207d
instruction Manual
Aphex Systems Ltd. Model 207D
Page 12
207d
2-ch tube mic preamp with adc
Page 13
Aphex Systems Ltd. Model 207D
phantom power is switched on can sometimes be
dangerous. Some microphones can be damaged by
power inrush. Good practice calls for switching off
the phantom power before changing or inserting a
microphone. Wait long enough to hear the mic go
silent before unplugging.
You should be aware of the shock hazard with the
phantom power system.
Long, open mic cables that
are disconnected from the preamp while phantom is
on can hold a d.c. charge for long periods of time,
sometimes days. They will act as a storage capacitor
and
you can get shocked most rudely
by holding
the XLR plug and touching the pins inside. Also
beware of microphone patch bays that may carry
phantom voltage. Don’t hold the patch cord by the
metal parts, only the plastic shell.
4.3.2 USING THE POLARITY SWITCH
There will be times when you need to reverse the
polarity (phase) of a mic signal. Vocalists monitoring
themselves on headphones will hear a different sound
when the phase is reversed, for example. The reversed
phase may sound fuller and more truthful. That is
because there is a cancellation of frequencies within
the ear when the external sound from the headphone
mixes with the sound directly conducted to the ear.
“Flipping the phase” can make the effect less notice-
able.
Another time when phase reversing can be helpful is
in mixing multiple microphones that are within each
other’s coverage areas. The “crosstalk” between mics
may come through different delays due to the dif-
fering distances from the sound. These sounds may
Using the 207D
interfering with the audio signal carried on the same
wires.
The industry standard phantom power source is posi-
tive 48 volts d.c. supplied to pins 2 and 3 through
precision low noise 6.81K
Ω
resistors. You may note
from spec sheets that many mics rated for phantom
power actually run at something less than 48 volts.
For example, the Audio Technica AT3031 small dia-
phragm condenser mic is rated for 11 to 50 volts at 3
milliamps. On the other hand, the Neumann TLM193
spec sheet states only that the supply voltage should
be 48 +/- 4 volts and the current consumption is
not stated. Don’t let these specifications confuse you.
They all run perfectly well off the standard phantom
power source described above. For the technically
interested, here’s why.
The 6.81K
Ω
resistors mentioned are shown in figure
4-1 below. Since the power to the microphone is car-
ried equally (a requirement carefully observed by all
mic manufacturers) on pins 2 and 3, it is like power-
ing the mic through a single series resistance equal to
the two resistors in parallel, which is a resistance of
half, or 3.405K
Ω.
A voltage drop will occur across the
series resistance equal to the microphone’s current
consumption times the series resistance. In the case
of the AT3031, consuming 3mA, the drop is .003 X
3405 = 10.22 volts. That means the voltage actu-
ally appearing on pins 2 and 3 is 48 - 10.22 = 37.78
volts. That is why, if you use a voltmeter to check
phantom power while the mic is plugged in, you will
always see something less than 48 volts. This is not a
fault with the mic preamp.
Plugging and Unplugging a microphone when
48 V.D.C.
SUPPLY
6.81K
Ω
1%
6.81K
Ω
1%
PREAMP
1
2
3
+
Figure 4-1 Phantom Powering
Distance
to Mic2
Distance
to Mic1
Σ
Sound
Cancelled
=
Figure 4-2 Phase Cancellation Effect