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Amplifier Circuit
The amplifier circuit consists of a few different, smaller circuits. The power supply converts 110-volt alternating current (AC) to a clean, regulated source of direct current (DC). A preamplifier section gives a boost to weak signals from sources such as turntables. The power amplifier has the four power transistors needed to drive the speakers. A stereo amplifier consists of two preamps and two power amps, one to drive each stereo channel.
Output Transistors
The output transistors have a high-power design. They are larger than small-signal transistors and have a full or partial metal body that bolts to the amplifier̵7;s metal chassis or other heat sink. As the transistors run, they become hot; the metal-to-metal contact conducts heat away from the transistors and into the larger chassis. If the transistors blow, they will either short-circuit, carrying excessive current and running very hot, or fail open and conduct no current. They may take on a charred appearance and produce a burnt-plastic odor.
Circuit Degradation
Some of the components in the amplifier circuit develop problems after 10 to 20 years. Mechanical parts such as variable resistors, used to control sound volume and right-left balance, wear out and produce scratchy electrical noise in the amplifier̵7;s output sound. Internal connections may corrode and break. The stresses of occasional abuse, running the amplifier at its maximum or connection to excessive speaker loads cause transistors to degrade. A good amplifier design has protections built in, preventing overloads from damaging the transistors, but if the circuit has deteriorated, the transistors may blow.
Heat
Transistors, being semiconductors, are heat-sensitive. Power transistors normally run warm to hot to the touch, but excessive heat will damage them. Unlike the behavior of metals, which have increased electrical resistance with increasing temperature, silicon in transistors has less resistance. The lower the resistance, the more current it carries, forming a condition called thermal runaway that ends with the transistor̵7;s destruction. A well-designed has thermal protections built in, but not all amplifiers do.
Poor Parts
Quality replacement transistors for a 20-year-old amplifier may be impossible to find; a repair technician must then settle for substitute parts. The manufacturing quality of substitute transistors varies; counterfeit parts are also a problem. A poor-quality transistor may function in an amplifier circuit but may blow in a few years.