first I earth the chassis for safety but why do I need to conect it to my ground...

[frank2395]

thank you,
First I earth the chassis for safety(well understood) but why do I need to conect my amp ground (zero) to chassis earth

i.e float or not to float if input and output grounds are isolated from chassis.

What are the reasons first and the advantages and disadvantages please

[jh6you]

My reason is:

I assume the chassis as a pool of fresh electrons, where I always have my star ground. I connect the input, output, circuit and PSU ground lines (coming separately) to this chassis star ground. And, from there, one line goes to the earth ground (power socket) through the bridge diode or thermistor, which blocks the poluted earth electrons, and at the same time secures safety in case.

Hope my reason will be reasonable.

Regards
jH

[Jennice]

1:
If the chassis and electronics do not share ground, you can have dangerous errors without knowing it. (Example: line voltage levels on output terminals, if the PSU fails - and you could still measure zero on the enclosure.)

2:
EMC. You'll get improved shielding = less noise and less likely to get HF oscillations, which can be triggered by HF signals picked up by your external connections (line in).

Jennice

[johnnyx]

I always connect the chassis to mains earth for safety, but I thought that if the signal paths were isolated, then there would be no opportunity for creating an earth loop.
In practice though, on a sub amp I built there was hum, which disappeared when I connected the signal ground to the chassis (at the input terminals, maybe that is important). The system used different mains outlets too. In a signal generator I built there were HF oscillations which disappeared when I connected the signal ground to chassis (at the output terminals, maybe that is important).
Now I always connect the signal ground to the chassis, and to hell with hum loops.

[northsiderap]

I'd say a good reason to ground a case is:

At your main circuit panel in your house, hopefully you have some 5-25 amp breakers from which your audio outlets are run. The neutral should only be grounded at the panel for best power distortion figures in your mains...

If you get a short, your mains circuit breaker is like a big ol' fuse. It protects you in case you get a short somewhere inside of the case that shorts the mains to ground.

If you didn't have a ground to the chassis, then to the main panel on your equipment, you might have a short from HOT to ground and have your chassis at 120v AC or 220v AC potential.

Picture yourself turning a knob on this high-voltage charged chassis and holding onto your guitar strings or touching another piece of equipment that IS grounded.

You would then become a conductor for all of this potential.... Straight to ground...


Establishing a good clean & 'neutral' ground potential is critical, especially when amplifying small signals... Your amp is probably designed to compare the potential of your power supply to the potential of the mains ground to feed the transistors. Don't want noise there.
Your piece of equip. probably also wants to use the mains ground for reference on loading your audio signal. Bad if ground is not 100% shunted into the earth.