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Author Topic: Safe and Efficient Grounding of Tube Microphones And Power Supplies?  (Read 7656 times)

River

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Hi Everyone -

We have 8 C12a mics all with N12a power supplies, but over the years they've all been repaired/modded in different ways.  Some have the ground pin on the AC plug removed, others don't.  You never know what you're going to end up with hum-wise and generally we end up having to lift the ground on the PS.

The N12a was apparently originally delivered with just a 4 conductor power cord with no plug attached.  The blue wire goes to circuit ground and the green and yellow to chassis ground.

What are everyone's thoughts on the best way to implement these?  I'd like to find a standard that works.

Tim

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klaus

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Re: AKG N12a Grounding
« Reply #1 on: April 17, 2014, 03:40:16 pm »

When the C12A was sold in the USA, there were no third-rail safety ground plugs. It was strictly a two-prong AC affair.

Sticking with that standard should not present grounding noise issues, provided that the mic 's grounding/shielding scheme is adhered to: all cable, connector, and mic chassis grounds need to be connected to the N12A's chassis/housing.

The safety of the original 2-prong approach is clearly lacking, especially in today's three-prong code environment, and there is a way to rewire the AKG power supply to accommodate a third safety rail. But AKG did not make that easy with its carefully isolated audio and power supply ground scheme.
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Klaus Heyne
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River

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Re: AKG N12a Grounding
« Reply #2 on: April 17, 2014, 03:55:53 pm »

Thanks Klaus!

So you just go with a 2 prong AC plug and let both the chassis and circuit grounds float?

That's the case with the majority of our power supplies at present.

-Tim
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Jim Williams

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Re: AKG N12a Grounding
« Reply #3 on: April 18, 2014, 10:56:27 am »

Chassis and power grounds usually connect at some point, just to minimize buzz pickup by the casing. In large consoles they connect at the console frame.

Other techniques are also used. Some float the power ground but it is connected via a small resistor (10 ohms) to eliminate any looping or connected with a small film cap to allow AC noise from about 250 hz up to be grounded. Some designers do both.

One or more of these techniques will work and create a safety ground. All AC connected audio gear should have that AC safety ground installed, that's why the US added that 3rd ground plug 50 odd years ago. Using high voltage tube gear without a safety ground can be risky, I'm not a gambler.

Eventually, the audio ground will connect to the safety ground if the mic is connected to a grounded mic preamp.
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radardoug

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Re: AKG N12a Grounding
« Reply #4 on: April 19, 2014, 04:25:00 pm »

It is NOT a good idea to go with the two wire cord. In the event of a mains fault, the ground current path is through the audio ground lead to the mixer. If the mixer grounding is not correctly done this path can be poor.
The best method is:
Ground the power supply metalwork to the a.c. ground with a standard 3 wire cable and plug.
Isolate the audio ground.
Connect the audio ground to the electrical ground with a small resistor and capacitor in parallel.
The resistor can be 33 to 100 ohms. The capacitor a .47 uF 50 volt.
Ideally all metalwork on the microphone should connect to the electrical ground.
On most microphones this wiring scheme is easy to acheive.
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klaus

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Re: AKG N12a Grounding
« Reply #5 on: April 19, 2014, 05:52:53 pm »

I agree with your basic premises. They are logical and seem self-evident, except:

1. Why would Neumann connect audio ground and chassis ground to B- ? In a NG power supply, we are talking about 300VDC+, worst scene scenario, and, on top of that, in a predominantly 220VAC (European) environment?

2. I have never heard of an electrical shock incident in our industry caused by any of the millions of "legacy" -prong plugs on vintage audio equipment out there.

3. I see lately more and more 2 prong plugs being used in modern equipment. (Just inflated my tires with a metal-housing Black & Decker AC-tire inflator. Where's the safety ground (it bears the UL seal of approval, by the way)?

Love to discuss this in more depth with anyone who may contribute some wisdom to the matter.
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Klaus Heyne
German Masterworks®
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David Satz

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Re: AKG N12a Grounding
« Reply #6 on: April 20, 2014, 12:06:38 am »

A safety ground (i.e. a three-wire AC power cord arrangement) is not, in itself, a UL requirement. Rather, UL has a separate set of safety standards for two-prong equipment, which are more stringent in some ways than those for three-prong equipment because the third wire isn't available.
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klaus

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Re: AKG N12a Grounding
« Reply #7 on: April 20, 2014, 04:15:03 am »

I am curious how to implement a stringent and safe 2-prong AC arrangement that avoids audio ground loops, incomplete shielding, etc. yet has shock protection.
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Klaus Heyne
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David Satz

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Re: AKG N12a Grounding
« Reply #8 on: April 20, 2014, 12:23:59 pm »

I feel the same way. It's an area that I think a lot of audio professionals are uninformed or (worse) misinformed about. And ignorance in this area doesn't only contribute to decreased audio quality; it leads to injuries and deaths.

