Universal Audio 4-710d Four-Channel Tone-Blending - Manual (Page 21)

Model 4-710d Overview
21
Phantom Power
Most modern condenser microphones require +48 volts of DC (Direct Current) power to operate. When
delivered over a standard microphone cable (as opposed to coming from a dedicated power supply),
this is known as “phantom” power. The 4-710d provides such power when the Phantom switch (
page 11) is engaged (placed in the +48V, up position), applying 48 volts to pins 2 and 3 of the rear
panel output connector.
While, in theory, this should result in no harm to the connected microphone even if it does not require
phantom power, problems can occur if the shield (pin 1) is broken or when using inexpensive
microphones that use the shield as their ground. The application of phantom power can even damage
those older ribbon microphones that have their output transformers wired with a grounded center-tap.
What’s more, the application of phantom power can often result in a loud pop (transient). For these
reasons, we strongly recommend that the Phantom switch be left in its off (down) position when
connecting and disconnecting microphones. Only turn the Phantom switch on if you are certain that
the connected microphone requires 48 volts of phantom power. If in doubt, consult the
manufacturer’s owner’s manual for that microphone.
Polarity Inversion
The occasional need for polarity inversion (changing the 4-710d front panel switch from IN ø to OUT ø)
is best demonstrated by a common example: recording an open-backed guitar amplifier with two
microphones, where one mic is placed close to the front of the amp's speaker and the other near the
back of the amp. The waveform display of the first mic will show an upward peak when the speaker
pushes outward, placing positive sound pressure on the mic. However, the waveform display of the
second mic (the one behind the amp) will show a downward (negative) valley when the speaker pushes
forward, because from the back of the amp the speaker moves away from the mic, thus creating
negative sound pressure. If these two signals are mixed, the positive waveform from the front mic
combines with the negative waveform from the back mic to result in cancellation of much of the amp's
sound and a "thinning effect" that is sonically disappointing. However, if the phase of one of the mic
signals is inverted, the two signals will combine instead of cancelling, and the result will be much
fuller and sonically pleasing.
Other double-mic applications often requiring phase inversion include piano soundboards, drum heads
(one mic on top of the drum and the other below it), and acoustic guitar miking, where one mic is
placed close to the soundhole and another further away or behind the guitar.
Low Cut Filtering
A common method for optimizing mixes is to apply low-cut filtering whenever possible. Excessive low
frequencies from microphones and instruments tend to build up in the mix, creating sonic “mud” that
masks musical detail, overloads or fatigues the listener’s ears, and sucks energy from power amps
and speakers. It isn’t uncommon to notice meters showing noticeably lower levels after low-cut
filtering is applied—a sure sign that such filtering was necessary. In addition, after low-frequency
mud is filtered, there is often more room in the mix to bring up important musical elements such as
vocals and lead instruments, resulting in a win-win situation (less mud = more music).
Typically, a low cut filter can be used to remove: vocal "B","P" and other popping sounds; moving-air
noise from close-miked vocals, drums, guitars and outdoor weather; instrument body noise from
handling guitars, basses, pianos, saxophones, etc; mic-stand vibrations; studio or stage floor
vibrations; air-conditioning; electrical hum; and unwanted proximity-effect bass boost.