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Old 5th July 2005, 06:36 PM   #1
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Default Earth again

I make only a slightest appology for asking this but its an issue of safety, and I alway tell people "ask the stupid questions first and often". This shoulb be in a F.A.Q. somwhere.

Am I correct in assuming that I should connect the mains earth to the chassis of my amp?
Is it correct that I should also connect it to the screen of my signal wire?
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Old 5th July 2005, 08:18 PM   #2
moamps is offline moamps  Croatia
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Here is what I think about the subject if you are interested.

Unbalanced cable "directionality"... mismatched instructions?!?!

post 10

Regards,
Milan
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Old 5th July 2005, 08:21 PM   #3
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I always "earth" the chassis of the amp. Then I form a connection
between the circuit ground and the chassis one of 4 ways:

1) Piece of wire

2) Power resistor

3) Power thermistor

4) Power diode bridge.

The last three offer some barrier against ground loops, but
should be designed so that the AC line fuse fails long before
they do.

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Old 5th July 2005, 08:38 PM   #4
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1. Yes, the chassis of your amp should be earthed.

2. Normally not. In balanced connections it may be a good idea, I don't know.

Edit: Some quick people here, the numbers refer to the initial question.
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Old 5th July 2005, 08:53 PM   #5
PRR is offline PRR  United States
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> ...I should connect the mains earth to the chassis of my amp?

Under many electrical codes, it is required. A strict reading of NEC calls for EVERY BIT of metal in the house to be Grounded. UL rules lead to metal cases on plug-in appliances being either solidly-grounded or a fail-safe "Double Insulated" scheme to ensure line-voltages can't appear on the outside of the device.

> Is it correct that I should also connect it to the screen of my signal wire?

Maybe.

Since you can usually touch it, Code requires bonding.

BUT. The Line is a potent source of garbage. Big garbage on the shield means small garbage in the signal. And if, as in most hi-fi, the "shield" is really also the Signal Common, garbage comes through directly.

However, if you float the audio system, the main outside interference (in most locations) is the Earth and the Power Lines. Totally floating operation can drown in buzz.

Also, if you have multiple grounds, you can get induced currents flowing in them, and inducing garbage voltages.

So we usually ground the audio common, but with great care.

If you don't know better, bond your DC power supply common to the Line Ground/Earth with a 10 ohm 1 watt resistor. This will suck-out most of the problems of totally-floating operation while also reducing ground-loop currents to very-small, and keeps the residual away from input stages. Much more elaborate grounds are sometimes needed, but very rarely.
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Old 5th July 2005, 11:37 PM   #6
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Extremely good advice, particularly PRR. My experience precisely.

Cheers,

Hugh
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Old 6th July 2005, 12:42 AM   #7
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all machines with metalic chassis/enclosures
must be earthed ground for the sake of owner safety
i have come across some japanese consumer electronic products
either with fuly metalic chassis or top half which have not been
earthed ground, for safety they rely on a clamp with short leads
close to final connection,how safe is that compared to earthed ground
machines is for safety experts and regulation authorities to judge

cheers
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Old 6th July 2005, 12:59 AM   #8
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For safety there is no question that the exposed metal chassis and heatsinks should be earthed/grounded to the mains ground.

With respect to signal earths, I always connect the supply ground to the chassis via paralleled 10R IW resistor and back to back 1A diodes.

Works for me.

Cheers,
Greg
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Old 6th July 2005, 02:41 AM   #9
jaycee is offline jaycee  United Kingdom
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This is exactly what I do and what Rod Elliot recommends in his article on grounding. He also suggests a 100nF capacitor as well but that's probably secondary.
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Old 6th July 2005, 02:52 AM   #10
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Quote:
Originally posted by jaycee
This is exactly what I do and what Rod Elliot recommends in his article on grounding. He also suggests a 100nF capacitor as well but that's probably secondary.
What the capacitor does is keep the impedance between the supply ground and the safety ground very high at power line frequencies, but much lower at RF so that the chassis can better serve its function of RF shielding.

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