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 91 
 on: May 22, 2021, 06:05:44 PM 
Started by Jim Williams - Last post by 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.

 92 
 on: May 22, 2021, 12:40:33 PM 
Started by Jim Williams - Last post by 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.

 93 
 on: May 21, 2021, 12:23:32 PM 
Started by Jim Williams - Last post by Kai
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.
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.

 94 
 on: May 20, 2021, 12:41:04 PM 
Started by Jim Williams - Last post by 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.

 95 
 on: May 20, 2021, 11:34:58 AM 
Started by Jim Williams - Last post by Jim Williams
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.

 96 
 on: May 19, 2021, 02:04:37 PM 
Started by Jim Williams - Last post by boz6906
IME, ground loops are caused by current flowing in the shield of the interconnect cable, caused by the gear having different chassis potentials with referenced to ground.

Yes, this may be cured by lifting the pin 1 connection but there is usually an underlying problem such as a leaky power transformer or AC bypass cap and that problem really should be corrected first.

My first step in addressing ground loops is to voltmeter the chassis-to-ground potential of the interconnected units.

And ground loops are a completely different problem then RFI/EMI.


 97 
 on: May 14, 2021, 08:54:19 AM 
Started by Thomas W. Bethel - Last post by Thomas W. Bethel
https://www.youtube.com/watch?v=9p5jyysloTw&t

 98 
 on: May 13, 2021, 02:37:49 AM 
Started by rdraudio - Last post by Kai
The other day I had a new car battery installed and the tech sprayed something from a spray can on the battery connections, he said if there was any corrosion left the color of the spray would have changed and I had the impression the spray also cleaned.

I don't know if that applies to NiCd corrosion.
No need to spray (never spray!) strange chemicals, probably not intended for this use which possibly make things worse.

To clean certain areas of printed circuit boards I use a thin hose, taped to a (water capable) vacuum cleaner.
While applying cleaning fluid with a small brush - usually demineralized water + isopropyl alcohol - I vacuum away the fluid from the board.

This way I can limit the cleaning fluid to the contaminated areas of the board, preventing it from spreading the contamination.
At the same time this technique removes ALL contamination, as the cleaning fluid isn‘t just drying on the board, but completely sucked away.
Always suck away the fluid from the brush before dipping it into the fluid for the next dose, to keep the fluid clean.

This works well even with very high impedance circuits, where every little bit of residue disturbs the function.

Warning: Don’t do this with microphone capsule’s diaphragms.

 99 
 on: May 11, 2021, 03:55:11 AM 
Started by Bo Hansén - Last post by Bo Hansén
Klaus,

Of course, I will keep you updated regarding these transformers.
These Ü35 and Ü54 versions seem to have come into the shadows when it comes to newly manufactured replacement transformers.

--Bo

 100 
 on: May 11, 2021, 02:49:24 AM 
Started by Bo Hansén - Last post by klaus
...and please update this thread, once you had a chance to compare fit and finish, and, most of all, sound!

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