I + V B
l
+ V C C
B A L A N C E
G A I N S T A G E
E R R O R A M P
CHANNEL 1
BALANCED
INPUT
N E G A T I V E F E E D B A C K
HIGH V O L T A G E
P O W E R S U P P L Y !
C H A N N E L
1
+ V B
• 4 - V C C
- V B
- V C C
LOW V O L T A G E
P O W E R S U P P L Y
+
15
- 1 5
B R I D G E
M O N O
C
B A L A N C E
G A I N S T A G E
+ L V A
V O L T A G E
T R A N S L A T O R
w u W V
C U R R E N T
L I M I T E R
- L V A
P O S I T I V E
O U T P U T S T A G E
N E G A T I V E
O U T P U T S T A G E
R
- V B
- V C C
N E G A T I V E
O.D.E.R
L
+ V C C
B R I D G E B A L A N C E
I N P U T
V . I N F
P O S I T I V E
B R I D G E B A L A N C E
O U T P U T S T A G E
P O S I T I V E
O . D . E . R
' — o
N E G A T I V E
B R I D G E B A L A N C E
O U T P U T S T A G E
' - V C C
P A R A L L E L
M O N O
S W I T C H
n a n n
CHANNEL 2
BALANCED
INPUT
E R R O R A M P
l + V B
l + V C C
+ L V A
V O L T A G E
T R A N S L A T O R
N E G A T I V E F E E D B A C K
HIGH V O L T A G E
P O W E R S U P P L Y !
C H A N N E L
2
+ V B
• + - V C C
• — V B
- V C C
C U R R E N T
L I M I T E R
- L V A
P O S I T I V E
O U T P U T S T A G E
N E G A T I V E
O U T P U T S T A G E
R
- V B
- V C C
N E G A T I V E
O.D.E.R
, + V C C
B R I D G E B A L A N C E
I N P U T
P O S I T I V E
B R I D G E B A L A N C E
O U T P U T S T A G E
P O S I T I V E
O . D . E . R
polarity f r o m the error a m p U 1 0 4 - A . T h e +LVA
(Q104,Q105) and the -LVA (Q110.Q111), with their
push-pull effect through the bias servo Q313, 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. D 3 0 1 , D302, D303, and D304 are
used to remove the charge on the unused portion of
the output stage, depending on the polarity of the
output signal.
CHANNEL 1 With the added voltage swing provided bytheLVAs, the
OUTPUT signal then gains current amplification through the
three-deep Darlington emitter-follower output stage.
T h e bridge-balanced c i r c u i t ( U 1 0 4 - B ) r e c e i v e s a
signal from the output of the amplifier, and differences
it with the signal at the V c c supply. T h e bridge-
balanced circuit then develops a voltage to drive the
bridge-balanced output stage. This results in the Vcc
supply having exactly one half of the output voltage
added to their quiescent voltage. D309, D310, D311
and a selected 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-
w o r l d c o n d i t i o n s . T h e s e c o n d i t i o n s a r e h i g h
instantaneous current, excessive temperature, and
A)
^ o p e r a t i o n of t h e o u t p u t d e v i c e s o u t s i d e s a f e
conditions.
Q107 and Q108 act as a conventional current limiter,
sensing current in the output stage. When current at
any one instant exceeeds 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 s p e c i a l l y
developed "ODEP" circuit is used (Output Device
Emulator Protection). It produces an analog output
proportional to the always-changing safe-operating-
area margin of the output transistor. T h i s output
CHANNEL 2 controls the translator stage previously mentioned,
OUTPUT removing any further drive that may exceed the safe-
operating-area of the output stage.
Thermal sensor S 1 0 0 gives the ODEP circuits vital
information on the operating temperature of the heat
sink on which the output devices are mounted.
N E G A T I V E
B R I D G E B A L A N C E
O U T P U T S T A G E
' - V C C
BRIDGE MONO OPERATION
By adding a jumper on the main board, the user can
convert the Micro-Tech into a bridge-mono amplifier.
With a signal applied to the Ch. 1 input jack, and the
load between the red binding posts on the back panel,
a double-voltage output occurs.
With the jumper added, the Ch. 1 output feeds the Ch.
2 error amp U204-A. Since there is a net inversion, Ch.
2 output is out-of-phase with Ch. 1 . This produces
twice as much voltage across the load. Each of the
channels' protection mechanisms work independently
if a fault occurs.
PARALLEL MONO OPERATION
With the "parallel mono" mono button ( S I ) pressed
out, the output of Ch. 2 is paralleled with that of Ch.
1
. A
suitable h i g h - c u r r e n t - h a n d l i n g j u m p e r must be
connected across the red binding posts to gain the
benefits of this mode of operation.
The signal path for Ch. 1 is the same as previously
discussed, except that Ch. 1 also drives the output
stage of Ch. 2. T h e b a l a n c e d input, error a m p ,
translators, and LVAs of Ch. 2 are disconnected and no
longer control the Ch.-2 output stage. The Ch.-2 output
stage and protection m e c h a n i s m s are also coupled
through S I and function as one.
In PARALLEL MONO mode, twice the current of one
channel alone can be obtained. Since the ODEPcircuit
of Ch. 2 is coupled through S I , this gives added
protection if a fault occurs in the Ch. 2 output stage.
The ODEP circuit of Ch. 2 will limit the output of both
output stages by removing the drive from the C h . - l
translator stages.
Each channel is powered by its own powertransformer
T100 or T200. Both channels share a common low-
voltage transformer T F - 1 . The secondary output of
T100 is full-wave rectified by D109 and is filtered by a
large computer-grade capacitor. D104-107 provide a
boosted voltage to power the LVAs and predrivers. A
t h e r m a l s w i t c h e m b e d d e d in e a c h t r a n s f o r m e r
protects it from overheating.
The low-voltage transformer T F - 1 uses a separate
winding on the fan motor. The T F - 1 output, rectified by
diodes D l - 4 , generates an unregulated 2 4 volts.
Monolythic regulators U l - 2 provide a regulated + / - 1 5
volts.
Fig. 4 . 1 Block Diagram
4-2
4-3
