230
instruction Manual
Aphex Systems Ltd. Model 230Page 10
230
master voice channel
Page 11Aphex Systems Ltd. Model 230
4.1 USING THE MICROPHONE INPUT
The Model 230 is perfect for all types of microphones,
either powered or not. We encourage you to try every
mic you own with the 230.
Many features of the 230 are standard with all profes-
sional preamps - polarity - pad - etc. Some features
are unique and we hope you will fully exploit them.
4.2 USING PHANTOM POWER
Active microphones that take power through the
standard mic cable fall into a class called “phantom
powered” mics. The power is called “phantom”
because it rides the mic cable invisibly, without
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 6.81KΩ 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 48V phan-
tom power source.
For the technically interested, here’s why. The 6.81KΩ
resistors mentioned are shown in figure 4-1 below.
Since the power to the microphone is carried equally
(a requirement carefully observed by all mic manu-
facturers) on pins 2 and 3, it is like powering the
mic through a single series resistance equal to the
two resistors in parallel, which is a resistance of half,
or 3.405KΩ. 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
4.0 Using the 230
FRONT PANEL VIEW
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
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, weeks or months. 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.
48 V.D.C.
SUPPLY
6.81KΩ
1%
6.81KΩ
1%
PREAMP
1
2
3
+
Figure 4-1 Phantom Powering
(15 pages)
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