Basics; Impendance; Balanced Vs. Unbalanced - Avalon VT-737SP Operation Manual

6.0 Basics

This chapter is intended to help the relatively
new recording engineer understand many of
the basics involved in recording. Included are
information on impedance and cables as well
as general information
preamplifiers, compressors, and equalizers.
The information does not necessarily apply
directly to the Vt-737sp, but many of the topics
discussed will help you understand how to
better utilize your Vt-737sp.
6.1 Impedance
Impedance is the total effective AC resistance
of a circuit, measured in ohms. The impedance
of an output or source is a measure of how
easily the power will flow (source impedance).
The impedance of an input is a measure of
how much power that input will tend to draw
(load inpedance).
Microphones are divided into two types of
impedance: high impedance and low imped-
ance. Most professional microphones are low
impedance devices, which means that their
source impedance is below 600 ohms. Piezo-
electric contact pickups, guitar pickups and
inexpensive microphones are usually high
impedance, meaning that their source imped-
ance is 2500 ohms or greater.
Low impedance microphones are preferred in
recording and sound reinforcement, since
when properly connected, they are far less
susceptible to extraneous noise pickup in the
cable. Low impedance mics can drive cables
hundreds of feet long, whereas high imped-
ance mics are limited to about 20 feet.
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on microphones,
High impedance microphones and pickups
require a transformer or buffer amplifier
when they are used with low impedance
inputs and/orlong mic cables. In this case,
the transformer converts the device's high
impedance to a low impedance suitable for
driving the connection.

6.2 Balanced vs. Unbalanced

An unbalanced connection is a two-wire
system. One wire carries the audio signal,
and the other (called the shield) is connected
to ground, or the electrical reference point.
A balanced connection is a three-wire
system. Two separate wipes carry the signal
- one inverted in polarity with respect to the
other - and the third is the shield, which
again is connected to ground.
Balanced connections are almost always
used for low impedance microphones. The
balanced system is less susceptile to
noise, and is the preffered method in pro-
fessional audio.
balanced connector found on pro audio
equipment is the three pin XLR-type, which
is chosen for several reasons: It has three
conductors, it is shielded, it locks in place,
and the ground pin makes contact first to
bleed static from the cable to avoid pops.
Unbalanced connections
high-impedance microphones and pickups,
and sometimes for low impedance mics in
consumer equipment.
system is more susceptible to noise pickup,
and is generally not preferred in professional
work. The most common unbalanced con-
nector is the 1/4-inch phone connector.
Avalon Vt-737sp Operation Manual
Basics
The most common
are used
The unbalanced
for