7 Hooking up
In live music there are several types of signal, roughly divided into small signal and large signal, requiring different processing and special kinds of cable.
Small signals can be subdivided into the following:
- Microphone: balanced, low impedance (150 Ω), low level (10 mV), XLR plug
- Instrument: unbalanced, high impedance(1 MΩ, usually low level (100 mV), 1/4" jack plug
- Line (consumer): unbalanced, medium impedance (15 kΩ), medium level (250 mV or -10 dBu), RCA or cinch plug
- Line (professional): unbalanced or balanced, medium impedance (15 kΩ), high level (1.25 V or +4 dBu), 1/4" jack (unbalanced), XLR plug or 1/4" TRS jack (balanced)
Microphone and professional line signals can be balanced, and usually are. This means that the signal is split in two at the signal source, and one of them is inverted (put in counterphase, reversed). The original signal is called "positive" or "hot", while the inverted signal is called "negative" or "cold". Both signals are sent across the cable, each through their own conductor. At the reception side, the negative signal is inverted back and both signals are added, effectively cancelling any noise, hum or interference that both signals may have picked up in the cable. The hot and cold signals were in counterphase, but the interference that might have been picked up, was in phase in both hot and cold, so when the cold signal is re-inverted at the receiving end, the interference is cancelled out when it's added to the hot signal.
Although not really a signal type as such, this is what an amplifier puts out, and is called large signal:
- Amplifier output: Very low impedance (.01 Ω), very high level (50 V), various connectors
- 100 W, 8 Ω load; P = V2/R; V = 28.3 VRMS
- 600 W, 4 Ω load; P = V2/R; V = 49.0 VRMS
© Joris van den Heuvel 2001-2009