11ULTRAGAIN PRO MIC2200 User Manual

microphone

 

microphone

 

microphone

preampli￿er

 

cable

 

ampli￿er

+48 V

microphone

microphone transformer

A

output

B

+48 V

Fig. 5.1: Functional diagram of phantom power supply

5.1.5 Phantom power

Condenser microphones need a specific supply voltage polarizing the condenser diaphragm. This voltage can be furnished from an internal battery, or an external power supply that is either connected directly to the microphone or supplies the voltage through the microphone cable. In practice, this technique is usually referred to as +48 V or phantom power supply, and uses the microphone cable to carry both the audio signal and the supply voltage required for the microphone.

Please read this chapter thoroughly: phantom power can damage the microphone, if used improperly.

We speak of phantom power when a microphone cable is used to carry several signals, with a DC voltage “layered” on the actual audio signal. The typical phantom DC voltage is +48 V, which is applied both to the positive (pin 2)

and negative inputs (pin 3) of the XLR connector, using current-limiting resistors. As the phantom voltage is split up in a balanced configuration among the signal leads, there is no need to apply it directly to the microphone transducer or the microphone itself, where it could damage the transducer and/or capsule. In an unbalanced configuration, DC voltage would be applied directly, which would inevitably lead to disturbing noise or could even damage the electronics.

To eliminate this risk, the BEHRINGER ULTRAGAIN PRO automatically disables the unbalanced phone jack in MIC mode. Microphones can thus only be connected to the XLR connector.

Never switch on the +48 V supply when you use unbalanced microphones, as this could cause electrical damage.

Some people hold that the sound of dynamic microphones is affected when the +48 V supply is on, or that ribbon microphones cannot be operated from an input equipped with +48 V phantom power. None of these statements is true. As a matter of fact, problems of this kind are caused by one of the following reasons:

1.When the output transformer of the microphone suffers from a short circuit or leakage current at any point, disturbing noise, clicks or hum can occur. In this case, you should have the microphone repaired.

2.When you connect a microphone while the +48 V supply is switched on, it cannot be guaranteed that both signal leads of the XLR connectors establish the electrical contact precisely at the same point of time. Therefore, detrimental current could flow because the connection is temporarily unbalanced. We recommend that you switch off the +48 V supply before you attempt to connect a microphone.

◊Please make sure that you are using an appropriate microphone before you switch on the +48 V supply. Read the operating instructions accompanying the microphone. Some condenser mics might need another type of power supply, older dynamic microphones could be damaged by the +48 V voltage, and unbalanced microphones should never be operated in combination with a +48 V power supply.

5.2 The ULTRAGAIN PRO as a level translator

Semi-professional devices in hi-fi and home recording environments are usually operated with a nominal level of -10 dBV (0.316 V), while the level used in studios is 0 dBu (0.775 V) or +4 dBu (1.23 V). So, when you connect devices of both types to each other, you should do this via some kind of level translator.

The BEHRINGER ULTRAGAIN PRO is excellently suited for this application. In LINE mode, the OUTPUT control allows you to raise or lower the input signal level by as much as 20 dB. The functions PHASE REV. and LO CUT are also enabled in this mode.

5.3The ULTRAGAIN PRO as a direct-injection box

When electrical signals delivered by instruments such as guitars, keyboards, etc. are transported over long unbalanced lines, the transmission quality may be affected by hum or other interference signals induced in the cable. This problem is usually encountered in studio or stage environments, where long cable lengths and magnetic fields of great magnitude may produce interference.

So-called direct-injection (DI) boxes are used to counter this effect: the DI box converts the unbalanced signal coming from the instrument into a balanced signal that is sent over the line. Interference induced in balanced cables is then eliminated by a subsequent differential amplifier (as explained in chapter 3.3).

Using the ULTRAGAIN PRO for this kind of application is easy. Simply connect the line output of your keyboard to the phone jack input on the ULTRAGAIN PRO. Then use the ULTRAGAIN PRO’s balanced output to send the signal to a stage box or other transmission chain. You can use both the XLR and phone jack connectors of the ULTRAGAIN PRO. Set the unit to LINE mode, with any additional functions switched off (an exception being the OUTPUT control which can be used to adjust levels, if necessary).