Unbalanced:
- Uses a single wire plus shield "guitar lead" cable in which the signal is transmitted as a voltage offset compared to the ground/shield wire. The braided shield is connected to the shield of the quarter inch mono jack plug, and the signal wire is connected to the tip of the quarter inch mono jack plug. Hence the plugs are refereed to as "Tip and Shield" plugs. If noise - for example mains buzz - is induced into the signal wire, the noise is added to the wanted signal. There is no noise cancellation.
- Uses a dual wire plus shield microphone cable in which the mono signal is transmitted in phase (ie positive) on one wire and out of phase (ie negative) on the other wire. The two signal wires are connected to the tip and the ring of a "stereo" jack plug or to an XLR connector's tip and ring pins. The quarter inch "stereo" plug is referred to as a TRS connector (Tip, Ring and Shield) when used for balanced mono signals.
If the letter V represents the signal, then the transmitter generates +V on one wire and -V on the other wire with respect to ground. The receiver subtracts the two signals, and so actually recovers 2V. The clever thing is that noise - such as mains buzz - is completely eliminated. How? Let's represent the noise signal with the letter N. During transmisssion one wire will carry (V+N) and the other wire will carry (-V+N). The receiver performs the subtraction (V+N) - (-V+N) = 2V, so the noise cancels out.
- Most microphones generate a very low level signal which is susceptible to noise (eg mains hum) pickup. Therefore nearly all microphones have balanced XLR connectors and balanced cabling so that the noise pickup will cancel out at the receiver. Also condenser microphones require 48V DC voltage applied to the "tip" signal wire. Many mixer desks provide this 48V DC on XLR inputs designed for condenser microphones. If you connect your keyboard's line output sockets to a 48V MIC XLR input on a mixer desk or a powered monitor speaker, you will cause a lot of damage to the keyboard.
- Case 1: Let's say someone connects the two line outputs from a Pa500 or from a CD player to a Stereo TRS connector, and plugs the connector into a powered monitor speaker.
On one wire you will have the left channel, L and on the other wire you will have the right channel R. The signals are almost identical most of the time. At the receiver it performs the subtraction R-L, leaving almost no signal at all. So the sound is terrible, it's very, very quiet and hopelessly distorted - since all you are hearing is the difference between the two channels instead of the sum of the two channels.
BALANCED and STEREO signals use the same connector. You have to understand the difference between the two because the signals are completely incompatible.
Case 2: Let's say someone uses a "Stereo" lead with TRS plugs at each end to connect the Right line output from the keyboard to a powered monitor speaker's balanced input. At the receiver, the input level will be half what was expected. Remember it is expecting to see a "2R" signal after performing the subtraction. Instead it will only see a "1R" signal. Also there will be an impedance mismatch which is another topic for discussion on a rainy day. The impedance mismatch will cause unwanted resonance and filtering of the sound.
So to connect UNBALANCED line outputs to BALANCED inputs, always use an active conversion box like the Behringer DI-120, using a pair of short guitar leads from keyboard to the DI-120 inputs, then use balanced XLR leads from the DI-120 to the balanced inputs on the mixer desk or powered monitor speakers.
Rob