Bill Whitlock (head of Jensen Transformers) gives excellent presentations on shielding, grounding, cabling, audio interfaces and (to some extent) AC powering at AES conventions in the U.S.; he keeps being asked back year after year. But I still feel that I haven't found the kind of systematic explanation of the topic that ought to be available to the audio world. Two years ago there was an AES workshop specifically on the ABCs of AC powering, but it focused mainly on power transmission and the interface between the power company's lines and the service panels in buildings--not so much on interior wiring or implications for the design of audio equipment.

http://www.jensen-transformers.com/apps_wp.html has some notes on Bill's talks, though (among other papers), and that's at least a good starting point.

--best regards
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Kai

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Re: AKG N12a Grounding
« Reply #9 on: April 21, 2014, 08:46:48 pm »

I am curious how to implement a stringent and safe 2-prong AC arrangement that avoids audio ground loops, incomplete shielding, etc. yet has shock protection.
Very simple - you might alread know as you might (should) use it in your lab (I hope, for your safety):
Use a high quality isolating mains transformer.
This way mains are no longer ground referenced so you have several advantages:

- Safety: Although it's not completely impossible to get an electric shock if something inside the PSU goes wrong, the amount of current flowing through you in this case is greatly reduced to a safe amount. Only some capacitive coupling to the mains through the transformer windings is present.

- Earth is lifted at the mains side, without loss of shielding, grounding is done as normal via the audio shield connection.

- The now balanced mains voltage can in some cases further reduce hum introduced into the audio path.

You still have to make sure that your system is coupled to earth at one point (e.g. on the preamp or mixer), otherwise you can get strange noise effects due to the voltage that builds up against earth on the whole system.
It's even necessary for safety, health and comfort - it's not too nice to get a little electric shock each time you touch your equipment.

These transformers are not expensive, you don't need an extremly strong one.
It should match the Mic's PSU wattage with some headroom.
You can use one transformer for several mic PSUs, as long as all of them use two-prong connectors.

BTW: for my lab I have a transformer with adjustable output voltage, so I can e.g. simulate different mains conditions.

Regard
Kai
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klaus

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #10 on: April 21, 2014, 09:05:00 pm »

Yes, Kai,
I use that system, but, frankly, as such, I am in a minuscule minority. Almost everyone plugs the tube microphone power supply into the wall, with no isolation transformer to be found far and wide…

Again: why would the geniuses at Neumann devise a system where AC, DC and audio ground are connected? What lack of respect for human life might they have had to neglect even a remote possibility of electric shock? Or did they know something about safe grounding with two prongs I have not discovered yet?



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Klaus Heyne
German Masterworks®
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Kai

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #11 on: April 22, 2014, 07:04:50 am »

Or did they know something about safe grounding with two prongs I have not discovered yet?
They knew nothing!
Remember these mics where build when it was still common to nail bare wires to the wall for mains supply!
People knew they better not touch that.

Regards
Kai
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radardoug

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #12 on: April 22, 2014, 04:32:27 pm »

Klaus, I point you to the power supply for the U67. This had pcb tracks and links so you could reassign the audio ground. So they have always been aware of the problem. In an ideal world, there is no problem, but often consoles have poor grounding of pin 1. The audio shield should not be used as a safety ground, and the mains ground should be! Connecting them all at one point in the chassis is not necessarily a problem.

The two wire power cord is not a safety option.

You can build the mains transformer to achieve low leakage, but that is what it will be, low leakage not zero leakage. A safety ground is always the preferred option.

I note that here in New Zealand it is supposedly mandatory that any item with a 2 wire power cord should have no exposed metal parts, and yet stereos are imported all the time with audio connectors exposed and connected to the internal grounds! The market rules!
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Jim Williams

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #13 on: April 23, 2014, 10:45:43 am »

The markey does indeed rule. For decades electric guitars have had the exposed metal parts grounded. Yes, several have been electrocuted.

Replacing the grounded bridge with a safety capacitor will prevent that, but Fender has more clout than OSHA.

BTW, all of my guitars are fitted with safety grounds. Many times playing in clubs I enjoyed the other acts getting zapped while I never did.
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klaus

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #14 on: April 23, 2014, 12:48:47 pm »

Jim,
You are always good with theory: It has been argued that grounding via capacitor is not as effective in regards to noise suppression as direct wire grounding. What say you?
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Klaus Heyne
German Masterworks®
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