DC Coupling (sometimes also known as AC-Blocking) is an electronic engineering arrangement that allows both AC (eg. audio) and DC (eg. control voltages) to pass into or out of an amplifier or other circuit. 

AC-coupled equipment effectively has high-pass filters at each connection, and these affect the phase response slightly, so some designers prefer to omit those and provide DC-coupled connections instead. The danger, though, is that any DC present on the connections can potentially cause damage and incorrect operation of connected equipment, as well as loud clicks or pops when making or breaking connections. An increasing mumber of audio interfaces now provide DC-coupled inputs and outputs to enable them to be used with synthesizer control voltages, which are effectively DC signals.

AC Coupling (sometimes also known as DC-Blocking) is an electronic engineering arrangement that allows an audio (or any other alternating) signal to be passed through a connection while simultaneously preventing any DC bias or offset voltage on the source signal from getting through. In other words, AC coupling rejects any DC components within a signal, passing only the AC elements. The simplest form of AC coupling is a series capacitor in the signal line and, in effect, it forms a high-pass filter with a very low turnover frequency (<1Hz).

AC coupling is employed widely in audio circuitry to isolate the DC operating condition of one stage of circuitry from affecting the next, and to protect parts of an audio chain from the potentially damaging effects of DC voltages.

Tonaderspeisung Power — more commonly called Tonader, T-power or A-B power — is a largely obsolete microphone powering system which was widely used on portable battery-powered audio equipment in the 1960s and '70s before the technically superior phantom powering system became more popular. T-Power operates with 9-12V DC with the positive rail connected to the hot side (pin 2) of a balanced audio connection, and the negative rail on the cold side (pin 3).

One of the potential weaknesses of T-Powering is that any power supply noise is inherently added directly to the wanted audio signal. T-Power is broadly similar in concept to the Plug-in or Bias Power arangement used on unbalanced consumer electret microphones, but with a higher supply voltage.

Plug-in (or Bias) Power is a method of providing power to the internal electronics of electret microphones, and is commonly used on consumer equipment. Plug-in Power is only ever provided on 3.5mm mini-jack input sockets as found on domestic sound recorders, 'phones, laptops etc. The format provides a low DC voltage of typically between 3 and 5V, with the positive side of the power supply connected to the unbalanced signal connection(s) in the mini-jack socket. So the tip connection for a mono input, or tip and ring connections for a stereo input. The negative return is via the sleeve connection.