Basic Audio is a free introductory textbook to the basics of audio physics and electronics. See the editorial for more information....

The Complete Amplifier

Author: N.H. Crowhurst

The complete audio amplifier requires a lot of "putting together." From the designer's viewpoint, first is needed a power output stage to deliver whatever power is required, with satisfactory performance. To make this work, it has two requirements: adequate audio input to control it and the various power supplies to operate the tubes or transistors. If the current or voltage requirement of the output circuit varies according to the audio input level, the power supply circuit must accommodate this variation properly, without changing the voltage or current more than can be permitted.

To get the audio input, voltage or current amplification (according to whether you use tubes or transistors) is needed so that the small input from a pickup or microphone delivers enough to drive the power stage. Then feedback will be added to get the best possible performance from this combination. In considering feedback, do not forget that supplying the different stages from a common power supply can result in output circuit audio fluctuations being fed back to input stages, which are more sensitive. This can cause instability if not properly controlled.

These are the main building blocks for a typical audio amplifier

Finally, you need certain controls, volume, and tone, as well as equalization. These have to be put outside the main feedback loop, although they may use some feedback of their own. But if you make the mistake of putting feedback around a volume or tone control, the feedback will carefully undo all the effect of the control! It holds the gain steady, and has no means of knowing that you meant it to change.

Last Update: 2010-11-03