Theory Of Operation; Overview; Features; Front End Operation - Crown Macro-Tech MA-3600VZ Service Manual

130366-1 Rev. A

3 Theory of Operation

3.1 Overview

It should be noted that over time Crown makes
improvements 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 amplifi ers, refer to the applicable Technical
Notes provided by Crown for this product.
This section of the manual explains the general
operation of a Macro-Tech 3600VZ power amplifi er.
Topics covered include Front End, Grounded Bridge,
ODEP, and VZ supply. Due to variations in design
from vintage to vintage (and similarities with other
Crown products) the theory of operation remains
simplifi ed.

3.2 Features

Macro Tech amplifiers utilize numerous Crown
innovations including grounded bridge and ODEP
technologies. Cooling techniques make use of the
what is essentially air conditioner technology. Air
fl ows bottom to top, and front to side. Air fl ows 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. 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 technique used is negative
feedback. Almost all power amplifi ers 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 amplifi ers have damping in excess of
1000 in the bass frequency range. This feedback,
along with our compensation and ultra-low distortion
output topology, makes Crown amplifi ers superior.
Features specifi c to the Macro Tech Series' include
two seperate power transformers (one for each
channel), a full time full speed fan which also serves
as the low voltage transformer, slew rate limiting,
and audio muting for delay or protective action. This
amplifi er 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
©2000 Crown International, Inc.
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 indicator for
each channel which shows the reserve energy 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 specifi cation section,
or to the applicable Owner's Manual.

3.3 Front End Operation

The front end is comprised of three stages: Balanced
Gain Stage (BGS), Variable Gain Stage (VGS), and
the Error Amp. Figure 3.1 shows a simplifi ed diagram
of a typical front end with voltage amplification
stages.
3.3.1 Balanced Gain Stage (BGS)
Input to the amplifi er 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 fi rst op-amp
stage. The inverting (negative) side of the balanced
input is fed to the inverting input of the fi rst 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.
3.3.2 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
Sensitivity 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 fi xed at 26dB (factor of 20), greater
amplifi er sensitivity 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 amplifi er gain is
26 dB, or is adjusted appropriately for 0.775V or 1.4V
input to attain rated output.
3.3.3 Error Amp
The inverted output from the VGS is fed to the non-
inverting input of the Error Amp op-amp stage through
MA-3600VZ Service Manual
Theory of Operation 3-1