Technical information

Macro-Tech 3600VZ Power Amplifier
Page 27
The balanced gain stage (U104-C,D) causes bal-
anced-to-single-ended conversion to take place using
a difference amplifier. From there, gain can be con-
trolled with a potentiometer. The error amp (U104-A)
amplifies the difference between the output signal and
the input signal from the gain pot, and drives the volt-
age translator stage.
The voltage translator stage channels the signal to the
Last Voltage Amplifiers (LVAs), depending on the sig-
nal polarity, from the error amp U104-A. The +LVA
(Q105,Q125) and the –LVA (Q110,Q126), with their
push-pull effect through the bias servo Q318, drive the
fully complementary output stage.
The bias servo Q318 is thermally coupled to the heat
sink, and sets the quiescent bias current in the output
stage to lower the distortion in the crossover region of
the output signal. D301, D302, D303, and D304 are
used to remove the charge on the unused portion of
the output stage, depending on the polarity of the out-
put signal.
With the voltage swing provided by the LVAs, the sig-
nal then gains current amplification through the
Darlington emitter-follower output stage.
The bridge-balanced circuit (U104-B) receives a sig-
nal from the output of the amplifier, and differences it
with the signal at the Vcc supply. The bridge-balanced
circuit then develops a voltage to drive the bridge-bal-
anced output stage. This results in the Vcc supply hav-
ing exactly one half of the output voltage added to their
quiescent voltage. D309, D310, D311 and a trimmer
resistor set the quiescent current point for the bridge-
balanced output stage.
The protection mechanisms that affect the signal path
are implemented to protect the amplifier under real-
world conditions. These conditions are high instanta-
neous current, excessive temperature, and operation
of the output devices outside safe conditions.
Q107 and Q108 act as a conventional current limiter,
sensing current in the output stage. The allowable cur-
rent level is also adjusted as a function of voltage.
When current at any one instant exceeds the design
criteria, the limiters remove the drive from the LVAs,
thus limiting current in the output stage to a safe level.
To further protect the output stages, a specially devel-
oped ODEP (Output Device Emulation Protection) cir-
cuit is used. It produces an analog output proportional
to the always-changing safe operating area of the out-
put transistors. This output controls the translator stage
by removing any drive that exceeds the safe operating
area of the output devices.
Thermal sensor S100 gives the ODEP circuits vital in-
formation on the operating temperature of the heatsink
on which the output devices are mounted.
Should the amplifier fail in such a way that would cause
DC across the output lead, the DC protection circuit
senses this on the negative feedback loop and shuts
down the power supply until the DC is removed.
5.3.2 Bridge-Mono Operation
By setting the back panel Stereo/Mono switch to
Bridge-Mono, the user can convert the Macro-Tech into
a Bridge-Mono amplifier. With a signal applied to the
Channel 1 input and the load between the red binding
posts on the back panel, a double voltage output oc-
curs.
The Channel 1 output feeds the Channel 2 error amp
U204-A. Since there is a net inversion, Channel 2 out-
put is out of polarity with Channel 1. This produces
twice as much voltage across the load. Each of the
channel’s protection mechanisms work independently
if a fault occurs.
5.3.3 Parallel-Mono Operation
With the Stereo/Mono switch set to Parallel-Mono, the
output of Channel 2 is paralleled with that of Chan-
nel 1. A suitable high-current-handling jumper must be
connected across the red binding posts to gain the
benefits of this mode of operation.
The signal path for Channel 1 is the same as previ-
ously discussed, except that Channel 1 also drives the
output stage of Channel 2. The balanced input, error
amp, translators and LVAs of Channel 2 are discon-
nected and no longer control the Channel 2 output
stage. The Channel 2 output stage and protection
mechanisms are also coupled through S1 and function
as one.
In Parallel-Mono mode, twice the current of one chan-
nel alone can be obtained. Since the ODEP circuit of
Channel 2 is coupled through S1, this gives added pro-
tection if a fault occurs in the Channel 2 output stage.
The ODEP circuit of Channel 2 will limit the output of
both output stages by removing the drive from the
Channel 1 translator stages.