Standoffs far apart that pass tech ground (chassis)

Wow thanks NM,  very helpful.  I appreciate your time and input.

I don't really understand what  "shielded freewire points" are ?

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Where long traces ran the length of the board, I replaced their endpoints with thru-hole pads, and will bridge them with wire that is shielded.

"And I still don't get why you want to put a 'Chassis Pour' on a pcb ?"

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Maybe chassis pour is the wrong name for it. It was  primarily meant to shield any stray emr from adjacent transformers, since they are geometrically on axis with each other (EDIT:  see photo in subsequent post) and the pcb cutout square where the bobbin is recessed means the card adjacent to the tx is what would 'shield' between channels.

"An 'AGND pour' will attenuate crosstalk between channels. As will any Copper area held at a low impedance fixed potential eg a voltage rail - but 0V is usually easily available and offers other electrical benefits."

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I suppose I have an incorrect running assumption that there may be slight differences in potential between adjacent card audio 0V since their will be differences in audio channel information. I figured chassis a separate shield, wholly independent of all circuits was the cleanest way to isolate channels uniformly. HA...to return to JR's metaphor about sewer pipe poo and dirty sink water, perhaps I'm needlessly making my sewer pipe out of gold or something?!?

So, to restate and summarize my primary concern: there isn't impedance or crosstalk issues to worry about with two AGND planes near each other sinking different audio information?

I think you've been trying to dispel that myth for me from the beginning?
(I should mention I will be running agnd and pgnd bus bars per 8 channels under the console to maximize low impedance.)



Frankly this is my first serious attempt at making some PCB's.  Forgive me for running on high-school physics level instincts here, and I know there's plenty of design history that proves what works and what doesn't. =/

If your major concern is cross-talk between adjacent transformers, and this can happen, you will need a much more substantial shield than a copper pour - you will need at least a steel sheet.

If you main concern is cross-talk due to other forms of coupling then an agnd pour is your simplest option but don't expect it to stop magnetic fields.

Cheers

Ian

ruffrecords said:

If your major concern is cross-talk between adjacent transformers, and this can happen, you will need a much more substantial shield than a copper pour - you will need at least a steel sheet.

If you main concern is cross-talk due to other forms of coupling then an agnd pour is your simplest option but don't expect it to stop magnetic fields.

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+1
You need high permeability to bother H fields (magnetic).
Means ferrous material. Steel will do something but there's a reason those input transformer cans are made of a mu metal material.
Don't look at the cost if you're nervous 

With H-field distance is your friend - it drops off as square law or cube law depending on near / far field - can't recall the detail now.
Also you can orientate for minimum interaction.

Ugh. Wow, yeah, permeability. That's a pretty basic oversight on my part. Thanks for taking time out to help and make suggestions.

I might need to have engraved a plate on the frame of this mixer with all the names of people that have been kind enough to nudge me back on track.