IF YOU HAVE OR USE PHANTOM POWER, READ THIS SECTION.
Never patch ANY MIC through a patchbay with phantom power on. This puts 48 Volts DC across pins
two and three, and magnetizes microphone output transformers. Faulty or miswired cables, or a poorly
designed phantom supply can also cause such subltle magnetiation damage. Good practice is always to:
A. Turn power off for at least two minutes, so the phantom power capacitors are all fully discharged.
B. Connect all your microphones to the inputs while phantom power is off.
C. Turn the phantom power back on.
Not all gear can deliver full-spec phantom power: 10 mA of to each channel simultaneously. This occurs
more often with battery powered equipment. The Edirol R44 user manual specifies a total of 25mA current
for all four inputs. Not everyone reports such useful details. Some studio mics initially need lots of current
to get their DC to DC converters operating properly. They still pass signal if starved for current at startup,
but also won’t sound right. Their specification sheet only quotes the normal operating current numbers. So
we suggest turning mic power on in stages, if you have concerns about current capacity.
If DC is applied across pins two and three of a passive ribbon such as the R88, the ribbon accelerates out
of the gap, and often stretches or breaks. Moving coil and condenser mics are damaged in a more subtle
way as DC flows through their output transformers. DC magnetizes the core, and because of residual
magnetization, even after the voltage is removed the still mic sounds different. Phantom Power is not
supposed to damage mics and if used thoughtfully this is nominally true. But phantom power can damage
microphones in various ways, and even for mics that use it, some cautions are appropriate.
Coincident Stereo Microphone Techniques
The R88 is a
coincident
microphone system. Coincident mic techniques use two or more transducers placed
closely together. Coincident systems have good
time coherence
between their channels. This means that
sounds from the same sound source all arrive at close to the same time. One advantage of phase coherence
is that when channels are mono summed for television or radio, there is no comb-filtering effect.
Spaced
techniques lie at the other end of an engineer’s continum of techniques. Spaced microphones
are noticeably separated, which can produce time-of-arrival differences for sound from the same source.
When mixed to mono these time delays sum in and out of phase, in a complex and identifiable pattern
called a “comb filter”. These frequency response aberrations change with the sound source location,
and equalization cannot correct them. Some engineers prefer spaced mic techniques regardless of
mono summing problems, because of the impression that the stereo field produced is “wider” and
more “enveloping.” A properly positioned Blumlein pair also conveys a strong sense of spaciousness,
reminiscent of spaced omnis, which we attribute to the left and right and left quadrants being out-of-phase.
Blumlein stereo uses two figure-8 microphones, vertically coincident, and 45 degrees from the
centerline. Blumlein stereo creates a
genuine
stereo image, where instruments have both width and depth.
Intensity stereo cues across the front and rear quadrants are exceptionally precise. To use the R88 for
Blumlein stereo, align the AEA logo and vertical marks with the center of the instrument or ensemble you
would like to record. Keep the sound source or ensemble in the 90 degree angle found between the channel
indicators “1” and “2” to ensure consistency of phase information and an accurate stereo image. This front
quadrant is the
in phase and positive polarity
quadrant. The rear quadrant, on the opposite side of the mic,
is
in phase, out of polarity and reversed image.
There are no phase problems between channels, but as seen
in the diagram, the rear lobes of figure-8 microphones are opposite in polarity and angularity . The left and
right side quadrants are the
out of phase
quadrants, as sound arriving from those quadrants is picked up out
of phase. The R88 is the only coincident mic to maintains directionality down to 20 Hz and this provides
