What means ground lift?

[Bassterror]

Hi,

sorry offtopic

I've got a PA Amp here. It has a little switch at the back
"Ground lift"

What exactly disconnects it?

[AndrewT]

Hi,
there are two versions of ground lift that I am aware of.
One is dangerous and the other is safe.

You should open up the unit and check what the switch is disconnecting.

The chassis MUST be PERMANENTLY connected to the safety earth if there are any exposed conductive parts on the unit and it is not double insulated.

1.) The audio ground can be connected to the chassis.
or
2.) The audio ground can be connected via disconnecting network to the chassis.

In case 2. (the safe version of the ground lift) switch is open. Closing the switch converts the amp to case 1.

The unsafe ground lift switch breaks the permanent connection from chassis to safety earth.

You can test this without dismantling the unit.

[Spirtos]

Hi,
Ground Lift circuit is a method which is been used mostly from PA amps designers in order to eliminate signal injection hum problems. The goal is the isolation of the signal ground from signal injection interference. I'll try to post a circuit with more details.

Regards

[Spirtos]

ok, it looks like this:

[AndrewT]

Hi,
post2 , case2 (the safe one) is like post4 schematic.

Earth ground & chassis is the Safety Earth.
the audio ground (star ground point) is on the right.

Thanks Spirtos.

However, there is a problem.
The 10r causes too high a voltage across the disconnecting network when mains fault current flows from the audio side to the safety earth.
A low voltage component must be added in parallel, capable of carrying the fault current.
A power bridge rectifier or a power thermistor can achieve this duty.

[Spirtos]

You are absolutely correct Andrew, I have seen a lot burnt resistors (5 or 10W) in ground lift circuits in PA amps.


Regards

[Bassterror]

Thank you guys

I'm going to measure wich case it is.

[Chris Daly]

Ground Lifting is one technique but there are better ones

Using a Full Wave Bridge Rectifier Block , connect + and - together
connect that to actual Gnd , then feed Ground returns to the AC poles
of the rectifier. Instantly you have isolation between ground paths,
for instance one may be power ground with the other audio ground.

You also gain the peak inverse diode voltage rating then protecting
equipment from voltage /currents on actual earth that can occur
as a result of lightning returning through earth.

Equipment using this inexpensive modification also sounds better too.

Hope this helps / Chris

[driller45]

is it the other way around ?

AC~ to gnd / + and - to circuit

as per :

http://sound.westhost.com/earthing.htm

or does it matter ?

thanks .

g.

[AndrewT]

Originally posted by driller45
is it the other way around ?

AC~ to gnd / + and - to circuit

as per :

http://sound.westhost.com/earthing.htm

or does it matter ?

thanks .

g.

Hi,
it does not matter.
the 4 diodes inside the bridge are coupled in inverse parallel and pass current in either direction.