Jon, sounds like you're the guy with experience that others come to for help when they're stumped. You mentored someone by demonstrating how a scope probe makes a great 'sniffer' for detecting radiated fields and how to use metal shields to block these fields. Good stuff to know.
Your colleague learned from you, and many reading this story may have just gained another incremental bit of knowledge that might be useful someday. Thanks for sharing.
Any idea on what would cause a typical sounding 60/120Hz buzz with a U-grounded Pro-crossover/power amp rack, driven by a TEAC CDZ500 directly?... but only when the CD player typical output section muting FETs are unmuted, as ONLY in Play mode. Also dead-quiet in Pause or Stop mode. Cd player sounds dead quiet into standard two prong AC powered Pioneer integrated amp plugged into the identical AC source!!! I have 30+ years in this and am scratching my head.
These are highly critical and for every application we need to solve them differently.A standard set of ideas or solutions are not sufficient. One need to work hard to find out the reasons and solve the conducted and radiated emmissions.
This reminds me of my first job as a EE Technician. One of my first assignments was to breadboard four copies of an audio processor that had both input and output transformers. These were rather large and heavy, so I positioned them at one end of the board and built the circuit in a "U" shape, with the digital controls elegantly at the other end.
One function of the circuit was to disable the output audio by means of a digital input. The engineer-in-charge was polite but disappointed when the in-out isolation test failed, and explained to me that putting two transformers so close to each other would inevitably lead to crosstalk.
I spent the rest of the afternoon moving the output stages to the other end of the PCB.
As someone who specializes in troubleshooting studio-grade audio equipment (and systems) for noise coupling issues, I amazed at how much basic physics is forgotten or ignored by most engineers. Coupling, whether capacitive or inductive, is proportional to the rate of change in voltage or current. A good example is a system that's free of audible hum and buzz ... that is, until an ordinary light dimmer causes fast-rising currents to flow in AC power wiring. The inductive coupling of this into the ground system will reveal "latent" coupling mechanisms (especially in unbalanced signal interfaces, which have no inherent ability to reject ground noise).
To auddoc: While I haven't seen the schematic of the CDZ500, I'll assume that its output is unbalanced and I'll guess that it's "muting" takes the form of a series switch on the high side of its output. When the switch is open (output muted), the crossover/amplifier input can't "see" the signal or ground reference (which has the buzz voltage). You can contact me directly if you like, with more details, at firstname.lastname@example.org.