R/E/P > Klaus Heyne's Mic Lab
XLR shell connections to ground in a microphone cable
klaus:
Since my last post, I received three more requests to troubleshoot RF in condenser mics - two solid state, one tube. In all three cases the clients were again able to solve the issue by following Neumann's (and my) grounding/shielding scheme.
P.S.: Not sure whether I had previously mentioned it, but in the current Neumann U67 power supplies AC, chassis and audio ground are one.
Kai:
--- Quote from: Jim Williams on May 20, 2021, 11:34:58 AM ---It's a bit more than that, you must first determine if chassis ground (AC ground) is internally connected to power ground (audio ground return path). If so it can be isolated via a 10 ohm resistor between them. A .1 uf cap across the resistor will provide an AC connection above 500 hz to remove buzz and any rf. I do this on rack mount mic preamps to avoid any issues when rack mounted.
Otherwise it's a crap shoot every time you interconnect various AC powered audio pieces.
--- End quote ---
ABSOLUTE NO-NO!
Don‘t anybody mess with the connection of mains (safety-) ground to chassis!
This is deadly dangerous.
If a device is designed that way, then this connection is essential for safe operation of the unit.
It’s possible to interrupt CIRCUIT ground from chassis / safety ground IF NO CONDUCTIVE PARTS OF THE CIRCUIT STICKS OUT AND CAN BE TOUCHED.
Few devices are built like that, and usually doing so takes some effort, as multiple connections need to be isolated. None of this, I suggest, is a layman's job.
In the case mentioned, I suggest to use Humfrees for rackmount.
Jim Williams:
I did not suggest that. AC chassis ground connections must be maintained unless it's a non-grounded AC feed like many common dbx compressors. The audio power ground is isolated in many designs. These devices use isolated TRS jacks and XLR connectors from the chassis. If audio power ground is connected to chassis ground via multiple connectors, isolation is not possible.
Yes, you need to know about this subject before you start probing. It's intended for designers, not end users.
Kai:
I was mislead by your terminology.
What you call “AC ground” is in fact “SAFETY ground”, “Schutzerde” in German (yellow-green wire in most countries).
The term audio power ground was unclear to me. I think you mean the PSU’s secondary DC ground.
A PSU has a multiple of those, one for each voltage, mostly, but not always, combined into -> (audio) circuit ground, often clearly visible as a starpoint on the PC-board.
On current devices, to avoid RFI problems, the audio circuit ground is connected to chassis on multiple positions, isolation is almost impossible and kills RFI immunity.
On devices with safety ground, doing so could be a safety problem too, i.e. imagine a short in the power transformer or other loss of isolation between primary and secondary.
I’ve seen this, not only on decades-old devices where isolation materials are far beyond their intended lifetime.
Even current devices can lose isolation, caused by over-voltage from mains or nearby lightning strike.
Humfrees are the better, safer and more user friendly solution.
Or even simpler: all my studio racks have wooden mounting rails, where isolation comes for free.
Jim Williams:
Here I use standard metal rack rails for support. Some devices like the Bricasti M7 have a lot of rear weight pulling on the rack screws, metal is far more secure than wood. The sag from behind can cause the device cases to touch even though there is isolation on the rack rails. I do use isolation washers, those with the ability to float the screw away from the rack rail.
I use a continuity meter to test isolation after an install. A strip of isolating plastic across the rear of the device will prevent sag connections.
However, that will not prevent possible ground loops simply because in some devices the audio supply power ground still connects directly to AC safety ground internally. Patch those devices together and you create dual ground return paths, one through the DC audio power ground via connectors and another through the AC connected safety ground. This is where the 10 ohm isolation resistor has its benefit.
I recall doing extensive testing on outboard pieces with the Audio Precision back in the 1990's. What I found was a crapshoot. It became apparent that audio gear designers failed at interfacing and testing their products in a real world app, isolated on the test bench they all tested ok. When interconnected, the issues arose.
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