Recording - October 2015 - 69

result: When the hum signal in the Hot
wire goes up, the signal in the Cold wire
goes up identically. The preamp's differential input sees no difference between
the signals on the two input pins, and,
because there's no difference, it ignores
those signals; as a result, there's no hum
at the preamp's output.
Like magic, the preamp amplifies the
microphone signal (wanted) but ignores
the noise signal (not wanted). The noise is
called common-mode signal, because it's
common to both wires. Ideally, the preamp will reject any common-mode signal,
whether it's 60 Hz hum or garbage at
radio frequencies (coming from taxi dispatchers, broadcast stations, cell phones
and the like).
That's why I laid such emphasis in last
month's mic cable discussion on the tight
wrap of the inner conductors; the closer
they are to one another, the more identical
the noise signals induced in them, and the
better the preamp's differential input will
reject the noise. (In practice, transformerless circuits usually do a good job rejecting
60 Hz hum, but transformers are better at
ignoring radio-frequency crud.) Balanced
circuits are found in high-level connections
too, with similarly useful rejection of noise.

frequencies more drastically. Lowcut filters are also useful for minimizing room
noise and rumble that comes into the mic
through the stand.
The polarity switch inverts the polarity of
the amplified mic signal. That can be useful
if you mike the top and bottom of a snare
drum with two different microphones; flipping the polarity of the bottom mic can
make them blend better. The switch (or button) is sometimes called "Phase" or labeled
with a Ø. Opinion time: I think this is a misnomer; the word "phase" should be
reserved for information about when signals arrive at a mic, not for polarity.
Sometimes it's not possible to design a
mic preamp with low enough gain to
avoid clipping when it's used to amplify
hot mics on loud instruments or voices.
Those preamps often incorporate a pad,
which is a network of resistors at the input
that drops the mic's signal level (usually
by 14-20 dB) when it's switched in.
One more thing: a mic preamp has an
input impedance, into which the mic
operates. This can sometimes affect the
mic's performance-see the free articles
"The Taming of the Shure" and "When
Mikey Met Ohmy" on the Recording website to learn more.

Hot stuff
Mic preamps will often include an
instrument input. This is usually a jack on
the front panel, convenient for the selfrecording musician. Electric guitars and
basses like to see a high impedance, and
the instrument inputs on preamps, channel strips and interfaces usually oblige
them, with input impedances in the range
of 1 megohm (1 million ohms).
Instrument inputs are also used for electric keyboards, synths and the like. Those
don't require the same high impedances
as electric guitars, but the high impedance doesn't hurt anything.
Manufacturers sometimes call these "DI"
inputs, but I think "DI" should be reserved
for boxes that convert high-level, highimpedance, unbalanced instrument signals
into mic-level, low-impedance, balanced
signals that can travel down long cables. I
prefer to preserve the distinction by calling
the jacks on mic preamps "instrument
inputs", but maybe that's just me.

Across the great divide
Mic preamps can be divided into two
broad categories. No, I do not mean tube
and solid state. One group includes preamps intended to add their own flavor of
sound to signals that pass through them.
My colleague Paul Vnuk Jr. often refers to
these as "colored" or "vibey" preamps. The
other is composed of preamps intended to
be neutral, adding no particular color. The
jargon name for these is "straight wire with
gain"; I prefer to avoid the word "clean",
because it implies that colored preamps are
somehow "dirty" as in noisy.
These categories don't line up with
tubes and transistors. Tube preamps can
be very neutral-I've designed and built
several preamps for my own use, and the
most neutral one used tubes as the amplifying devices. Contrariwise, some of the
most famous "color" preamps have been
built from transistors.
A few examples of "colored" preamps
are the Universal Audio 610 designs, the
API preamps (whether in consoles or in
the famed 500 Series modules), and the
Little Labs LMNOPre. Pretty much anything with a Neve pedigree is also considered to have a particular color; Rupert
Neve sold preamps and consoles under
his own name, but also designed consoles for AMEK and Focusrite, and his
1073 preamp design has spawned
dozens of third-party imitators.
In the "straight wire with gain" category you'll find preamps like those from
Grace Design and Millennia Media, the
Solid State Logic (SSL) preamps (whether

More bits and pieces
More bits often found on mic preamps
are lowcut filters, polarity switches, and
pads. A lowcut filter, when switched in,
cuts the level of the signal's low frequencies; the filter usually begins to roll off
around 80-100 Hz (see Figure 2), and
it's normally used to compensate for the
proximity effect found in directional
microphones-see Article 5 in this series,
May 2015. A filter that slopes down at 6
dB/octave is gentle, while 12 dB/octave
and 18 dB/octave filters chop the low

RECORDING October 2015

http://www.t-funk.com

Table of Contents for the Digital Edition of Recording - October 2015