Recording - October 2015 - 68

By Paul J. Stamler

Part 10: Beyond The Boost-Mic Preamps II
Figure 1a

Figure 1b

Last time we talked about the physics of mic preamps: how they change signal levels,
and what lurks in the shadows when we do (clipping and hiss). But there's more to preamps than just providing gain, and it's time to look at these handy extra features now.
Power games
What else do preamps do? Well, they provide phantom power to power condenser
(and some ribbon) mics. They do that by including a source of +48V of DC (direct current) that connects to the Hot and Cold input terminals of the preamp (we talked about
them last month) through a pair of well-matched resistors. The phantom power can be
turned on and off; sometimes this can be done for each channel on a console. On cheaper consoles, and on channel strips and interfaces, all channels of phantom power get
switched on and off at once.
I've mentioned this before, but I'll say it again: never plug in or unplug either end of
a cable with phantom power turned on! Most of the time, nothing will happen (aside
from a pop in the audio), but if you happen to have a defective cable, you can destroy
an expensive microphone or console. So just don't do it.
A question of balance
Here's something else important to know about mic preamps: Their mic inputs are balanced. What does that mean?
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RECORDING October 2015

Let's take a short detour. If you play
electric guitar, you know that the cord
connecting the guitar to the amp has two
conductors: the Hot conductor, which is
wired to the tips of the 1/4" plugs, and
the Ground conductor, which is wired to
the shafts (sleeves) on the plugs. The Hot
wire carries the varying signal from the
pickups, while the Ground is defined as
maintaining a constant voltage of zero
volts. When you plug the cord into an
amp, the Hot signal goes to the amplifying circuit, while the Ground signal connects to the sleeve of the input jack and
thence to the amp's chassis, which is
grounded through the wall plug.
You also know that, if you lay your guitar cord near an AC power line, or something else which generates hum, the cable
will pick up the hum and you'll hear it
through the amp, which amplifies the hum
just like it was a signal from the guitar.
Microphone inputs on mic preamps
are different. They are differential, which
means they will amplify the difference
between the Hot connection (pin 2 on
the XLR connector) and the Cold connection (pin 3). Some mic preamps do the
differential amplifying thing by using a
transformer at the input, while others do
it with an electronic circuit that's transformerless, but they all do it-mic input
circuits are always differential.
Here's how that makes a difference
(pardon the expression). Most microphones put out a balanced signal, meaning that the changing voltage on the Hot
pin (2) is matched by a mirror-image signal on the Cold pin (3), with the polarity
inverted. When Pin 2 goes up, Pin 3
goes down; the differential circuit in the
preamp sees that they are different, and
amplifies that difference (Figure 1a).
But what happens if you lay the mic
cable in a hum field, like next to a power
cable, or in front of a refrigerator motor?
As we talked about last month, the Hot
and Cold conductors in the cable are
twisted tightly around each other (extratightly in a Star-Quad cable), and the
same hum signal gets induced in the Hot
and Cold wires. Figure 1b shows the

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