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avid 20th August 2006 07:37 AM

chassis ground vs. earth ground
 
I'm currently finishing one of Peter Daniel's PCB LM3875 chipamps and I'm stumped on the chassis ground/star ground concept. Is the star ground a collection of the chassis ground, signal ground and earth ground all connected at one common point?

My chassis is made of wood so I don't really have a chassis ground. Where do I connect the chassis ground wire coming off of PCB? Directly to the Earth ground coming from the mains?

Obviously I need to read more about ground loops and their causes...

Let me know what you think.

Thanks,

Eric

AndrewT 20th August 2006 08:47 AM

Hi,
You need a safety earth on all exposed conductive parts.

If you are sure the transformer fixing bolt can never be touched while your amp is plugged into the mains and sure all other conductive parts cannot be touched then you do not need a safety earth.

However I have never seen or read the standard for double insulated electrical equipment and I suspect the conditions for omitting the safety earth are quite onerous.

Do not take the decision to omit the safety earth lightly.

What happens if a mains live or neutral connection breaks loose and the free end touches the audio ground somewhere and you end up with a direct connection from RCA ground to the broken neutral? It will kill the person that touches the interconnect on any OTHER part of the interconnected system. Except if the other parts have a connection from audio ground to safety earth. Then your other equipment provides the fuse blowing route. Do you see the problem? What if all the other equipment were double insulated? There would be no escape route and your fuse would not blow until someone touched the interconnect, TOO LATE! :dead:

Back to your wooden chassis with no exposed conductive parts.
The audio ground can be a central star ground.
This has:-
power ground from the PCB,
signal ground from the PCB,
RCA ground from the input terminal (usually via the PCB),
decoupling grounds from the PCB (usually combined with the power ground),
the speaker return,
the Zobel/Thiel network return,
relay and/or digital grounds
and finally the PSU common (0volts).
Normally you would have one more connection off the audio ground. This additional link would go to chassis earthing point via a disconnecting network consisting of any or all of the following:-side by side high current diode pair (inverse parallel), high frequency capacitor, low value high current resistor, ground lift switch (more for experimenting with problem hum than day to day use).
Fitting the earthing wire from the audio ground should NOT require the safety earth to be dismantled. The safety earth must be a permanent fixture preferably welded to the exposed part or securely bolted with an extra pair of nuts for additional connections.
If the ground lift switch needs to be closed to improve the hum then you have a grounding fault elsewhere.
Each of the grounds in the list should ideally have it's own separate wire with solder tag to allow you to bolt all the grounds together. Someone else once pointed out that the order in which the solder tags are bolted together can affect the performance of the amplifier. I have thought about it, he is right! Keep all the dirty grounds at one end followed by the cleaner high current grounds moving towards the clean grounds at the opposite end. This avoids most of the voltage spikes running through the dirty grounding system affecting the sensitive ground reference needed by the front end of your amplifier.
It's simple once you have grasped the separation principle, but hum and buzz are avoidable when the grounding is designed correctly.

One final bit of advice. DO NOT take all your audio grounds to the PSU common and bolt everthing to the common plate no matter how thick or thin it might be. It is NOT a good voltage reference.

CarlosT 10th November 2006 05:07 AM

Quote:

Originally posted by AndrewT
Hi,
You need a safety earth on all exposed conductive parts.

If you are sure the transformer fixing bolt can never be touched while your amp is plugged into the mains and sure all other conductive parts cannot be touched then you do not need a safety earth.

However I have never seen or read the standard for double insulated electrical equipment and I suspect the conditions for omitting the safety earth are quite onerous.

Do not take the decision to omit the safety earth lightly.

What happens if a mains live or neutral connection breaks loose and the free end touches the audio ground somewhere and you end up with a direct connection from RCA ground to the broken neutral? It will kill the person that touches the interconnect on any OTHER part of the interconnected system. Except if the other parts have a connection from audio ground to safety earth. Then your other equipment provides the fuse blowing route. Do you see the problem? What if all the other equipment were double insulated? There would be no escape route and your fuse would not blow until someone touched the interconnect, TOO LATE! :dead:....

This is very interesting. I'm kinda struggling with this as well from a design standpoint. A lot of commercial amps just use polarized 2-prong (no earth ground) plugs. This would seem to keep some of the potential noise outta the box. Then again this creates a problem since the power side needs to be either double insulated or totally isolated from the signal side. I can't seem to easily achieve either in my design but I did come up with an interesting solution...a GFCI!

A ground fault interrupt breaker (or as you call them in Europe...RCD...residual current device) does not require a ground to operate yet it can very quickly shut off whenever there's any leakage of current (i.e, a person touches a live wire or chassis or there's any other current differential between hot and neutral). This is actually much safer than an earth ground for a few reasons.

My biggest problem so far (in the US) is finding a GFCI or RCD device that would easily integrate into the amp design. There seem to be no PCB or miniature GFCI breakers. The closest things I found were inline devices that are part of the power cord. This may have to do.

AndrewT or anyone else...care to comment on the use of GFCI devices as an alternative to earth grounding?

AndrewT 10th November 2006 07:23 AM

Hi,
RCCB, (RCD) are not alternatives to the safety earth.

In the UK the RCCB is usually fitted at the distribution board although local units are used in socket outlets and as plug in accessories.

All equipment down stream is still built to the usual double insulated or safety earthed regulations.

There are special rules for wiring up more risky areas eg. wet rooms, medical etc. where the rules are more onerous.

Never depend on an RCCB to protect life in lieu of correct safety earthing practice. It is against the regulations.

CarlosT 10th November 2006 12:56 PM

The thing that hits me kinda odd is that...

I'm very familiar with double-insulation in power tools. These have grown chunky and plasticky versus the old days of cast pot metal cases...plastic is now abound. However, when I look at let's say my Adcom amps and most other audio gear, these just got polarized two-prong plugs (no earth). Yet these don't appear plastickied out...there's still a lotta metal around...the case...the transformer is clearly visible and nominally isolated from the case...never mind much physical separation between power and signal. Aren't these things then terribly dangerous? Why do I not hear horror stories of audiophile deaths? Wouldn't Adcom be weary of us litigious Yanks and ground the sh_t outta everything?

Besides the reliability issue of the RCD/RCCB/GFCI that you pointed out, is the basic function of the device not inherently safer than relying on a ground alone? Grounds are also subject to the quality of the home wiring and outlet and small yet lethal currents could be leaking through the chassis to ground and not setting off any fuses or breakers.

BTW as you know...I'm not an electrical engineer but I play one on TV :D

Thanks again, AndrewT.


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