A preamp accepts millivolts and puts out a volt or so,
whereas a power amp accepts a volt and puts out tens
of volts. But a more important difference is that the
power amp can drive a lot of current through a low
impedance load like a speaker (4 or 8 ohms), whereas
a preamp is only supposed to drive a high impedance
like the input to the power amp, typically 10000 ohms
or so. There are no numerical rules; the distinction
is usually pretty obvious in practice because there aren't
a lot of applications for things in between these extremes.
An exception might be driving headphones; they are
relatively low impedance (say, 8 ohms to 600 ohms or so)
but they don't take much voltage to get high levels. So
a preamp may inlcude a headphone driving stage (separate
from the stage that drives the big power amp), and this
headphone stage is really just a little power amp.
So to make a long story short, the main distinction is
probably the impedance they drive. (But somebody
is sure to bring up tube amps driving high-Z electrostatic
speakers or something...)
Bob Masta
dqatechATdaqartaDOTcom
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com