Macro-Tech 24x6 Reference Manual - page 22
Page 22
Macro-Tech 24x6 & 36x12 Power Amplifiers
supply becomes larger, the power transistors must
dissipate more heat. Also, the lower the resistance of
the power transistors, the more voltage you can de-
liver to the load. But when you lower the resistance of
the transistors, you increase the current passing
through them, and again increase the amount of heat
they must dissipate.
5.2.2 The VZ Supply
An articulated power supply like
VZ avoids much of
this problem by reducing the voltage applied to the
transistors when less voltage is needed. Reducing the
voltage reduces the heat, so the amplifier runs cooler
and more power can be packed in safely.
The
VZ supply is divided into segments to better
match the voltage and current requirements of the
power transistors. Remember that audio signals like
music are complex waveforms.
When the voltage
requirements are
high,
VZ supplies
switch to a
series
mode which pro-
duces higher volt-
age and less
current. The ampli-
fied output signal
never misses a
beat and gets full
voltage when it
needs it—not when
it doesn’t need it.
Fig. 5.2 Complex Musical Waveforms
For music the average level is always much less than
the peak level. This means a power supply does not
need to produce full voltage all of the time.
The
VZ supply
is divided into
two parts. When
the voltage re-
quirements are
not high, it oper-
ates in a
parallel
mode to pro-
duce less volt-
age and more
current.
Fig. 5.3 VZ Supply in Parallel Mode
Fig. 5.4 VZ Supply
in Series Mode
Sensing circuitry watches the voltage of the signal to
determine when to switch
VZ modes. The switching
circuitry is designed to prevent audible switching dis-
tortion and yield the highest possible dynamic transfer
function—you hear only the music and not the ampli-
fier. You get not only the maximum power with the maxi-
mum safety, but you also get the best power matching
to your load.
5.3 Circuit Theory
Each channel is powered by its own power transformer,
T100 or T200. Both channels share TF-1, a low
voltage
transformer. The secondary outputs of each trans-
former are full-wave rectified by heavy duty bridge rec-
tifiers and are filtered by large computer grade
capacitors. A thermal switch embedded in each trans-
former protects them from overheating.
The low
voltage transformer TF-1 uses a separate fan
motor winding. The TF-1 output is rectified by diodes
D1-4, generates an unregulated 24 volts. Monolithic
regulators U1-2 provide a regulated ±15 volts.
For simplicity, the following discussion of the circuitry
and operation will refer to one channel only. Please re-
fer to the block diagram in Figure 5.5 and the schemat-
ics provided at the back of this manual.
The input signal at the phone jack passes directly into
the balanced gain stage (U104-C,D). Use of a
P.I.P.
module for input signal causes the input signal to pass
through the
P.I.P. and then to the balanced gain stage.
The balanced gain stage (U104-C,D) causes balanced
to single-ended conversion to take place using a dif-
ference amplifier. From there, gain can be controlled
with a potentiometer. The error amp (U104-A) amplifies
the difference between the output signal and the input
POWER
TRANSISTOR
POWER
TRANSISTOR
SPEAKER
LOAD
VZ
STAGE
+
–
VZ
STAGE
+
–
VZ POWER SUPPLY
The power transistors stay cooler and are not forced to
needlessly dissipate heat. This is the normal operating
mode of the
VZ power supply.
POWER
TRANSISTOR
POWER
TRANSISTOR
SPEAKER
LOAD
VZ
STAGE
+
–
VZ
STAGE
+
–
VZ POWER SUPPLY