A preamplifier takes the source voltage (about 1 volt, from a CD player, LD player, VCR, Tuner) and amplifies it to about 3 volts, adding any treble and bass, DSP, and volume controlling that you wish. The function of modern preamplifiers is not so much to increase the voltage but to allow switching between inputs, controlling the volume, and maintaining a constant output impedance. The output of the preamplifier is fed to the power amplifier where the signal is increased to about 20 volts – 60 volts, which is sufficient to drive the speakers at high levels. Voltage amplification itself is not a complicated process, but keeping noise and distortion from being added along the way is difficult. Tube amplifiers are a product of the early twentieth century, whereas the transistor was invented in the 1940s. Tubes are very simple devices, and the designs of decades past are still being used in amplifiers of today. Many audiophiles treasure the tube sound for its warmth, and they are willing to pay high prices for the small number of tube amplifier designs available, compared to solid state (transistor) amplifiers.
Other Parts in an Amplifier
There are many other parts in the amplifier besides the rectifying and amplifying transistors (or tubes). These include resistors (which resist the flow of current), capacitors (which store electrons), inductors (for example, transformers), conductors (the wiring which conducts electrons from one part of the amplifier to another), and insulators (plastic coating on wires and other parts which prevents electrons from flowing where they are not wanted). These parts are all placed in a chassis (metal box), some of the parts having been mounted on a circuit board, and connected to jacks (sockets), controls (on/off, volume, tone controls, input/output selectors), and indicator lights. An inexpensive amplifier will use inexpensive parts, and an expensive amplifier contains nothing but the best. Somewhere in between is what most of us are looking for.