Keep in mind that for our purposes all of those terms are functionally equivalent. You might see any number of permutations of those terms. There are so many that it is needlessly confusing as they all mean essentially the same thing. For example, you might hear, "Circuit Common".
Audio ground is the reference by which the audio is amplified. All analog audio circuits establish a zero volt DC reference used as the circuit ground. Audio ground can be established between two pieces of audio equipment in two ways.
- Via the interconnected (common) AC ground.
- Via an interconnected (common) shield or signal ground. The interconnection of a signal ground between units forces them to be the same "reference". In unbalanced, ungrounded, two pronged AC plug equipment such as consumer audio gear, this is almost always the case because there is no AC ground.
The next two pages show how the AC ground is used for the audio reference in pro audio equipment instead of an incoming signal ground.
Bipolar DC Power Supplies in Audio Equipment
Audio circuitry is powered directly by DC voltage. When a piece of audio equipment is plugged into AC power, it makes the DC power it needs directly from the AC line. In professional audio equipment this is accomplished in the DC power supply.
The AC power is first run through a step down transformer inside the unit (generally the heaviest single component of any piece of gear). It is fed by the hot and neutral of the incoming AC power line while the ground wire is generally diverted upon entry and electrically attached or "bonded" to the chassis of the unit. When the incoming AC earth ground is attached to the chassis it "earth grounds" the chassis of the unit for safety reasons first and foremost. The power transformer isolates the unit from the AC line and reduces the 100-240 AC line voltage (depending on the country you are in) to some specified voltage as measured across the full secondary winding of the power supply transformer. Secondary voltages differ unit to unit with respect to the power needs of the particular device. Power amps, for example, have bigger transformers that produce much higher secondary voltages as they need to dissipate more power. The secondary winding of a center tapped transformer is actually two windings. The ends of the two equal secondary windings that meet in the middle are the "center tap" that provides the reference of half the voltage of the full secondary.
Let's say we had a power supply transformer and we measured 60 AC volts across the full secondary. If you then measured from the center tap to each of the other secondary wires you would measure approx. 30 AC volts. The full secondary 60 volts AC is converted to DC by the bridge rectifier then filtered (smoothed) by large filter capacitors to produce positive and negative (bipolar) DC voltage to power the audio circuitry, lights, meters etc. The positive (+) terminal of the positive filter capacitor is connected to the positive (+) terminal of the rectifier. The negative (-) terminal of the negative filter capacitor is connected to the negative (-) terminal of the rectifier. The negative (-) terminal of the positive filter cap is connected to the positive (+) terminal of the negative filter capacitor. This connection establishes a midpoint between the positive and negative voltages of the filter capacitors. The center tap of the power supply transformer secondary is connected to the electrical point between the positive and negative DC filter capacitors and this establishes the bipolar DC Zero Volt (0V) reference for the audio circuitry. This is where audio ground starts in any piece of AC powered pro audio equipment. Again, the center tap of the power supply transformer secondary is always electrically bonded at the electrical midpoint between the positive and negative filter capacitors that smooth the DC output of the bridge rectifier. The positive (+) and negative (-) regulators hold the DC output at the specified voltage for the audio circuitry to accommodate reasonable fluctuations in the AC line voltage. The output of this DC power supply is typically seen as +(n)VDC , 0VDC, -(n)VDC after DC regulation. It is from this 0V point that the audio circuit reference is established and is known as "Audio Ground", "Circuit Ground", "Audio Reference", or " 0V" for the internal audio circuitry.