Preamplifier - Rockford Fosgate SYMMETRY EPX2 Installation & Operation Manual

Preamplifier

The preamplifier section is the heart of all EPX
activity. It provides source selection from one of two separate input
sources, volume control covering an 80dB range, independent bass
and treble controls with 12dB of boost or cut, and balance and fade
capabilities.
The EPX preamplifier has "differential inputs" to help prevent noise
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problems from creeping into the audio system. A circuit that incorpo-
rates differential inputs will only amplify the difference between two
conductors or wires. In car audio, most components are connected
together with RCA patch cables. Under ideal circumstances, the
difference between the hot center conductor and the outer shields is
the musical signal. Cars are anything but ideal, however. Electrical
noise is generated by the alternator, fan motors, windshield wipers,
brake lights, etc. These noises can be inductively coupled into the
RCA patch cables. When this occurs, the noise spikes appear on both
the center conductor and its outer shield. Since differential input
circuitry amplifies only the difference between the two conductors,
the noise spikes are NOT amplified, and they are consequently
eliminated from the system.
CENTER
CONDUCTOR
OUTER
SHIELD
Differential inputs only amplify the difference between two conductors.
Note: the noise spikes appear on both the center conductor and the outer
shield and therefore are not amplified.
To further reduce the possibility of noise, each set of RCA inputs has
its own independent audio ground. This is critically important when
dealing with systems that contain multiple source units. In every
system there should be one, and only one, point where audio ground
RCA INPUT
– 4 –
signal processing
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DIFFERENTIAL
OUTPUT