Output Device Emulation Protection (Odep); Vz Power; Background - Crown Macro-Tech MA-3600VZ Service Manual

MA-3600VZ Service Manual
and are biased AB+B in the same manner as the
HS.
When the amplifi er output swings positive, the audio
is fed to an op-amp stage where it is inverted. This
inverted signal is delivered directly to the bases of
the positive (NPN) and negative (PNP) LS predrivers.
The negative drive forces the LS PNP devices on
(NPN off). As the PNP devices conduct, Vce of the
PNP Darlington drops. With LS device emitters tied
to ground, -Vcc is pulled toward ground reference.
Since the power supply is not ground referenced
(and the total voltage from +Vcc to -Vcc is constant)
+Vcc is forced higher above ground potential. This
continues until, at the positive amplifi er output peak,
-Vcc = 0V and +Vcc equals the total power supply
potential with a positive polarity. If, for example, the
power supply produced a total of 70V from rail to
rail (±35VDC measured from ground with no signal),
the amplifier output would reach a positive peak
of +70V.
Conversely, during a negative swing of the HS output
where HS PNP devices conduct, the op-amp would
output a positive voltage forcing LS NPN devices
to conduct. This would result in +Vcc swinging
toward ground potential and -Vcc further from ground
potential. At the negative amplifi er output peak, +Vcc
= 0V and -Vcc equals the total power supply potential
with a negative polarity. Using the same example as
above, a 70V supply would allow a negative output
peak of -70V. In summary, a power supply which
produces a total of 70VDC rail to rail (or ±35VDC
statically) is capable of producing 140V peak-to-
peak at the amplifier output when the grounded
bridge topology is used. The voltage used in this
example are relatively close to the voltages of the
PB-1/460CSL.
The total effect is to deliver a peak to peak voltage to
the speaker load which is twice the voltage produced
by the power supply. Benefi ts include full utilization
of the power supply (it conducts current during both
halves of the output signal; conventional designs
require two power supplies per channel, one positive
and one negative), and never exposing any output
device to more than half of the peak to peak output
voltage (which does occur in conventional designs).
Low side bias is established by a diode string which
also shunts built up charges on the output devices.
Bias is adjustable via potentiometer. Flyback diodes
perform the same function as the HS fl ybacks. The
output of the LS is tied directly to chassis ground
via ground strap.
Theory of Operation 3-4
3.6 Output Device Emulation Protection
(ODEP)
To further protect the output stages, a specially
developed ODEP circuit is used. It produces
a complex analog output signal. This signal is
proportional to the always changing safe-operating-
area margin of the output transistors. The ODEP signal
controls the Voltage Translator stage by removing
drive that may exceed the safe-operating-area of
the output stage.
ODEP senses output current by measuring the
voltage dropped across LS emitter resistors. LS NPN
current (negative amplifier output) and +Vcc are
sensed, then multiplied to obtain a signal proportional
to output power. Positive and negative ODEP voltages
are adjustable via two potentiometers. Across ±ODEP
are a PTC and a thermal sense (current source). The
PTC is essentially a cutoff switch that causes hard
ODEP limiting if heatsink temperature exceeds
a safe maximum, regardless of signal level. The
thermal sense causes the differential between +ODEP
and –ODEP to decrease as heatsink temperature
increases. An increase in positive output signal output
into a load will result in –ODEP voltage dropping;
an increase in negative output voltage and current
will cause +ODEP voltage to drop. A complex RC
network between the ±ODEP circuitry is used to
simulate the thermal barriers between the interior
of the output device die (immeasurable by normal
means) and the time delay from heat generation at
the die until heat dissipates to the thermal sensor. The
combined effects of thermal history and instantaneous
dynamic power level result in an accurate simulation
of the actual thermal condition of the output transis-
tors.

3.7 VZ Power

VZ means Variable Impedance and is the name of
Crown's patented articulated power supply technol-
ogy. It enables Crown to pack tremendous power into
just 3½ inches of vertical rack space.

3.7.1 Background

A power supply must be large enough to handle
the maximum voltage and current necessary for
the amplifi er to drive its maximum rated power into
a specified load. In the process of fulfilling this
requirement conventional power supply designs
produce excessive heat, are heavy, and take up
precious real estate. It's no secret that heat is one of
a power amplifi ers worst enemies.
130366-1 Rev. A
©2000 Crown International, Inc.