Input Circuits - Orban OPTIMOD 6300 Operating Manual

6-8
TECHNICAL DATA

Input Circuits

This circuitry interfaces the analog and digital inputs to the DSP. The analog input
stages scale and buffer the input audio level to match it to the analog-to-digital
(A/D) converter. The A/D converts the analog input audio to digital audio. The digi-
tal input receiver accepts AES3-format digital audio signals from the digital input
connector and sample rate-converts them as necessary. The digital audio from the
A/D and SRC is transmitted to the DSP.
1. Analog Input Stages
Located on input/output/DSP board
The RF-filtered left and right analog input signals are each applied to a floating,
balanced amplifier that has an adjustable (digitally controlled) gain. Analog
switches set the gain. The outputs of a latch set the state of the switches. By writ-
ing data to the latch, the control circuits preset the gain. This circuit feeds an RC
low-pass filter that applies the balanced signal to the analog-to-digital (A/D)
converter.
The digitally controlled gain circuitry was included on the circuit board
for possible use in future products. In the 6300, its gain is preset so that
the A/D will clip at +27 dBu with respect to the 6300's analog inputs.
Note that the small RFI "tee" filter assemblies connected to the input and output
connectors are socketed and user-replaceable.
Component-Level Description:
The left channel balanced audio input signal is applied to the filter / load net-
work made up of L100-103 and associated resistors and capacitors. (There are
solder pads available in the PC board to accept an optional 600
load [R106] on the input signal if the user wishes to install one.) A conven-
tional three-opamp instrumentation amplifier (IC100 and associated circuitry)
receives the input signal. R110-114 and quad analog switch IC101 make up the
circuit that sets the gain of IC100. The switches in IC101 set the gain of the in-
strumentation amplifier by switching resistors in parallel with R104. (Smaller
total resistances produce larger gains.)
IC100 feeds IC104 and associated components. This stage balances, DC-biases,
and scales the signal to the proper level for the analog-to-digital (A/D) con-
verter IC107. IC105A and associated components comprise a servo amp to cor-
rectly DC-bias the signal feeding the A/D converter. R137-139, C109, C110
make an attenuator / RC filter necessary to filter high frequency energy that
would otherwise cause aliasing distortion in the A/D converter.
The corresponding right channel circuitry is functionally identical to that just
described.
IC100, 102 are socketed for easy field replacement. All other circuitry is sur-
face-mounted and is not field-replaceable.
ORBAN MODEL 6300
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termination