Chapter 4: Integrated push-pull vacuum tube amplifier

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.

Integrated push-pull amplifier
Figure 15: Integrated push-pull vacuum tube amplifier.
The input signal is first handled by the input stage, which pre-amplifies the signal. The signal then goes to the phase splitter stage, which produces two phase-inverted copies of the same signal. These are passed to the two power amplifiers that compose the push-pull stage. The output amplified signal is fed to the output transformer, then to the output speaker. Output signal can also be fed back to the input stage to reduce distortion and noise, at the expense of a reduced gain.

Next sections discuss more in details all these components. We will proceed backward from the output stage to the input stage.

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