"Ground lift" - what is it, and why/when would we use it?
. (My DI box also has switches ... to "ground lift", a concept I still don't grasp.)
Allow me to explain "ground lift", or "earth lift", as it is known outside N. America
Long story short: you should only ever have one item of grounded/earthed equipment in your audio setup (be that stage amplification, or home audio). Every other item of equipment needs to run to ground via that one piece of kit.
'Why?!' I hear you cry. 'And what's this got to do with 'ground lift?!!'
Well, if more than one grounded/earthed items are connected together, a loop is created amongst the ground/earth wires (aka shield wires), via the ground/earth itself (see diagram):
or in real life:
Once that loop is made, it forms a (potential) induction loop: it starts generating a current (just like as in a transformer). The source of that induced current is the AC input. So if your AC reverses polarity at 50Hz(60Hz), you will get a 50Hz(60Hz) current induced in your ground loop. Now, while this induced current operates at very low power (not nearly enough to blow a fuse, after all), it is still powerful enough to create a 50Hz(60Hz) hum in your audio signal - the dreaded 'earth hum'.
The solution to the problem is simple, if seemingly hazardous (it's not hazardous at all, btw, just seemingly so). 'Lift' (i.e., 'remove) every single ground/earth connection
but one from your audio setup (what's known as
Star grounding).
In the old days, this would involve disassembling equipment or plugs, finding the ground/earth lead and pulling it out. We would always, where possible, leave the ground/earth lead poking out of the equipment so that everyone could see that its ground/earth had been lifted (particularly important from a safety point of view on mains power extension leads!).
These days, audio kit often comes supplied with a 'ground lift' switch, so 1) you don't have to disassemble the kit to lift the earth, and 2) no need to leave a piece of wire dangling through the housing! So this is what your 'ground lift' switch is for - to help eliminate ground hum (I say 'help', because figuring out where actually the ground loop has formed is very often a matter of much trial and even more error. Toggling the ground lift switch is very often only the first step of very many!).
Which single item to leave earthed? We would always choose the single most powerful amp in the rig - and come the end of the evening, this would always be the last item to be powered down and disconnected (so we'd go in a sequence: mixing board; fx racks, guitar amps, monitor amps, PA amps, grounded amp(s)).
Further reading:
UNDERSTANDING, FINDING & ELIMINATING GROUND LOOPS
Advice on setting up a hum-free safe PA system
Author's note: redpaul1 spent most of the '80s paying his way through grad school by doing sound and lights for rock'n'roll bands, including, The Bangles, Blur, Butthole Surfers, Dinosaur Junior, Einstuerzende Neubauten, Happy Mondays, Henry Rollins, Hole, Humphrey Lyttelton Big Band, Inspiral Carpets, Julian Cope, L7, Lush, Lydia Lunch, My Bloody Valentine, Napalm Death, New Order, Sandi Shaw, Sonic Youth, Swans, They Might Be Giants, Tom Robinson, Wilco Johnson and many more...
He gave up rock'n'roll to go work for the UN on human rights.
