User Manual

46
User's Guide MADIface XT © RME
20. Analog Inputs / Outputs
20.1 Mic / Line In (XLR / TRS)
The MADIface XT has 2 balanced XLR/TRS inputs on the front panel.
When using unbalanced cables make sure to connect pin 3 (-) to 1 (ground) (TRS: ring to
ground). Otherwise noise may occur, caused by the unconnected negative input pin.
The pin assignment follows international standards. With XLR, pin 2 is + or hot, pin 3 is – or
cold, pin 1 is ground. Pin 1 is connected to the chassis directly at the socket (AES48).
XLR and TRS Line have a gain range of 51 dB in steps of 3 dB, and another step of 9 dB, a
total range of 60 dB. The TRS Line input gain range is shifted by about 12 dB to support +24
dBu balanced operation (SMPTE). TRS also has a higher input impedance (6.6 kOhm).
The AD-converter in the MADIface XT reaches full scale already at an input level of –48 dBu
(Gain 60 dB, XLR input), but also at +24 dBu (Gain 0 dB, TRS input). Therefore the inputs are
both sophisticated microphone and line types.
20.2 Phantom Power
The LED +48V indicates activated phantom power for the XLR input. Phantom power should
only be activated when using condenser microphones which require such a power supply.
Connecting and disconnecting microphones while phantom power is active causes a high
voltage surge, which can destroy the microphone input stage! Switch phantom power off
before connecting/disconnecting any external device.
The MADIface turns on the phantom power smoothly during one second, from 0 to 48 Volts.
This technique is advantageous for the connected microphone as well as the MADIface.
The phantom power of the MADIface is short-circuit proof. With a maximum load the internal
voltage from the power supply does not drop below 47 Volts.
20.3 AutoSet
Some preamps include limiters in order to prevent clipping, especially of the A/D converter
stage. Such a circuitry is not feasible for the MADIface, because it would spoil the mic preamp's
excellent technical data.
But as the MADIface's gain is controlled completely digitally, the device can set it automatically,
thus providing perfect protection from overload with no degradation of the audio signal, which
does not have to pass any additional electronic circuits.
Since AutoSet operates as overload protection and not as 'compressor', there is no automatic
gain increase. AutoSet only reduces gain. And with AutoSet activated, the gain can still be
changed manually. The currently highest possible value can not be exceeded, because AutoSet
will reduce the gain in real-time during the manual change.
In practice, there are two possible ways to work with AutoSet:
• Gain of all channels is set to a rather high value, e.g. 60 dB. Then a rehearsal with maximum
acoustic level is performed. Thereafter AutoSet is switched off.
• As above, with AutoSet permanently active.