Before the advent and popularization of transistors, vacuum tubes and coupling capacitors ruled the day. And what a good sounding day it was.
Vacuum tube preamplifiers, with their requisite signal coupling capacitors, represented the sound of music reproduced in the home. Sweet and lush with a touch of floppy wooliness—topped off with a flabby bottom end—vacuum tube preamps, tuners, and peripherals of that bygone era established a tough benchmark for the fledgling transistor technology to reach. Fortunately, their solid state rivals had an easy ace. Their bipolar nature meant designers could eliminate the need for coupling capacitors.
Perhaps a bit of ‘splainin’ would be in order. First, we’d rather not have more parts in the signal path—especially capacitors: passive parts that color and rob music of its purity. *If we are forced to have coupling capacitors, it’s incumbent on the performance-conscious designer to ear-select the very best for the circuit.
Second, we want our preamplifier to have its input and output sitting at the same place; ground. Any deviation from ground on the input can cause nasties like hum and buzz (or worse) while output deviations from ground can wreak havoc on succeeding amplifier stages.
In some circuits, especially tubes and single-ended designs, achieving ground on the output requires the help of a coupling cap. This is because all gain stages must sit higher or lower than their inputs to do their job. If the designer wants to eliminate coupling capacitors and have gain, two amplification elements are required: each sitting away from its input but in opposite directions. Together we can get back to where we started. And this is where the trouble begins for vacuum tubes.
Vacuum tubes wouldn’t do well in nature because they have only one gender (fortunately, they don’t need to self replicate). What this means, when it comes to eliminating coupling capacitors, is simple: if the goal of an amplifier is for its input and output to sit at the same place—and a gain stage’s input and output cannot be in the same place at the same time—only one of the requisites can be met (either the input or the output sitting at ground, but not both). What’s needed is a second gain stage moving in the opposite direction. Here, the second device has its input sitting away from ground enabling its output to be right where we want it, at ground.
Here’s a picture I snagged off the internet. It’s not entirely accurate because it only has one power supply. For direct coupled you need a + and a – supply with ground sitting in the middle. But, it shows the two-stage amplifier example as well as the two-gender approach (Q1 is an NPN gender and Q2 is a PNP gender, each moves in opposite directions).
We could do this with vacuum tubes too if they had both a positive and negative variety to them, but alas, they do not. Thus, vacuum tube preamplifiers are forever saddled with coupling caps.
