Crown Macro-Tech 2402, Руководство по обслуживанию

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Circuit Theory 4-1
130445-1 Rev. A MA-2402 Service Manual
©2000 Crown International, Inc.
4 Circuit Theory
4.1 Overview
It should be noted that over time Crown makes improve-
ments and changes to their products for various rea-
sons. 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 grounded bridge power ampli-
fier. Topics covered include Front End, Grounded
Bridge, and ODEP. Due to variations in design from vin-
tage to vintage (and similarities with other Crown prod-
ucts) the theory of operation remains simplified.
4.2 Features
Macro Tech amplifiers utilize numerous Crown innova-
tions including grounded bridge and ODEP technolo-
gies. Cooling techniques make use of the what is es-
sentially air conditioner technology. Air flows bottom to
top, and front to side. Air flows a short distance across
a wide heatsink. Output transistors are of the metal can
type rather than plastic case. This allows for a signifi-
cantly 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 feed-
back loops for maximum stability and greatly improved
damping. Most Crown amplifiers have damping in ex-
cess of 1000 in the bass frequency range. This feed-
back, along with our compensation and ultra-low dis-
tortion output topology, make Crown amplifiers supe-
rior.
Features specific to the Macro Tech Series include three
seperate power transformers (one for each channel and
one for low voltage), a full time full speed fan, slew rate
limiting, and Crown's “Quad-Mute” protection circuit for
muting delay or protective action. This amplifier can op-
erate 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 front panel and are of the rotary
type. Front panel indicators let the user know the status
of the low voltage power supply (enable), an ODEP in-
dicator for each channel which shows the reserve en-
ergy status, and a SPI/IOC indicator for each channel
which indicates signal output and distortion. 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 Reference Manual.
4.3 Front End Operation
The front end is comprised of three stages: Balanced
Gain Stage (BGS), Variable Gain Stage (VGS), and the
Error Amp. Figure 4.1 shows a simplified diagram of a
typical front end with voltage amplification stages.
4.3.1 Balanced Gain Stage (BGS)
Input to the amplifier is balanced. The shield may be
isolated from chassis ground by an RC network to inter-
rupt 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 in-
verting (negative) side of the balanced input is fed to
the inverting input of the first op-amp stage. A potenti-
ometer is provided for common mode rejection adjust-
ment. 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.
4.3.2 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 Sensi-
tivity Switch, and level is determined by the level con-
trol. VGS is an inverting stage with the input being fed
to its op-amp stage. Because gain after this stage is
fixed at 26 dB (factor of 20), greater amplifier sensitivity
is achieved by controlling the ratio of feedback to input
resistance. The Sensitivity Switch sets the input imped-
ance to this stage and varies the gain such that the
overall amplifier gain is 26 dB, or is adjusted appropri-
ately for 0.775V or 1.4V input to attain rated output.
4.3.3 Error Amp
The inverted output from the VGS is fed to the non-in-
verting 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 con-