Even Further Implications from Listening to the DR.

I've written 2 Blogs around a response the QST DR. Joel Hallas W1ZR (now SK) wrote in
2014 in response to my question regarding the cascading of units that process received audio.  

• [X] - The DR Validates Cascading Filters
• [X] - More Lessons Learned from Listening to the Dr.
• [X] - Radiosport: Learning - It's ALL about Learning - Part 2

Rather than out right answer my question, he gave us information allowing evaluation
of the cascade situation for ourselves.  For cascading to actually match the theoretical,
we assume there is no D.C. interaction between the devices being cascaded.

The equipment setup I use with the portable setup @W7AYT's QTH brings together a morass
of disparate Analog and DSP equipment; each with its own power requirements, as well as its
own approach to signal ground paths.  This can lead to what are known as "Ground Loops", essentially D.C. interaction between devices.  Ground Loops can introduce hum, signal 
attenuation and occasionally "PoPs" and "crackling" noises.

Because most analog audio filters utilize OP-amp IC's, they can be prone to overloading, resulting
in distortion and unintended feedback.  A major downside of cascading different style devices is that
[so-called] "ground loops" can be introduced unexpectedly, exacerbating this problem considerably.  While attempting to resolve a time-delay artifact problem between analog and DSP devices, it occurred to me that the device order might be contributing to the ground-loop problem.

Earlier this month, I put the Radio Shaft 32-2059 15-levels/ch equalizer back into the FT-2000
audio line.  Of course, I am again confronted with a similar sort of inquiry regarding the proper equipment cascading.  Which is the "correct" order?:

• FT-2000 ===> Stereo Equalizer ===> External Audio Filters.
  - OR -
• FT-2000 ===> External Audio Filters ===> Stereo Equalizer.

Eliminating "ground loop" hum is a major goal.  While we can set the audio cutoff in each channel to <200 Hz to 0 (ZERO) just prior to the output mixer, the REAL question is whether it would be more advantageous (effective) to EQ-out the ground loops BE-4 the external filter audio chain or AFTER. 
I guess it all depends on "where" in the audio line the ground loops are being introduced.

When I tested the filter configuration at my Alameda office, (this Blog's opening picture), there were NO ground loops.  The Equalizer comes with Low-CUT buttons for Left and Right stereo channel, which theoretically, is all we should need.  

Altho I originally purchased the Radio Shaft equalizer for $49 a dozen years ago, they can still be found for sale around $100 and are WELL worth the investment.  The company produced over a dozen different equalizer models, often twice or more as expensive as the 32-2059, however the others are all too KLUNKY and frankly not as aesthetically pleasing as this unit.

In the Blog entries on Stereo CW, I could have just as easily used a stereo equalizer instead of the Autek QF-1A filter pair that I currently prefer.  Those filters take the concept to the next level thanks
to their overall PEAK/Notch capabilities.

Have you ever played around with external Stereo Equalizers with receive and/or transmit audio?

What Discoveries DiD YOU Discover?