Plug-in power is designed to power electret microphones through same audio cable that carries audio. Most lavalier (tie-clip) microphones, consumer video camera microphones and microphones used with computer soundcards are electret microphones. Typical electret condenser microphone capsule is a 2 terminal device which approximates to a current source when biassed with around 1-9 volt and routinely consumes usually 0.5 mA or less current (there are also 3 pin capsules with separate power and audio out pins). This power is consumed by a very small preamplifier built into the microphone capsule which makes the conversion of very high impedance source of the electret element itself and the cable which needs to be driven.

Usually the microphone is powered from 1.5V-5V power source through a resistor which has resistance of few kilo-ohms. There are several variations how this power is connected to 3.5 mm jack.The power is typically wired to the same pin that carries the audio, and this being the tip for mono mics and tip+ring for stereo microphones. The mic input in PC sound cards and computers with separate mic connector the power is connected to the ring and audio is carried on the tip. For more details read my Powering microphones web page. Plug-in-power (PiP), is the low-current 3 V to 5 V supply provided at the microphone jack of some consumer equipment, such as portable recorders and computer sound cards. It is also defined in IEC 61938.

Microphone: Phantom power

Phantom powering is the most common microphone powering method used in professional audio applications. Phantom powering deltails are covered by DIN spec 45596. Initially, it was specificed as 48 volts (P48) provided thru 6.8k resistors. There are now defined systems at 24 and 12 volts (P24 and P12), but those are more rare than traditional 48V feed. Most modern condensor microphones will work off a wide range of phantom power voltages. The 48V (+10%..-20%) phantom power is the de-facto microphone powering method supported by practically all major sound mixers.

Typically 48-volt phantom-powered condenser microphones had simple circuitry and required only small amounts of operating current (typically less than 1 mA per microphone). The original DIN 45596 phantom-power specification called for a maximum of 2 mA. Later condenser microphones designed for 48-volt phantom powering often require much more current (up to 5-10 mA), so the IEC standard gives 10 mA as the maximum allowed current per microphone. Besides microphone itself in some cases the same phantom power can be used to drive small indicator LEDs. Also many active Di’s are supposed to be powered with Phantom power. If you need to check if devices provide phantom power, check out DIY Phantom Power Checker.

Links to more material:

T-powering and A-B powering the same thing, A-B being the old term for what is now referred to as T powering. T-power (short for Tonaderspeisung, also called AB or parallel power, and covered by DIN spec 45595) was developed for portable applications, and is still common in film sound equipment. T-power is usually 12 volts, and the power is connected across the balanced pair through 180 ohm resistors. Newer recorders and mixers are phasing out this option.

Microphone: Digital microphones

Digital microphones are another story. Digital microphones complying with the AES 42 standard may be provided with phantom power at 10 volts, impressed on both audio leads and ground (up to 250 mA ). USB microphones are powered with normal USB power (5V 500mA).

Speakers cables

Speaker level signals are post amplification signals that drive passive loudspeakers. The signal is AC signal (typically 20Hz-20kHz)that has same waveform and the audio signal had. The signal level and current depends on the power being fed to the speaker element. For example, 1,000 watts feeding an 8-ohm load would require an 89.4 volt level (with a complementary current of 11.2 amps). There is limited cable length and longer the cable needs to be the thicker it needs to be to avid too much resistance causing issues with speaker performance.

There is also 70 volt speaker system in use. 70 volt systems are generally used in commercial applications where many speakers needs to be run from one amplifier. It is also called high impedence speaker system. It allows using  long cable runs of relatively small gauge (usually 20-24 gauge) do not significantly affect the output You would want to use this type of system if you plan on powering say 10 speakers with one amplifier(one 100W amplifier can power ten 10W speakers).  A 70 volt system is used in restaurants, small bars, department stores etc. 70 volt is common in the US, but in other countries there  other high impedance system, including 25 and 100 volt.

USB

USB (abbreviation of Universal Serial Bus), is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication, and power supply between personal computers and their peripheral devices.

The Universal Serial Bus connection is commonplace and allows you to connect and record instruments, microphones, and audio inputs to a computer. USB interfaces can be mains or bus powered depending on the model.

The USB 2.0 max current is 0.5A for normal USB use, while USB 3.0 is 0.9A. Higher currents can use used for charging applications using normal USB connectors (typically up to around 2A) and with USB type C connector (up to 20V 5A).

Intercom

Intercom is a private telecommunciation system that allows typically two or more locations to communicate with each other like telephone does. Many productions which needs co-operation of more than a few people need special intercoms that cover many users. Intercom systems used in TV and stage productions are usually headset type intercoms connected to one line using party line arrangement.

The most commonly used intercom system I have seen in use is Clear-Com party line system that uses 3 pin XLR connectors for the line. It uses the following pin-out on XLR conector: Pin 1: Common; Pin 2: PL Power (+28VDC); Pin 3: Line (Audio). The power supply can typically supply power maximum up to 2A.

Related links:

http://www.epanorama.net/links/intercom.html

http://www.windworksdesign.com/blog/comclone-shop-built-intercom-interface/

http://www.rcrowley.com/rcrowley.com/comclone/?