An integrated amplifier includes all components needed to produce an amplified signal having enough power to drive a speaker, starting from a signal produced by an input source (CD, DAC, radio, etc.). Figure 15 shows the basic building blocks of a push-pull integrated vacuum tube amplifier.
The input signal is first received by the input stage, that operates a pre-amplification of the signal. Push-pull amplifiers, discussed in Section 4.1.4, require two phase-inverted copies of the same signal to be amplified. The phase splitter stage, after the input stage, takes a signal and produces two output signals, one 180° phase inverted with respect to the other. The two phase-inverted signals are passed to the push-pull stage, composed of two power amplifiers that amplify the two received signals. The output stage also includes an output transformer, where the two phase-inverted amplified signals are combined and adapted to be given to the output speaker. The output signal can also be used as a global negative feedback, to reduce distortion and noise, at the expense of a reduced gain of the amplifier.
Next sections discuss in more details all these components. We will proceed backward from the output stage to the input stage.