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Author Topic: Safe and Efficient Grounding of Tube Microphones And Power Supplies?  (Read 7193 times)

Jim Williams

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #15 on: April 24, 2014, 10:57:59 am »

That depends on what you want to ground. An electronic audio device, yes, always. I use a combo resistor and small cap to prevent looping and the cap shorts to ground above 250 hz. That way lethal AC voltage is blocked but the high pass function allows the upper harmonics (buzz) to be bleed off to the chassis ground.

On an electric guitar, only the cap is used because using the human body as a grounding rod is not advisable.
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J.J. Blair

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #16 on: April 25, 2014, 11:14:52 am »

Jim, sorry, but I'm trying to grasp this.  You're saying you use a cap between the ground wire and the bridge?  I'm confused.
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Jim Williams

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #17 on: April 25, 2014, 12:14:13 pm »

When you have an audio/power ground that is seperated from the chassis/safety ground, or if there isn't a safety ground provided I use a resistor/cap combination to connect the two. A .1 uf 250 volt film cap (Wima MKP-2) paralled with a 10 ohm resistor, 1/2 watt. You would need to replace a two prong AC cable with a grounded cable with the AC ground tied to the metal chassis.

This provides a safety ground connection so any lethal AC voltage leakage will be drained off to ground.

The 10 ohms is almost a complete connection with just enough resistance to prevent any ground loop problems that can occur if there are two or more ground return paths available. The cap drains off higher frequency "buzz" frequencies like dimmer and motor noise.
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Kai

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #18 on: April 25, 2014, 02:24:42 pm »

Please everybody technically experienced here, keep in mind not all readers understand the difference between safety ground, power ground and audio ground.

Please be very careful giving advice for mods done to equipment because misunderstanding can be very dangerous.

Every exposed metal part of electrical equipment MUST BE EARTHED to safety ground.
This means a direct wire connection (no caps, resitors or anything else) to the 3rd prong of the power connector, period.
Only exceptions:
- if you use a dedicated isolating transformer,
- or if a (modern) device is specially build for use with a 2 prong connector, this means the in- and /or outside is properly isolated to prevent electric shock even if something goes wrong.

This is NOT the case with vintage equipment, so an isolation transformer or a mod to 3-prong power line connector is mandatory to use them.

There is a 3rd way, you can use an automatic switch in the powerline that immediately cuts power when a little amout of electric current gets lost somewhere (e.g through anyone's body).
I found "ground fault interrupter" and "residual current protective devices" as translation to the german "Fehlerstrom-Schutzschalter", please correct me if that's wrong.
Don't mistake this for a fuse!

I will not go into details here, ask your local skilled electrician to install these.
They are not expensive!
All professional PA companies use them in their mobile installations too, at least here in Germany.

During this you can even add an overvoltage protection to your electric system, saves a lot on repairs and again is quite inexpensive.

Regards
Kai
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David Satz

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #19 on: April 25, 2014, 09:07:37 pm »

Kai, for U.S. usage at least, "ground fault interrupter" (GFI) or "ground fault circuit interrupter" (GFCI) is correct.
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Jim Williams

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #20 on: April 26, 2014, 11:57:23 am »

Best advice if you do not understand completely what is decribed and done here, do not try this at home.
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River

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #21 on: April 29, 2014, 07:13:37 pm »

So, to clarify, in the case of the original question of the C12a's.

These mics use a 4 wire power cord with Blue to power ground and Green/Yellow to chassis ground.  They can be joined at either the AC plug, inside the supply itself, or obviously not at all. 

So are you recommending that the safety and power grounds both be connected to the third prong and that the resistor/cap be inserted between power ground and pin 1 of the XLR out? 

Thanks!

Tim

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radardoug

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #22 on: April 30, 2014, 03:53:53 am »

Yes, that's what I would recomend.
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polypals

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #23 on: October 04, 2014, 06:47:29 pm »

Earthing goes further than connecting older PSU's in studio's.

To prevent hum loops there are two systems that will work.
Operate all equipment that is connected to the power grid without connecting the safety ground to the ground in the mains socket.

Connect all metal casings with a secure lead to the mixer and connect the PSU of the mixer to a clean earth connection. That way you operate according to safety rules used in Europe.
Please note all gear must be connected permanenty to the console or connectingboard.

The other method allows to connect all power operated gear to the ground of the socket they are plugged in. To avoid hum loops the ground from the circuits must be disconncted from the casings and connected to the mixer were the mixers PSU is connected to a secure earth connection

In general companies that supply power like to connect their ground to the earth connection the studio has. If the earth connection proofs to be better than the ground the power grid supplies you get the ok to use the studio's earth connection.

Connecting the studio's earth to the ground supplied by the power grid will result in drawing large amounts of unwanted spikes etc from the grid to the xstudio's former clean earth connection.
This should be avoided.

I apologise for using the terms earth and ground. If their use is incorrect Klaus please edit my post.

Some engineering terms are difficult for me manage, English is not my mothers tongue.
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Kai

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #24 on: October 06, 2014, 11:52:25 am »

Earthing goes further than connecting older PSU's in studio's ... ...
Please Paul, do not give that type of advice here.
It is very dangerous to tell technically unexperienced people to mod their line powered equipment in a way you describe.(Bold added by the moderator- K.H.)


For those people who need a solution that works "out of the box" I suggest an power line isolation transformer.

BTW: your idea to use separate wires as safety ground sounds very strange and impractically to me and wouldn't prevent ground loops either.
Disconnecting the internal circuit from the safety ground where the manufacturer had connected it makes a device unsafe and is not recommended.

Regards
Kai
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polypals

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #25 on: October 06, 2014, 05:43:01 pm »


I am sorry if my post leads to confusion on the subject of safe and problem free operation of mains powered equipment.

The fact that not all readers have an engineering background was overseen by me.

Edited

The system where all ground connections are brought togehther was used at Polygrams Wisseloord Studio from the start in 1977.
As far as I know there have not been safety incidences with that set up.
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boz6906

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #26 on: October 07, 2014, 10:53:06 am »

Regarding ground loops...
In my experience ground loops are caused by the chassis of a unit being at a different potential above ground, as compared to a connected unit.  This causes a current to flow between the units on the ground/sheild of audio interconnect cables.

You can lift the sheild (pin 1 on XLR) at one end and sometimes remedy this condition.  A better way is to find the problem unit by using a voltmeter between the chassis and earth ground.

Older gtr amps can have 30-40Vac or more on their chassis with respect to earth/power co ground. usually a leaky power transformer or ceramic bypass cap.

Or, different ac  circuits can have shorter/longer paths to ground, causing different ground potentials outlet to outlet, this is why all audio gear should be connected to the same ac circuit.

I always recommend any questionable ac circuits be checked by a licensed Master Electrician familiar with audio/video installs.

(Especially on live gigs where I want a licensed. bonded/insured electrician to approve the ac before I connect the distro... An improperly connected 60kW diesel generator can make short work of a $10,000 amp rack or console.)
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klaus

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Re: Safe and Efficient Grounding of Tube Microphones And Power Supplies?
« Reply #27 on: October 07, 2014, 12:44:34 pm »

All good points to be considered for guitar amps and other processing equipment, but ground looks in condenser mic systems are extremely rare. The fact, that the mic is at the beginning, not in the middle, of the audio chain helps. And: NEVER lift ground on a mic cable connector! RF will ensue.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com
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