The AC hum issue can be this problem. Electricians, take note:
In the vast majority of cases where 'dedicated lines are installed to feed audio systems, huge ground and hum issues develop.
In the USA, AC power is delivered to domestic loads as a 240 volt system, that is center tapped into two 120 volt rails that are actually out of phase relative to one another.
When 'dedicated lines' are run by the average electrician, the usual practice is to just line up a row of breakers and hook up wire to the room....BAD AND WRONG!
What happens is that any asymmetry in the AC voltage waveform, and there IS asymmetry, is feeding the system and difference will flow as ground currents that produce hum.
- This practice will feed the room with 120 volt lines that are sourced from a mixture of both sides of the 240 as it is split into the pair of 120 volts.
There is only one fix for this....the multiple AC lines that feed a given system MUST and ALWAYS come from only ONE of the AC rails...never a mixture of the two.
- The first step would be to have an electrician come in that understands how US AC power is split phase, and have him move all the feeders to breakers that are on the same AC polarity. In all breaker panels made for USA market since the 1950s, there are 2 rows of breakers. On each row, every other breaker has the opposite phase feed. This is so that a double width breaker can draw from both sides, and create a 240 volt output. In order to get all the lines on a single AC phase, the breakers that are feeding those lines MUST be alternating. and from the same row. As an example....lets start at the end of one row of breakers.....pick either end and call that 1, and the next breaker 2 and the next 3 and so on.....now, all the ODD number breakers will be on the same phase, and all the EVEN number breakers will be on the opposite phase. All the feed lines to your system MUST AND ABSOLUTELY come from only ONE SINGLE POLARITY...it does not matter which phase is chosen, but all the feeds must come from only one phase...this means that using adjacent breakers is a NO!
Time and time and time again, over and over, we see incorrectly wired multiple dedicated lines that cause hum and buzz issues that are rarely resolved, until the AC feed is corrected to source from a single phase of the AC system.
In the normal course of training for electricians, they are instructed to 'balance the loads' ..meaning that if you have 30 thousand watts of consumption over the layout of a house, lighting, outlets and such, that the goal is to spread out that 30 thousand watts so that approximately half of it is fed from one polarity and half from the other polarity. This is fine for light bulbs, small appliances and such, but when opposing phases are feeding a single audio system where ground is part of the signal and connected to all the gear, all hell breaks loose.
Electricians are NOT educated in the fine art of what is correct for high end audio...only for balancing the lighting and small appliance loads throughout the house.
Audio gear that interfaces with no ground connection, as we see in most professional systems, and some consumer gear, VIA a real balanced differential connection interface, are usually immune to such issues, as the ground and AC neutral are NEVER used as any sort of reference point from which the audio signal is created or sensed.
However the reality is that most consumer gear is single ended RCA jack interface and if that is the case, GROUND IS IN YOUR SIGNAL PATH!!! ....any ground currents flowing between equipment WILL and DO add to the signal. We hear this as hum and buzz. No AC noise remover on earth can correct for this condition. The only cure is to FIRST correct the AC feed problem... Even after this is done, hum and buzz can still exist, but the causes are different...but first things first...get the AC fixed correctly.
Updated Jan 6th 2015