The basic building block in audio is the amplifier. When the word is mentioned most of us have the image of a power amp pop into our heads. They get all the press because they are the largest and most glamorous of the species, but what about the myriad of smaller and forgotten gain stages that occurred before the signal arrives at this last power stage? They are largely anonymous and taken for granted, but determine the quality of recorded sound.
Almost every knob on a piece of audio equipment is controlling a specific amplifier stage. And the farther we get away from the basic understanding of this simple entity, the farther we get away from knowing how to maximize its sonic potential.
The steady advancement of technology has served to obscure their very existence. Amplifiers have become so small and commonplace that they have virtually disappeared from human consciousness. Just look at the iPod nano – that thing is loaded with amplifiers, all crammed onto a little chip and powered by another sliver of technology.
The implementation of each gain stage, individually, and then as a complete amplifier, determines the sound quality of a piece of audio equipment. This fact seems to have been largely lost in the mysteries of time. Most people don’t even realize the devices they use even contain an amplifier. It just works.
In recent years, there has been an onslaught of multi-function units, recording channels, and the like. With the recording business moving from a professional to a consumer market, manufacturers are trying to offer the most features for the price. This looks great on the outside, but there is a large cost on the inside. The quantity of functions within a unit is usually inversely related to its sound quality. Fundamentally, it is difficult to design a good-sounding, multi-function unit, because every gain stage comes with the constraints of its implementation.
The requirements of a gain stage are:
- Its gain coefficient. (With a coefficient of 10, 1 V input will give 10 V output.)
- Bandwidth. For audio we generally want to double the range of human hearing (20 Hz to 20 kHz) so that would be 40 kHz to insure good transient response.
- Input impedance.
- Output impedance. (Generally we want to have the output impedance of the previous stage low in relation to the stage that it is driving to minimize the losses in the coupling between the two stages.)
- Maximum output signal before clipping.
- Maximum input signal before clipping
Pick up the August issue of PS for Part II.
Bryan Martin owns Sonosphere Mastering. Over his 20+ year career he has worked with David Byrne, Rufus Wainwright, Max Roach, Run DMC, and White Zombie. He can be contacted via e-mail at firstname.lastname@example.org or on the web at www.sonosphere.ca.