Symetrix 528 - Microphone Signal Processors; Microphone Preamplifier Details; Phantom Power for Condenser Microphones

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1. Microphone Signal Processors
1.1 Introduction
The 528 Voice Processor contains five functions needed for microphone signal processing:
mic preamp (with phantom power), de-esser, downward expander, compressor/limiter, and
parametric equalizer. The 528's "normal" si~al chain includes all five functions, but the
de-easer, the dynamics processor (expander/ co mpre ssor/limiter), and the parametric
EQ/notch filter may be individually bypassed with front panel switches.
In addition, the versatility of the 528 is a-reatly enhanced by its patching capabilities.
Each section is brought out to its own set of normalled rear panel terminations, so any
si~al routing possibility is provided for, and patch bay installations are supported.
1.2 Microphone Preamplifier
The ultra low noise , low distortion mic preamp provides a direct-coupled balanced input,
for optimum transient response and phase coherency. Its positive going and negative
iroinir slew rates are symmetrical, ensurinir sonic intergrity. The mic preamp's balanced
input .rain may be adjusted with the front panel gain trim control, allowing it to handle
very hi1rh signal levels -- up to -3dBV without overload.
1.3 Phantom Power
All condenser microphones require some kind of electrical power for operation. This
power may be supplied to the mic by internal batteries, an external power supply that's
connected to the microphone by a special cab le, or through the microphone cable by
phantom or "T" System powerin1r.
The 528 provides phantom powering for condenser microphones. The rear panel PHANTOM
POWER switch applies +48VDC to pins 2 and 3 at the mic input XLR connector. Phantom
power is so named because it is "invisible" to audio signals, even though the microphon e
cable carries both direct current (DC) phantom power, and alternating current (AC) audio
si~als. Specifically, the term phantom power means a positive DC volts.re applied to
the microphone via both audio leads, u sua ll y pins 2 and 3 of a 3-pin XLR type
connector. This volta.re is applied thrOUIZ'h current limiting resistors, which also serve to
isolate the audio leads from one another.
The phantom power technique uses the mic cable to deliver the power required by the
microphone , eliminatinri the need for an additional external power supply or internal
batteries. In this situation +48VDC is applied equally to both sides of a floating
balanced ci rcuit , so no current flows throurih the micr ophone's transformer, or through the
microphone element itself. However, if the circuit is unbalanced, current will flow. As
a
result, unbalan ced mies can never be used with phantom power.
Phantom powerinri is not to be confused with ''T'' System powering, sometimes called
modulation lead or AB pow erinri. ''T'' System powerinri applies power to only one audio
lead, usin1r the other as power a-round. Phantom powerin4 and 'T" Syste m powerin4 are noc
compatible without special adapters.