Earth again

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
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.

:cool:
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> ...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.
 
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
 
jaycee said:
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.

se
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> a 100nF capacitor as well

If you don't do a solid-ground on signal, a small cap bleeds radio interference around your resistor or diodes, and swamps the few-pFd general stray coupling to The World and its garbage.

0.1uFd frightens me. I remember un-grounded systems with a 0.047uFd cap from chassis to one side of the line. 2-pin plug; you would flip it over for best buzz/radio rejection. In theory this would put the cap to the groundED side of the line, but sometimes it worked the same both ways. If you got it wrong, you have 53K reactance to 120V: 2mA into very-sweaty skin. Usually not fatal, but stings. I don't think that much leakage is allowed today.

> metalic chassis/enclosures must be earthed ground for the sake of owner safety

Codes vary in different lands.

Codes vary particularly between 100V-120V lands and 200V-240V lands. Edison had no good wire insulation, but wanted high voltage to reduce copper cost. He picked 100V because he observed that few of his workers died around <100V machines, many more died around >100V machines. (This also guided his thinking on the Electric Chair, but he hadn't enough test cases to know that current in the body is VERY unpredictable.) In the US, we've bent his legacy up to 110V-120V. Good plastics do not change the fact that we have VERY stupid plugs; yet fairly few people die. UL does test for shock hazard, but NEC code is fairly casual about electrocution, obsessed with handling the high currents we need at 120V. And there are a LOT of 2-pin outlets (and a lot of miswired 3-pins) still in use.

In much of the rest of the world, electrification didn't get big until good rubber and other insulators were developed. Common sense suggested higher voltages, safe with good rubber and shrouded plugs. There the codes tend to talk a lot about shock hazard.

Most consumer HiFi in the US is still 2-pin, no ground. A good transformer, properly insulated, properly wired, is very unlikely to break-down to the point that it makes the chassis "live". And I grew up in 2-pin days, learned my lessons, and survived; so I generally build with 2-pin cords. BUT.... in DIY work, mistakes happen. I'm willing to repeat my mistakes (though I try not to); I am not willing to advise others to dance like a jerk, fall and hit their heads, or stop their hearts. I'd much rather see solid-grounded cases, and deal with the signal issues, than deal with dead friends or even bench-fires. With a solid-grounded case, the worst-case wiring mistakes mostly lead to a popped fuse, not fire or death.
 
Originally posted by PRR
0.1uFd frightens me. I remember un-grounded systems with a 0.047uFd cap from chassis to one side of the line. 2-pin plug; you would flip it over for best buzz/radio rejection. In theory this would put the cap to the groundED side of the line, but sometimes it worked the same both ways. If you got it wrong, you have 53K reactance to 120V: 2mA into very-sweaty skin. Usually not fatal, but stings. I don't think that much leakage is allowed today.

Over here it is strictly forbidden to use the neutral conductor as any sort of earth, even though they both do return to the same point eventually. Our plugs are also shaped such that it is impossible for them to be inserted such that an L-N phase reversal occurs. There are exceptions, eg the "figure of 8" connectors found on tape players, but as these are only used on double insulated gear it is of little issue.

Most hifi gear here seems to have either hardwired leads, or uses IEC connectors the same as computers. My own stuff with use IEC connectors.

Originally posted by PRR
Good plastics do not change the fact that we have VERY stupid plugs; yet fairly few people die.

Yes, I do not like the US power plug standard one bit.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> Over here it is strictly forbidden to use the neutral conductor as any sort of earth, even though they both do return to the same point eventually.

True in the US; but "leakage" has to be allowed. The 0.05uFd cap sneaks in because its 60Hz leakage is small, but at AM Radio frequencies it is dead-short, and at mid-high audio frequencies it is low enough impedance to help somewhat.

> Our plugs are also shaped such that it is impossible for them to be inserted such that an L-N phase reversal occurs.

I should mention that the old 1960s plugs are no longer sold here. There is enough flare where the plug meets the wall that you have to try very hard to touch a live halfway-in pin; not like the old ones where there was almost nothing between your finger and the half-in pin.

Also we now have a fat/narrow blade scheme that is supposed to force correct L-N connection. However many new plugs do not fit some old sockets and vice-versa. For a decade I carried tools to "modify" new plugs to fit old outputs, primarily the accessory power outlets on the back of receivers. And although my kitchen had 3-pin outlets, none of the 3rd-pin holes were grounded (the green wires were in the wall, cut-off neatly) and about 7 of the 12 outlets had the Hot and Neutral reversed. This house has evaded the Electric Inspector for a century; the US average is better but there are many many places worse than mine. (BTW: I did fish-out all those cut green wires and do everything up proper, though for an obscure subparagraph not perfectly up to Code. Kitchen outputs should be on a 20 Amp circuit; I can't be sure of all the wire gauges so I left it at 15A. Much better to blow a fuse than start a fire in the wall. And I now have enough new good 20A circuits around the kitchen counters that I'm not under-wired.)
 

EDUM

Member
2003-10-02 9:21 am
Almere
It is in fact remarkable that the knowledge regarding earthing- and safety is not wide spread.
In the interest of public safety great care was given to make available standards covering those subjects.
Every manufacturer has follow these standards to ensure that no unsafe products apear on the consumer market.
The next step however, making these standards publicly available and part of educational programs, does not seem to happen. This leads to the lack of knowledge and to potential dangerous situations.

A "nice" example is the IEC 60065 standard. This standard regulates the "Audio.video and similar electronic apparatus - Safety requirements"

Unfortunately obtaining that document here in the Netherlands cost over 180 Euro....

I believe that knowledge about safety leads to improved safety. Shielding / hiding this info works contra productive.

Ward
 
Thanks to everyone for the advice. I agree with EDUM. Its should be more widely known. I aught to be a sticky or an FAQ here.

I did the work in 2 stages:-
1/ bolt the metal to earth. Result :- mains hum.
2/ Connect the line-in shield to earth via 10R 1 W. :- Mains hum 98% gone.

This is good enough for a stage monitor. There remains, however, an amount of hiss, even when its running on the charged caps ( i.e. no mains). I don't think its possible to be rid of this. Its a 100W amp after all.
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
There remains, however, an amount of hiss, even when its running on the charged caps ( i.e. no mains). I don't think its possible to be rid of this. Its a 100W amp after all.

Maybe not without doing some board-level modifications, but it's worth checking to make sure you don't have an oscillation. I've got a 200W amp running here that is dead-silent.
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
Use a wide-bandwidth oscilloscope, 100MHz or better. Look for high-frequency stuff (Megahertz range) at the output. Check with all cabling and loads in place.

You shouldn't have more than a millivolt or two of noise. "Real" noise is randomish, you won't see anything resembling a sine wave. If you can see discrete frequencies (and do play around with sweep time and trigger level/holdoff on the scope to spot this) or anything like a repetitive waveform, you've got some repairs to do.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> hiss, even when its running on the charged caps

That (probably) has nothing to do with power-line grounding.

I have ~100 watt power-amps and high-efficiency speakers that are extremely quiet. Gain is more the issue than grounding: do you have excess gain that you have to turn-down in normal use?