Ever wondered why the background ‘Hum’ is generated and found any methods to resolve it? It is just because of the Ground Loop or otherwise called Earth Loops.
This is a common problem being faced by Sound Engineers in any complex audio system installation as well as in car audio installations, where there are multiple earth loops between piece of equipment and the main supply ground. If you connect them together there is a chance of getting background ‘hum’.
All cable have an electrical resistance, if an electrical current is passed through any material that has an electrical resistance, a voltage will be produced between the two points of contact, and strength of voltage depends on the current and resistance of the material (conductor). The current is passed through the scree of cable, there will be difference in voltage between 1 end of the screen and the other which can cause problem with ground loops.
If you place the closed loop of wire near to the transformer or choke or heavy light it can pick up the hum of 50 / 60 Hz.
In provisional studio where everything balanced, disconnecting the screen at one end of the signal cable will usually cure any hum problem because the screen is not used as a return path for the signal. It is purely a protective screen.
In unbalanced to balanced systems, disconnecting one end of a screen can cause difficulties because you will then be relying on the mains cable earth to act as return path for the audio signal. This can lead to pick up RF signal. If the main cable is removed this will leave you with no return path at all, you will be greeted by a very loud hum.
In unbalanced to balanced connection for tackling ground loop : connect a small resistor (100 ohms) in series with the screen at the unbalanced end of the cable. It will reduce any induced hum currents.
If you hear high frequency whistles or any radio station connect a (100 picoFared) capacitor in parallel with the resistor