AC & DC
The terms ”A.C.” and “D.C” are used to describe two different types of electric current. Alternating Current can be abbreviated as “A.C.” or “AC” and Direct Current can be abbreviated as “D.C.” or “DC”. In reference to voltage, Volts Direct Current can be abbreviated as “V.D.C.” or “VDC” and Volts Alternating Current can be abbreviated as “V.A.C.” or “VAC”
An electric current which flows in one direct in a conductor is referred to as direct current. Common examples are the current from a battery or the power supplied to electronic circuitry like computer logic or audio amplifiers. It is possible to modulate the DC power to form signals like those in a computer or to control electromechanical devices like motors or the light output level of LED’s. The voltage of a DC signal may also vary without it being considered an “AC signal” as long as the current only flows in one direction.
In contrast, an electric current which changes direction in a regular or periodic manner is referred to as alternating current. This type of current is used to transmit high voltage currents commonly referred to as “electricity.” It can also refer to much lower power audio or radio frequency signals. Unlike power transmission which contain one predominant frequency (50 or 60 Hertz), AC signals can contain two or more frequencies.
AC Modulation of DC
There is also the special case of an AC signal being superimposed on a DC voltage. In these cases the current only flows in one direction while the voltage varies by the same total voltage as the original AC signal. For example, an AC signal which varies between plus 1 volt and minus 1 volt would appear as a signal that varies between plus 3.5 VDC and plus 1.5VDC at the output of the DC level-shifting circuitry operating on 5VDC. The AC voltage variation is centered on one-half the DC voltage or 2.5VDC.
Historically this was the case in early audio amplifiers which operated on single-ended DC power (typically only a positive voltage). After the AC audio input signal passes through a capacitor, it is “centered” at approximately one-half the single DC power supply voltage within the amplifier circuitry. The internal DC voltage was prevented from reaching the input or output of the amplifier by being “blocked” by a capacitor and/or audio transformer.
In contrast to single-ended designs, an amplifier operating on a bipolar DC power supply can amplify AC audio signals without the need for DC level shifting. This type of amplifier is power from a positive and negative DC voltage with the same nominal voltage; for example most IC Op-amps operate on +/-15VDC. One big advantage of this approach is the possibility of Direct Coupled (“DC”) operation without the need for coupling capacitors between amplifier stages. This type of audio amplifier will amplify any DC voltage present at the input and present it to the output; which means they can be used for the type of DC level shifting mentioned above.
In contemporary digital audio, DC level-shifting circuitry is used to allow signals to pass between converters operating on only positive DC voltages and audio amplifiers operating on bipolar DC power supplies. In AD converter units, the signal is typically bipolar at the input, and is converted to single-ended by the level shifting circuit to feed the input of the AD converter IC. In DA converter units, the single ended output of the converter IC is level shifted to bipolar for output. With careful design, the audio circuitry can remain Direct Coupled through the entire chain.