Theory; Overview; Front End Operation; Balanced Gain Stage (Bgs) - Crown Com-Tech CT-1610 Service Manual

Rev. 0

5 Theory

5.1 Overview

Com-Tech amplifiers incorporate several new technologi-
cal advancements, including real-time computer simula-
tion, low-stress output stages, an advanced heat sink
embodiment and the Programmable Input Processor
( P.I.P. ) expansion system.
Custom circuitry is incorporated to limit temperature and
current to safe levels, making it highly reliable and toler-
ant of faults. Unlike many lesser amplifiers, it can oper-
ate at its voltage and current limits without
self-destructing.
Real-time computer simulation is used to create an ana-
log of the junction temperature of the output transistors
(hereafter referred to as the output devices). Current is
limited only when the device temperature becomes ex-
cessive (and by the minimum amount required). This
patented approach, called Output Device Emulation Pro-
tection (or ODEP ) maximizes the available output power
and protects against overheating—the major cause of
device failure.
The amplifier is protected from all common hazards that
plague high-power amplifiers, including shorted, open
or mismatched loads; overloaded power supplies; ex-
cessive temperature and chain-destruction phenomenon;
input overload; high-frequency blowups, internal faults;
and input and output DC.
The four-quadrant topology used in a Com-Tech
amplifier's output stages is called the grounded bridge .
This patented topology makes full use of the power sup-
ply, providing peak-to-peak voltages to the load that are
twice the voltage seen by the output devices.
As its name suggests, the grounded bridge topology is
referenced to ground. Composite devices are con-
structed to function as large NPN and PNP devices to
handle currents which exceed the limits of available de-
vices. Each output stage has two composite NPN de-
vices and two composite PNP devices.
The devices connected to the load are referred to as
"high-side NPN and PNP" and the devices connected to
ground are referred to as "low-side NPN and PNP." Posi-
tive current is delivered to the load by increasing con-
ductance simultaneously in the high-side NPN and
low-side PNP stage, while synchronously decreasing
conductance of the high-side PNP and low-side NPN.
Com-Tech 810 and 1610 Amplifier Service Manual
The two channels may be used together to double the
voltage (Bridge-Mono) or current (Parallel-Mono) pre-
sented to the load. This feature gives you flexibility to
maximize power available to the load.
A wide bandwidth, multiloop design is used for state-of-
the-art compensation. This produces ideal behavior and
results in ultra-low distortion values.
Aluminum extrusions have been widely used for heat
sinks in power amplifiers due to their low cost and rea-
sonable performance. However, measured on a watts-
per-pound or watts-per-volume basis, the extrusion
technology doesn't perform nearly as well as the heat
exchangers developed for Com-Tech amplifiers.
Our heat exchangers are fabricated from custom convo-
luted fin stock that provides an extremely high ratio of
area to volume, and area to weight. All power devices
are mounted directly to massive heat spreaders that are
electrically at the Vcc potential. Electrifying the heat
spreaders improves thermal performance by eliminating
an insulating interface underneath each power device.
The chassis itself is used as part of the thermal circuit to
maximize utilization of the available cooling resources.

5.2 Front End Operation

The front end is comprised of three stages: Balanced
Gain Stage (BGS), Variable Gain Stage (VGS), and the
Error Amp. Figure 5.1 shows a simplified diagram of a
typical front end with voltage amplification stages.

5.2.1 Balanced Gain Stage (BGS)

Input to the amplifier is balanced. The shield is iso-
lated from chassis ground by an RC network to inter-
rupt ground loops. The non-inverting (+) side of the
balanced input is fed to the non-inverting input of the
first op-amp stage. The inverting (-) side of the bal-
anced input is fed to the inverting input of the first op-
amp stage. A potentiometer (R103) is provided for com-
mon mode rejection adjustment. Electrically, the BGS
is at unity gain. (From an audio perspective, however,
this stage actually provides +6dB gain if a fully bal-
anced signal is placed on its input.) The BGS is a non-
inverting stage. Its output is delivered to the Variable
Gain Stage.

5.2.2 Variable Gain Stage (VGS)

From the output of the BGS, the signal goes to the
VGS, where gain is determined by the position of the
Sensitivity Switch, and level is determined by the level
control. VGS is an inverting stage with the input being
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