Earth lift

[bumpy]

An amp I have is fitted with an earth lift. This is very effective at removing the last trace of hum in very sensitive speakers, but is it making the amp electrically unsafe?

[Firebottle]

No it will not be making the amp unsafe.
The 'lift' will only be from the audio circuitry, not the safety chassis earthing.

:)

[bumpy]

No it will not be making the amp unsafe.
The 'lift' will only be from the audio circuitry, not the safety chassis earthing.

:)

Thanks Alan

It seems such a simple and effective cure for earthing hum. Are there reasons many amp manufacturers don't do it?

[anthonyTD]

I wouldnt be too sure about that, unless the amp in question has been tested while in both modes, some amps do infact lift the earth completely!
No it will not be making the amp unsafe.
The 'lift' will only be from the audio circuitry, not the safety chassis earthing.

:)

[bumpy]

I wouldnt be too sure about that, unless the amp in question has been tested while in both modes, some amps do infact lift the earth completely!

Can I check for that simply by looking for continuity between chassis and mains plug earth with the switch in either direction?

[struth]

Think a fwbr used as an earth block is safer. As for saftey the amp still has to be earth saftey grounded.

[bumpy]

Think a fwbr used as an earth block is safer. As for saftey the amp still has to be earth saftey grounded.

fwbr?????

[anthonyTD]

Hi Chris,
Yes, that would be the best way to test it for sure!
Can I check for that simply by looking for continuity between chassis and mains plug earth with the switch in either direction?

[bumpy]

Hi Chris,
Yes, that would be the best way to test it for sure!

Thanks, much appreciated.

[bumpy]

Hi Chris,
Yes, that would be the best way to test it for sure!

The bad news is, that with the earth lift switch 'up' the chassis is not earthed. This will need to be changed. :rolleyes:

The good news is, that with the earth lift switch 'down' the chassis and all associated cabling etc are full earthed. This is the position that leaves the amp perfectly quiet. :cool:

[struth]

fwbr?????

Full wave bridge rectifier.

[Arkless Electronics]

An earth lift should leave the chassis earthed!!

[struth]

Yup +1000:)

You can do a quick test by lifting an earth to see if its the problem, as long as your careful, but under no circumstances should it be left that way

[anthonyTD]

Result! :)
It should never be taken for granted that where an earth lift switch is fitted, it leaves the chassis conected, always check!
The bad news is, that with the earth lift switch 'up' the chassis is not earthed. This will need to be changed. :rolleyes:

The good news is, that with the earth lift switch 'down' the chassis and all associated cabling etc are full earthed. This is the position that leaves the amp perfectly quiet. :cool:

[Light Dependant Resistor]

Full wave bridge rectifier.

A full wave bridge indeed can be used as a earth lift. It places circuitry otherwise connected to chassis safety earth, then at the current capability and forward voltage of the diodes.
Although good for low voltage and current, a resistor and capacitor are added in parallel where higher currents are involved.

Star earthing. where individual sections return to chassis, though remains good practice and should not be ignored where diodes are used, as a combination of
the two is most beneficial.

The emphasis in replies, on chassis earth being always connected to your safety earth return, is critical to proper earthing of equipment.

A diagram shows the three methods. The first can be interpreted as 1 half of a full wave bridge connected this same way, useful where you need an individual
section of circuitry lifted this way, the second and third using full wave bridge rectifiers are equally applicable, although the one to the right is usually seen.
Note, two earth seeking inputs are available with a full wave bridge. A typical way of using these in a unbalanced audio system might be one for circuitry that is left
and one for right

A capacitor and resistor are often added to a full wave bridge where greater currents are used, as seen here at Rod Elliot's website http://sound.whsites.net/earthing.htm Figure 4
however just diodes are quite applicable, where voltage and current is low.

Further techniques in the same basket are to use opto- isolation for circuitry sections internally. Discussed here, gives some insight into how high isolation
can be achieved. https://www.fairchildsemi.com/application-notes/AN/AN-7511.pdf

Cheers / Chris

[Reffc]

You can buy a 15A to 35A rated bridge rectifier for under £3. You connect your signal earths to this with a 10R resistor and small value capacitor (100nF is fine) in parallel. This maintains a low impedance signal ground lifted from the chassis with the chassis safety earth separated from the signal by the 10R resistor. The signal takes the shortest lowest impedance path to ground via the bridge rectifier. Job done, costs a fiver and most importantly, doesn't compromise the chassis earth to mains ground.