HiFi Grounding Hum and Earth Issues - The great Challenge
Of all problems, hifi earthing and hum issues are the most time consuming and troublesome. It can seem almost impossible to eliminate hum altogether even given the best equipment and interconnects, especially when you have unsucessfuly tried all combinations. However, a logical, progressive approach can get rid of this problem entirely. Your system is OK when (and only when) cranked up to listening levels ++ you are unable to hear any trace of hum / interference and noise at all - none.
A duplication of the mains electricity voltage (or some fraction) added to your hifi output. Or, it could be a symptom of Radio Frequency (RF) oscillation in some part of your setup. The former is characterised by a buzz of constant tone (frequency) and the later by random noises and fluctuations of the first and is usually modified by you touching the equipment cases.
For electricity to do anything useful, it must exist in a circuit - a circle - a closed loop where the input of one circuit is connected to the output of another. Hum problems arise where two circuits overlap by some amount and one of them is carrying either a large current or an earth return. Worst of all is when a very sensitive input circuit (ie your cartridge pickup) is overlaped by a high current circuit. The larger the overlap, the worse the hum and the only way to eliminate the problem is to separate out the circuits such that no overlap exists. Easier said than done! The human ear is amazingly sensitive to hum and hum harmonics - even the tiniest amount can be plainly heard and it is fiercely annoying. When heavy speaker currents are added to sensitive inputs, rf instability can arise because of positive (additive) feedback into the input circuit. This is particularly nasty and can fry your speakers, power supply, Zorbel network and cause problems that seem totally baffling ie touching the amplifier case alters the buzz or stops it completely!
Tracking it down - Try to eliminate where the noise is NOT coming from. Disconnect all inputs by physically unpluging them and see if the noise has gone. If it has - then you have a problem with input connections. If not - your power amp or preamp is generating the noise and may be faulty. Indeed some power amplifiers seem designed to be noisy and can only be quietened down by internal modification (though this is very specialised)! Note: This is true of Maplin's excellent Mosfet Modules which can be used to make fine hifi systems....Maplin Mosfet Modification.
Draw out on paper the physical input circuit starting with where the signal enters and ending where it leaves. Add to your drawing any high current circuits ie the speaker output circuit being careful to include the return to 0V path. Look critically for any overlap where currents combine to flow down a single track or wire and seperate these out. A single point contact (ie the star 0V point) is OK but mulitiple connections are not. Above all do not mix under any circumstances analogue and digital 0V lines unless at one point only - the star point otherwise you will get horrendous trouble. For both analogue and digital systems you may need to adopt individual star points for each earthing system and connect these together in one place only.