8.3 The audio amplifier, stage by stage

Outcome 3 of 12 in domain 8 · Systems, reading and designing a complete device

The Yamaha finally readable: the blocks every power amp contains.

My targets

I declare:

Name the three stages of a classic power amp (differential input, voltage amplification, output) and say what each does to the signal.
Explain the differential input pair and why it’s the one that receives the feedback.
Explain the class AB push-pull output stage, its biasing, crossover distortion and thermal compensation (the transistor on the heatsink).
Spot the global feedback loop on the schematic and calculate the gain it sets.
Spot the protections (current, DC at the output, temperature, speaker relay) and explain what each watches.
Read a commercial amp’s schematic (Yamaha, NAD, Marantz) in full and find each block on the board.
Measure an amp: power, distortion by ear and on the scope, bandwidth, noise floor.
Diagnosis: an amp that runs hot at idle or has DC at the output, biasing, unbalanced input stage or leaky output transistor; the order of measurements, load disconnected.