AudioControl SA-3052A, Руководство по эксплуатации

Страница: 59 / 64

SA-3052A Owner’s Manual
9-1
®
This section of the SA-3052A manual discusses the theory of operation to a func-
tional block diagram level. This section is intended to familiarize you with the major
circuit blocks that make up the SA-3052A’s circuitry.
The circuitry of the SA-3052A is contained on two printed circuit boards (PCBs).
The analog circuitry and part of the digital circuitry are contained on the main PCB,
while the microprocessor and it’s related circuitry is contained on a second PCB.
The discussion begins at the input connectors to the SA-3052A.
Input Circuitry
Microphone level signals are applied to the preamplifier circuit. The preamplifier
consists of three op-amps connected as an instrumentation amplifier. This converts the
balanced input signal into an unbalanced signal, and supplies enough gain to bring the
input signal up to the SA-3052A’s standard operating level. The overall gain of the pre-
amplifier circuit is set by the
D
B i
nPut switCh
. Simplex (phantom) powering for 12 volt
condenser microphones is also provided at the microphone input connector.
Line level signals applied to the ¼” tip-ring-sleeve phone jack are first attenuated to
microphone level, and then sent to the preamplifier circuit. Signals applied to the RCA
connector are applied to an op-amp that provides any voltage gain needed as well as im-
pedance buffering. The overall gain is set by the
D
B i
nPut switCh
.
The output signals from both preamplifiers are then summed together. The gain of
the summing amplifier is also set by the
D
B i
nPut switCh
. The last stage of the input cir-
cuit is another variable gain stage. The gain of this stage is also controlled by the
±10
D
B
ControL
. Lastly, the signal is scaled to the correct level by the
D
B/s
teP switCh
.
One-Third Octave Filters
After the input circuitry, the signal drives the inputs of thirty one-third octave filters
and the broadband SPL circuit. Since the SPL circuit is simpler, we’ll discuss it first.
The input to the SPL circuit is a broadband signal. It is rectified, filtered, and applied
to one input of the multiplexer (see the next section).
Each of the thirty one-third octave filters is the same, except for part values. Each of
the filters is made up of two cascaded, two-pole, bandpass filters. The Q (bandwidth) and
center frequency of each filter is precisely set by precision resistors and capacitors. The
center frequencies of the two filters are split on each side of the ANSI specified center
frequencies. The proper combination of filter Q and the offset center frequencies com -
bines to give an overall curve that meets the ANSI specification for a one-third octave,
Class II, type E filter.
Each of the thirty filter outputs is then rectified, filtered, and sent on to the multi -
plexer.
Signal Multiplexing and Analog to Digital Conversion
The multiplexer switches between each filter output, in sequence, for a fraction of a
second per filter. Each filter output is scanned for the same period of time. Another multi -
plexer input reads the output of the broadband SPL circuit discussed earlier.
Chapter 9 - Theory of Operation