Grounding ?

Which raises another question, why do some amp manufacturers include a ground lift switch. The answer is very simple because they know it can be very convenient to flip a switch to lift the ground to avoid hum caused by a ground loop rather than buy and use a sound degrading cheater plug or cutting the ground pin off the power cord.

Note: When using some tube preamps, I have had to lift all the grounds on all components except for the preamp to eliminate hum or considerably reduce it. The other thing to check is to make sure all power cords are not too close to the interconnects.

Mike Elliot talks extensively about ground loops and hum on his Alta Vista website and recommends all except one component be grounded and if a ground fault occurs in any system component it can be safely drawn away by the interconnects via the one grounded component. I think we can trust this excellent designer who has been producing quality gear for many years.

Charles Hansen of Ayre also advocates floating the grounds.

Which raises another question, why do some amp manufacturers include a ground lift switch.
08-09-09: Phd

The switch only lifts the signal ground from the chassis. Never the safety equipment ground from the chassis/enclosure.
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Jea48, this could be true, but before agreeing I would like to research it. You already know that many preamps as well as power amps are two wire, that don't provide a third wire for grounding. In addition there are non-shielded interconnects that only use the center wire. Not observing how your interconnects are designed as well as your gear could produce the perfect storm.