSP unit at one point which currently processes the left audio line Channel 3 of the output mixer.  The MFJ-784 contains a crude form of DNR and several flavors of notch filtering.  For the most part, it simply gives me more knobs to twiddle.
   
   
6. All of the filter and DSP devices eventually send their audio to the appropriate channel
   of a classic Rockville 8-channel mixer, allowing a blend of the various filter effects, which
   can easily be turned to 0 should they not contribute to audio intelligibility.  This mixer has no built-in output amplification, requiring some sort of output amplification in order to properly drive the speakers, or even the wireless headphones.
   
   
7. Over the years, experimentation has been done with different forms of audio amplification
   for this multi-filter arrangement.  Some approaches have ended up in overloading feedback,
   while others have required a jumble of wires to make it all work.  The current solution is a $23 10-watt per-channel amplifier purchased on e-Bay.  The challenge with this setup is keeping hum-creating ground loops out of the final result.
   
   
8. While it is possible to switch in one of three speaker arrangements, the current favorite is a pair of LEKATO mini guitar amp speakers.  A pair is used by shorting both speaker channels
   of each unit and feeding them with the left/right amplifier channels.  Like the input mixer,
   these speakers are powered by way of a USB hub, providing the required 5V needed
   to produce adequate audio.

While this current audio filter arrangement seems to synchronize symphonically, it has come
to fruition over 15 years of cut-and-try of the various filter units, eventually landing themselves into
this reasonably practical arrangement.  Then again, at any time, I reserve the right to rip the cables out and restring the audio into a completely different configuration.  The above scribble sheet crudely documents how it currently flows.

Do YOU use any form of external audio filtering?

What unique ideas have YOU created?