This may not be possible.....
I want to use 4 - 5 transistors and a small antenna to amplify all electrical noise that's detected... now if the power goes out, the circuit springs to life? Possible?
How near/ far from the mains cable do you want to detect the hum?
If you can be very near to it, say within an inch, a small flat coil (like a very small metal detector) should pick up sufficient induced current to be able to detect if the supply is on.
If your device is only to detect one particular point, you could try wrapping the coil around the power cable.
But any non-invasive device will only detect if there is a live supply in the cable, not whether the device is switched on or is on standby.
I usually know when there is no power. The lights go out and my 'puter stops working! ]
The entire house 
I'm detecting the total lack of power, the idea is to place the circuit inside a flashlight have it detect a power outage.... super super sensitive
The question is whether, given the physical constraints of the flashlight body (and hence limited antenna or sensor size), you can pick up enough electric or magnetic field to distinguish it from noise and VLF radio waves. Two ideas spring to mind:
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Use a coil with a very large number of turns, feeding an op amp configured as a bandpass amplifier.
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Use an antenna feeding a high impedance input (e.g. mosfet-input) op amp configured as a bandpass amplifier.
You'll probably need at least 2 stages of bandpass amplification, followed by a peak detector. One of the problems I can see is that it would be draining the flashlight battery all the time. But you can get micropower op amps that take very little power.
so in short, it's possible....
thanks for the input
cjdelphi:
so in short, it's possible....
I didn't say that. It may be the case that under some conditions, there is too little mains frequency electric or magnetic field to detect. Also it is likely that there will be some orientations of the flashlight in which the measured field is zero, unless you use multiple sensors with different orientations.
While sensing the mains would be the coolest approach you might be able to get away with sensing another radio signal. For instance, at my house my WiFi would die when the power goes out, and sensing that would be simple enough.
how about having a low wattage (15w) light running continuously. not quite fool-proof but good enough to prove a fool.
cjdelphi:
so in short, it's possible....thanks for the input
But definitely not easy.
Much easier to connect something to a mains plug.
Atmel connects their Arduinos directly to mains AC: http://www.avrfreaks.net/modules/FreaksFiles/files/346/doc2508.pdf
cjdelphi:
The entire house
I'm detecting the total lack of power, the idea is to place the circuit inside a flashlight have it detect a power outage.... super super sensitive
So you want the flashlight to come on when the power fails. My approach was to install 3 strategically placed 12V 50W lamps, run from an old car battery. The battery is constantly being trickle charged, in parallel with a 12V relay coil. When there's a mains failure, the charger stops charging and the relay drops out, switching the battery from 'charge' to 'lamps on'. There's one hefty (the charger can supply up to 15A!!!) diode between the relay coil and the battery, to stop the battery holding the relay in when the power goes off. When the power comes back on, the relay pulls in and the battery recharges. It gives me about 30 minutes of usable light, plenty time to check the fuseboard or to evacuate the house (in the event of a fire or flood). I've never had to find a flashlight in the dark since I installed it. 
Well.. i already have one of those systems in place, this was the idea to turn the flashlight body into an antenna.. but the circuit would have to draw a small amount of current ..
Hi, do I understand that what you want is a PORTABLE mains detector, so if you are somewhere in the house and the mains fails, you are not left in the dark?
Tom
Sorry, I do development and modification work and a solution can so easily be found if requirements are set out plainly first.
cjdelphi:
Well.. i already have one of those systems in place, this was the idea to turn the flashlight body into an antenna.. but the circuit would have to draw a small amount of current ..
How about a centrally located, mains powered, small transmitter with a receiver in the flashlight that holds the light off whilst it's receiving a signal. You may have to modulate the signal at, say 1KHz, nothing fancy, just using a 555.
Don't go too far with the flashlight, otherwise you won't be able to turn it off 
cjdelphi:
Well.. i already have one of those systems in place, this was the idea to turn the flashlight body into an antenna..
Oh, now you're changing the job specifications on us (and blaming us for proposing wrong solutions).
I understand.
What you are trying to do is extremely difficult.
The problem is what is electrical noise?
Electrical noise is everywhere, not only when there is mains power connected to your house.
Unless you lived somewhere like the middle of nowhere, with nothing anywhere near you, you will always
pick up some kind of electrical noise.
If you try to detect only the mains frequencies like 50 or 60 hz, this will only work when your detecting device
is very close to a power carrying wire in your house, like a few inches away.
mauried:
What you are trying to do is extremely difficult.
The problem is what is electrical noise?
Electrical noise is everywhere, not only when there is mains power connected to your house.
Unless you lived somewhere like the middle of nowhere, with nothing anywhere near you, you will always
pick up some kind of electrical noise.
If you try to detect only the mains frequencies like 50 or 60 hz, this will only work when your detecting device
is very close to a power carrying wire in your house, like a few inches away.
that's precisely what i want to detect (using as little current as possible)...
I just need to work out what kind of noise difference between total blackout and regular with power noise, when it drops low enough, power's out!...
That is going to be tough, since the wires are carrying the same current both ways, magnetic and electric fields cancel out pretty well.
You might look up some EMF detector circuits. With the problem of orientation with magnetic sensors, you may need three coils at 90 degrees to each other.
Electric field detection has the same problem of orientation. With an ungrounded sensor, you need essentially a dipole sensor. But if it is aligned to a null, the flashlight turns on.