Theory - Crown Com-Tech CT-200 Service Manual

Overview
It should be noted that over time Crown makes im-
provements and changes to their products for various
reasons. This manual is up to date as of the time of
writing. For additional information regarding these
amplifiers, refer to the applicable Technical Notes
provided by Crown for this product.
This section of the manual explains the general opera-
tion of a typical Crown power amplifier. Topics covered
include Front End, Grounded Bridge, and ODEP. Due
to variations in design from vintage to vintage (and
similarities with other Crown products) the theory of
operation remains simplified.
Features
Com-Tech amplifiers utilize numerous Crown innova-
tions including grounded bridge and ODEP technolo-
gies. Cooling techniques make use of the what is
essentially air conditioner technology. Air flows bottom
to top, and front to side. Air flows a short distance
across a wide heatsink. This type of air flow provides
significantly better cooling than the "wind tunnel" tech-
nology used by many other manufacturers. Output
transistors are of the metal can type rather than plastic
case. This allows for a significantly higher thermal
margin for the given voltage and current ratings. All
devices used are tested and graded to ensure maxi-
mum reliability. Another electronic technique used is
negative feedback. Almost all power amplifiers utilize
negative feedback to control gain and provide stabil-
ity, but Crown uses multiple nested feedback loops for
maximum stability and greatly improved damping.
Most Crown amplifiers have damping in excess of
1000 in the bass frequency range. This feedback,
along with our compensation and ultra-low distortion
output topology, make Crown amplifiers superior.
Features specific to the Com-Tech Series' include slew
rate limiting, and audio muting for delay or protective
action. This amplifier can operate in either a Bridged or
Parallel Mono mode as well as dual (stereo). A sensi-
tivity switch allows selection of input voltage required
for rated output. Level controls are mounted on the rear
panel and are of the rotary type. Front panel indicators
let the user know the status of the low voltage power
supply (enable), signal presence, distortion, and an
ODEP indicator for each channel which shows the
reserve energy status. In general, the packaging of this
model is designed for maximum watt/price/weight/
size value with user friendly features.
For additional details refer to the specification section,
or to the applicable Owner's Manual.
Com-Tech 200 Amplifier Service Manual

Theory

Front End Operation
The front end is comprised of three stages: Balanced
Gain Stage (BGS), Variable Gain Stage (VGS), and the
Error Amp. Figure 1 shows a simplified diagram of a
typical front end with voltage amplification stages.
Balanced Gain Stage (BGS)
Input to the amplifier is balanced. The shield may be
isolated from chassis ground by an RC network to
interrupt ground loops via the Ground Lift Switch. The
non-inverting (hot) side of the balanced input is fed to
the non-inverting input of the first op-amp stage. The
inverting (negative) side of the balanced input is fed to
the inverting input of the first op-amp stage. A poten-
tiometer is provided for common mode rejection ad-
justment. Electrically, the BGS is at unity gain. (From an
audio perspective, however, this stage actually pro-
vides +6dB gain if a fully balanced signal is placed on
its input.) The BGS is a non-inverting stage. It's output
is delivered to the Variable Gain Stage.
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 Sensi-
tivity Switch, and level is determined by the level
control. VGS is an inverting stage with the input being
fed to its op-amp stage. Because gain after this stage
is fixed at 26dB (factor of 20), greater amplifier sensi-
tivity is achieved by controlling the ratio of feedback to
input resistance. The Sensitivity Switch sets the input
impedance to this stage and varies the gain such that
the overall amplifier gain is 26 dB, or is adjusted
appropriately for 0.775V or 1.4V input to attain rated
output.
Error Amp
The inverted output from the VGS is fed to the non-
inverting input of the Error Amp op-amp stage through
an AC coupling capacitor and input resistor. Amplifier
output is fed back via the negative feedback (NFb)
loop resistor. The ratio of feedback resistor to input
resistor fixes gain from the Error Amp input to the
output of the amplifier at 26 dB. Diodes prevent
overdriving the Error Amp. Because the Error Amp
amplifies the difference between input and output
signals, any difference in the two waveforms will pro-
duce a near open loop gain condition which in turn
results in high peak output voltage. The output of the
Error Amp, called the Error Signal (ES) drives the
Voltage Translators.
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