What is Ground Lift

From daver–(at)–r.hp.com Wed Mar 15 21:06:49 CST 1995
From: daver–(at)–r.hp.com (David Roach)
Newsgroups: rec.music.makers.guitar
Subject: Re: Ground Switching in Amps
Date: 16 Mar 1995 02:52:24 GMT
Distribution: world

Rennie Selkirk (selkir–(at)–elsci.arc.nasa.gov) wrote:
: In article <3k5u9c$ec--(at)--xnews1.ix.netcom.com> sstrou–(at)–x.netcom.com (Scotty Stroud) writes:
: >
: >The center position (if present) lifts ground. The other two positons
: >AC couple (via a capacitor usually .047uf) one side or the other of the
: >power line to ground.

: What does “ground lift” mean? And what is the function of the coupling
: capacitor?

: Thanks, HJB

To lift the ground is to disconnect it, i.e. like there was no
capacitor to earth ground connected. This is not the same as
disconnecting your amp’s middle prong of the AC line cord, which
connects the amp’s chassis to earth ground (it is not recommended
to disconnect earth ground for safety reasons).
The capacitor passes high frequency energy to ground
while blocking low frequencies (60 Hz line voltage). The cap
to ground arrangement forms a simple low-pass filter, with its
cutoff frequency selectable by the value of capacitor chosen–
more capacitance equals lower cutoff frequency (with this simple
filter, the “cutoff” is actually a very gradual rolloff).
If you were to gradually increase the capacitance value of this
part, more and more of the AC line current would pass through it
and the part would burn up.
One of the things I’d point out about power supply hum (typically
from poor filtering or regulation)– on designs using full-wave
rectifiers (4 diodes), the frequency of the hum is actually doubled
to 120 Hz because of the way full-wave rectifiers work. So often,
what people are calling “60 Hertz” noise is really 120.

Dave R.