Gary,
I came upon this post after the others; many thanks to Peter and Charlie for answering your query so well.
Earthing would fill a book, and has done on several occasions. There are extensive laws about it in Australia, Europe, and the US, and no-one seems to agree. I thought I could evade this nonsense when I designed the power amplifiers and preamplifiers, but it seems impossible!
The most important safety issue is mains earth to chassis. Get that in place, wire it as though it's a circus trapeze supporting four people, and you (should) be abiding by the law.
Then there is signal and star earth.
Star earth supports amp power grounds, speaker cold terminations, and transformer center taps. These are all high current nodes, and they need a healthy termination equidistant from the reservoir/filter capacitors in the power supply. The caps perform two functions, both crucial; they filter the power supply ripple, but significantly, they serve as ground return for the speaker currents.
All these currents balance. That is, by Kirchoff's Law, what goes in must come out. So, taking account of direction, speaker currents, transformer currents, power amp module ground currents and filter cap currents all sum to precisely and exactly zero. If this were not so, there would be a slow accretion of charge at the star earth, and eventually there would be a large arc to the chassis, a minor thunderbolt. Of course, this never happens, so by conservation of charge (and energy) the sum of all currents at this node must be zero.
The final consideration is signal input ground. This is a very low current node, because the input signal to the AKSA is tiny, and flows almost no current. In fact, the peak current from a CD player into the AKSA is around 50 microamps, and if this ground is mixed up with the star earth where currents of the order of four amperes are flowing - 80,000 times as much again - there will inevitably be interference, which would of course be amplified and likely produce hum in step with the power supply operation. So, to isolate this node, we ground lift it with a 10R resistor from amplifier ground (to keep it localized to its respective module), and hope that the heavier currents flowing at star earth at the power supply do not in any way interact across this resistor with those at the signal earth. This approach is widely used and very safe and effective, but there are other techniques, using capacitors and diodes.
Now, it happens that mains earth can often be noisy, since we share our mains earth with some very high current beasts; particularly reactive loads like electric motors in washing machines, hair dryers, TV sets, arc welders, microwaves - the list is endless and does not even consider the many 1000 hp electric motors used widely in industry all around us. Mains earth does not always have a flawless communication with planet earth, and consequently spikes and other peturbations can appear on mains earth which militate against a completely quiet voltage node. In the world of medical electronics, where tiny currents are picked off the surface of the skin for EEGs and ECGs, such a noisy earth is verbotten - and not just because it might swamp the tiny signals we seek to recover and amplify for charting. There is a remote possibilty of electric shock; a counter productive outcome indeed for a sick patient!!
So, much complex equipment, and particularly medical electronics, is double insulated, which means that the power supplies within them are fully isolated via a transformer and there is no direct connection between mains earth and device earth, which operates on the 'safe' side of the secondary of a small transformer, just as in the AKSA power amplifiers.
So, the AKSA star earth need not be connected to mains earth, though chassis MUST by law be connected to mains earth. However, in rare cases, you might just find that hum problems present using this regime; so a judicious wire from star earth to chassis earth MIGHT scotch it. The point is that you must test this empirically; you can't really predict. Most AKSA builders have found that star earth should not be connected to chassis earth for best hum performance, however, but YMMV.
I hope this gives the background to the earthing design on the AKSA. In five years I've not seen fit to change it; the amp is extremely quiet.
On another note, there are now quite a few Nirvana Plus upgrades out there, anyone had a good listen and would like to pass on their opinion of the improvements?
Cheers,
Hugh
