There appears to be some confusion concerning the ground lift switch implemented in this design. So I thought it might be good to go over a simple review and explain exactly why this switch is important in modern equipment.
Traditionally there has not been a need for a “ground lift” capability in audio gear except in a few specific situations with professional equipment. The primary reason for this has to due with the evolution of modern power systems and modern design. Traditionally, most electrical wiring systems were two wire and contained no inherent ground reference. In some 120vac systems the “neutral” lead was connected to earth ground at the power panel but not always. This philosophy of power distribution (as well as the need for voltage translation) led to the widespread use of power transformers for system isolation. In this way the power leads for the equipment could be electrically isolated from the electronics (from a ground reference perspective) and the AC power could be supplied via the traditional two wire interface.
The figure below shows a simple signal/preamplifier/amplifier system wired with this traditional philosophy.
Attachment:
old system.png
In this situation, each unit has a collective ground which is isolated from the facility power system. This prevents the generation of any large ground loops via the facility power system. This is important because a ground loop will inject signal signals into the audio chain by coupling stray electromagnetic fields from other equipment in the environment. And the strength of these signals will be proportional to the area of the ground loop. In the above setup the signal ground reference is provided entirely by the interconnect cables and everything works out alright.
In modern systems things are not quite so simple. Modern wiring system are typically three wire systems with one wire dedicated as a safety ground. In addition in polarized systems, one wire is dedicated as “hot” and the other as “neutral”. Slowly but surely most power systems worldwide are moving toward these polarized, three wire standards. This is a good thing for both safety and compatibility, but it does generate some complications for those of us building new equipment.
Below is a figure like the one above, but modified for the polarized three wire standard. I have included in this diagram, both a dedicated amp ground path and a preamp ground lift switch.
Attachment:
modern system.png
In this case, there are four possible conditions under which the system may operate. Lets start with the amplifier. There are some situations under which two wire power cords are still allowed. These conditions have to do with the chassis construction methods and the isolation of voltages preventing them from coming into contact with the chassis (and hence people). So the amplifier in our system above, may have a ground tie between it’s respective grounds, or it may not (it may or may not have a facility safety ground at all). This uncertainty is were the trouble begins. Because we do not know the ground state of the amp (or because it may change as we change equipment), we need to be able to adjust to whatever the situation may be. Below is a table showing the possible permutations.
Attachment:
fault table.png
The two green conditions assure that fault currents are properly grounded (good safety) and that the signals path has no ground loops (good fidelity). The upper right hand red condition has no earth path for fault currents (bad safety) but consistent single flow signal ground path (good fidelity). The lower left hand red condition has ample earth paths for ground currents (good safety) but large ground loops contributing to the signal path (questionable or poor fidelity).
So there is the purpose of the ground lift switch on the preamp. By properly setting the switch, depending on the system, it allows us to maintain both good safety and good fidelity.
As always, questions and or comments are more than welcome.
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FAQ
... damn me! and it have low resistance compare to the other cap, varying around 20k ohm, the first time i powered it up, i had the ground switch open and my signal side ground had a 325v potential difference in respect to earth. So a bad can be it