Circuit Theory; Front End Operation; Balanced Gain Stage (Bgs); Variable Gain Stage (Vgs) - Crown CE-1000A Service Manual

Rev. D

3 Circuit Theory

3.0 Overview
This section of the manual explains the general opera-
tion of the CE power amplifier. Topics covered include
Front End Operation, Voltage Amplification, Output
Stage Topology, Protection Circuitry and Power Sup-
plies. For simplicity, the circuit theory will only refer to
channel one. It may be assumed that channel two is
identical to channel one.

3.1 Front End Operation

The front end is comprised of three stages: Balanced
Gain Stage (BGS), Variable Gain Stage (VGS) and the
Error Amp. These front end stages are shown along
with the rest of the amplifier in block diagram form in
Figure 3.1.

3.2 Balanced Gain Stage (BGS)

Input to the amplifier is balanced. The shield is iso-
BGS VGS
-
-
+
+
Audio
Input
Figure 3.1 Block Diagram of CE Amplifier Circuit Operation
CE Series Power Amplifier Service Manual
Error
Amplifier
+
Signal
Translator
-
lated from chassis ground by an RC network to inter-
rupt ground loops. The inverting (-) side of the bal-
anced input is fed to the non-inverting input of the first
op-amp (U500-A) stage located on the input card. The
non-inverting (+) side of the balanced input is fed to
the inverting input of the first op-amp stage. Electri-
cally, the BGS is at unity gain. (From an audio per-
spective, however, this stage actually provides +6-dB
gain if a fully balanced signal is placed on its input.)
The BGS is a non-inverting stage. Its output is deliv-
ered to the Variable Gain Stage.

3.3 Variable Gain Stage (VGS)

From the output of the BGS, the signal goes to the
VGS where it is inverted and gain is determined by the
position of the sensitivity switch (26 dB or 1.4V), and
level is determined by the level control. The
sensitivity switch allows for R505 to be either in or out
of the local feedback path of U500-B. When R505 is
out of the path (i.e. sensitivity switch in 1.4V position),
and the gain of U500-B is greater than 1, the amplifier
will produce full rated output when the amplitude of the
input is 1.4 volts. Likewise, when the sensitivity switch
is in the 26-dB position, R505 is in parallel with R506.
This sets the gain of U500-B at 1.0 volt/volt. In
Positive
Driver
Stage
Bias
Servo
LVA
Positive
Driver
Stage
Negative Feedback (NFb)
Positive
Output
Stage
Amplifier
Output
Mute
Protection
Circuitry Circuitry
Positive
Output
Stage
Circuit Theory 3-1