Technical information

out of phase. Causes of phase shift include filters (especially those in loudspeaker
crossover networks) and the (lack of) time alignment of drivers in a loudspeaker.
Phono From phonogram. An adjective relating to systems for playing vinyl
recordings, such as LPs. Thus the phono input of an amplifier is the input into
which a turntable is plugged. This is usually equipped with an RIAA equalization
circuit. RCA plugs and sockets are sometimes called ‘phono’ plugs/sockets in the
UK.
Phonogram An old name for a record player, that is, a combined system with
turntable, amplifier and speakers, usually all contained in one cabinet.
Pink noise Random audio noise where the average amount of power is the same
for each octave. The power level of pink noise falls away by three decibels for
each increasing octave. Frequently used as a test signal, it more accurately
reflects the energy content of real-world audio than white noise.
Plasma display A flat panel display technology which consists of three tiny cells for
each pixel. Each cell contains a gas which, when electrically stimulated, emits
ultraviolet light which, in turn, activates a phosphor coating on the edge of the cell
facing the viewers.
PMPO Peak Music Power Output. This is a ‘fairyland’ measure of power output,
sometimes quoted for home stereo systems and should be ignored.
Port The hole in the enclosure of a bass reflex loudspeaker.
Power The amount of energy expended per unit time. The common unit is the
watt. Power (in watts) in a DC circuit equals voltage times current (in amps).
Things are a bit more complicated with AC.
Power amplifier A device that increases a line level signal (typically around two
volts into a high impedance) and boosts it in voltage, while allowing sufficient
current to be supplied to drive low impedance loudspeakers (that is, impedances
nominally in the four to eight ohm range). Most power amplifiers are analogue in
design and use transistors or valves, or a combination of both. New digital
amplifiers are appearing which effectively use a form of pulse width modulation,
low pass filtered to reduce the ultrasonic noise, to drive the loudspeakers.
Power supply All active audio products require a power supply, the section that
provides a suitable voltage and current for the operation of the device. This
typically consists of a transformer, rectifier (to turn AC into DC) and regulator (to
smooth out fluctuations in the voltage). Power amplifiers in particular are heavily
dependent upon the quality of their power supplies, which contribute a significant
proportion of their cost and weight. Of the transformer regulated supply designs,