But some posters here and elsewhere stated that I should have an isolated ground installed outside the house and at least 6 feet from the existing ground.Bad advice, forget the dedicated earth ground. The earth does not possess some magical mystical power that sucks nasties from our audio gear. The safety equipment ground main purpose is to provide a low impedance, resistive, path for ground fault current to return to the source, the electrical panel.
I did speak with two electricians today, and neither seemed to have much familiarity with the considerations for audio quality that I mentioned to them. Therefore, I would like some input on what to ask of them.
First off it depends on the local electrical code in your state and city what type of electrical installation can be used in your area. The electricians in your area will know what is allowed.
Because you are using an existing convenience receptacle outlet circuit to power your audio equipment I would suggest installing at least two new dedicated branch circuits. One dedicated circuit for the new power amp and one for the other equipment. With two dedicated circuit there is a less chance of a ground loop hum problem. This will lessen the chance of a difference of potential, voltage, existing between the two equipment grounding conductors of the two dedicated circuits.
If it were me I would feed my preamp and power amp from the same dedicated circuit. Use the other dedicated circuit for the other associated equipment.
You said the length of the run is around 50 to 60 feet. Did you figure up and down over and around in your calculation? If not that can add another 20 feet to the length.
IF you decide to plug the preamp into the same receptacle as the power amp I would install #10 AWG wire for this 20 amp dedicated circuit. With #10 AWG wire you won't have to worry about any dynamic power demand fluctuations from the power amp effecting the power being fed to the preamp.
For the other dedicated circuit use #12 Awg wire.
Type of wiring and wiring method used.
Probably the worst type of wiring method is to install conduit then pull the wires loosely randomly into the conduit. This method will assure you ground loop hum problems. Worse yet is to install more than one dedicated branch circuit in the same conduit.
If conduit is used, or must be used to meet local code, use aluminum armor MC Cable. One cable for each dedicated branch circuit. I would recommend solid core wire over stranded wire. MC is made both ways.
If code in your area allows NM-B cable,( Romex is a trade name of NM-B cable), works well providing long parallel runs are kept separated by at least 6 inches to reduce induced magnetic fields of the current carrying conductors from inducing a voltage onto the one another most importantly onto the equipment grounding conductors of the NM-B cable.
Make sure all the dedicated branch circuits are fed from the same Line, leg, of the electrical panel for all audio / video equipment that is connected together by interconnects.
A must read before you call the electrician.
Bill Whitlock, President of Jensen Transformers Inc.
http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf
Pay close attention to pages 31 through 36
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Jim