Audio Basics: A Is For Ampere Page 6
A transformer is two or more inductors placed so that one picks up on the other's magnetic fields. The source winding, to which input is fed to the transformer, is called the primary winding, or just primary; the output winding is the secondary. Usually, the windings are wrapped around opposite legs of a closed-M-shaped laminated magnetic-metal core (fig.11). Varying electrical currents in the primary sets up magnetic fields in the core, and these are induced into the secondary winding, which responds by putting out an AC voltage into any electrical circuit connected across it. (Without a load, no current can flow in the secondary, so the primary doesn't "see" it at all.) Because there is no electrical connection between the windings, the transformer cannot pass DC from one winding to the other.
The relationship between primary and secondary voltage depends on the relative numbers of wire turns on each winding (the turns ratio). An equal number provides no voltage change; 1 AC volt in equals 1 AC volt out. The transformer acts roughly like a capacitor, passing AC but no DC. If the secondary has more turns than the primary, voltage output is increased (called a step-up), while fewer secondary turns causes a voltage step-down. A transformer's voltage ratio is equal to its turns ratio. Transformers will also change impedance from winding to winding, but the relationship is different: Impedance changes as the square of the turns ratio, not on a 1-to-1 basis. A 1:9 turns ratio gives a 1:3 impedance ratio.
Transformers are used in audio mainly for power supplies, when it is necessary to change the value of the AC voltage or to provide a balanced source for full-wave rectification. They are also used as output transformers in vacuum-tube amplifiers, to prevent the high output-tube plate voltage from frying the speakers, and to match the tubes' high output impedance to the speaker's low load impedance.
Amplifiers are necessary for audio because real-world sounds are powerful and audio signals start out being exceedingly weak. A typical moving-coil cartridge, for example, delivers about 0.1mV (10 thousandths of a volt) of output, while it takes around 40V to drive a typical audiophile-type loudspeaker system to symphonic-music levels in an average living room.
Amplification involves what is called "current gating"---using a small voltage to vary the current flow from a larger DC power supply. The supply can be from a stack of batteries---indeed, it always was in the first amplifiers---but today the power supply is more likely to be one drawing its energy from the household AC mains via a transformer, then rectifying and filtering it. For now, let's assume it's batteries.
Your basic black-box amplifying device---tube, transistor, or FET---consists of a DC current path that behaves like a variable resistor, with a controlling gate or valve somewhere along it. (Hence the British name for the vacuum tube: "valve.") All the electrons from the DC source passing through the "resistor" must go through that gate, and the peculiarity of amplifying devices is that voltages applied to the gate have an exaggerated effect on the device's resistance. A small voltage at the gate causes a large change in resistance, and when this resistance is the bottom half of a simple voltage-divider network (fig.12), the result is a large change in voltage division.
Essentially, what happens is that an increasing voltage at the device's gating element---a decrease in the number of electrons dumped into that gate from our small signal source---acts to make it easier for electrons passing through it from the DC source, increasing the current flow through it just as though the resistor had decreased in ohmic value. The result, of course, is that the voltage division is increased, so that the higher the voltage from our source applied to the gate, the lower the voltage that appears at the point where the resistor and the amplifying device join. Conversely, the lower the value of the input voltage, the higher the voltage at the voltage divider point. And as the DC source voltage can be any value you want, you can arrange things so that a change in the small input voltage results in a large change in the output voltage---voltage amplification!
If you're following this, a light bulb must have gone off over your head by now. You're looking at polarity inversion: a high input voltage results in a low output voltage and vice versa. That's classic polarity reversal and---like the speed of light---it's immutable: amplification with a single device used in this manner always inverts polarity. You want a non-inverting amplifier? Either you make sure it has an odd number of amplifying stages, or an output transformer that you can connect either way.
A single amplifying stage is limited in the amount by which it can magnify a signal. As distortion tends to be lower when it isn't amplifying full-tilt, so several stages must usually be operated in series one after the other (cascaded) in order to do the job. But that isn't just as easy as connecting the output of one to the input of another. Nothing is ever that simple.