Crown Com-Tech CT-400 Manuel de service télécharger pdf (Page 7)

Com-Tech 400 Amplifier Service Manual

Theory

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 cov-

ered 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 bot-

tom 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”

technology used by many other manufacturers. Out-

put 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 maximum reliability. Another electronic tech-

nique used is negative feedback. Almost all power

amplifiers utilize negative feedback to control gain

and provide stability, but Crown uses multiple nested

feedback loops for maximum stability and greatly

improved damping. Most Crown amplifiers have damp-

ing 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 sensitivity 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 pres-

ence, 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.

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

potentiometer is provided for common mode rejection

adjustment. Electrically, the BGS is at unity gain.

(From an audio perspective, however, this stage

actually provides +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 Sen-

sitivity 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

produce a near open loop gain condition which in turn

results in high peak output voltage. The output of the

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Crown Com-Tech CT-400 Manuel de service Page 7

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