AN ASIDE: "The Great Quasi-comp
Hype
n
Some major amplifier manufacturers use
all
NPN
output
devices promoting
NPNs
as more
rugged than
PNPs. That*s true.
But think about it.
What
they
don’t tell you
is that they're
also cheaper.
So...
the logic
goes
in
the
Engineering
Department;
if we could use
all
NPNs
we
can squeak
by
with
fewer output
devices,
and
they'll be cheaper, too.
"Boy I That
ought to
be
good for my next raise." (And if I
can
put
enough feedback
around it I might be able
to
cancel
most
of
the non-match
linearity problems).
At JBL
we don't work that way. We know
we could save
10
or
15% on output devices if
we
gave
up
fully
complementary operation
but
good design
sense
won't
let us compromise
our design for
a
small monetary reward. The
ears are the
final judge.
Now,
back
down off of the soapbox we have...
The output short circuit protection
is
the simplest
kind.
Only non-feedback type current sensing is used. Under
overload
conditions,
current
is shunted
from
the
collectors of the
"transconductance" stage to the output line and
into the load.
This
quite
effectively protects the output stage from
destruction
with any possible load.
No
computers,
time delays or other
such
nonsense are
used. Simple, clean, fast, rugged design is
all
that is necessary
or
desirable unless
the design goal is
a
minimum of output
devices in order to save cost. We
don't
subscribe to such techniques.
4.4 RELAY DRIVER/OUTPUT DC
DETECTOR
The output relay
K1
is driven
by transistor
Q23.
Diode
CR1
1
shorts out back
EMF
from the relay
coil. Resistor R38 provides a
current source for the
STANDBY LED when the relay is open.
The
emitter of
Q23
is
clamped
at
-0.6
volts by R42 and
CR14.
Therefore,
the
transistor will turn
ON when the voltage on
its
base becomes positive with respect to
ground. When the trans-
istor is
ON
the collector becomes
approximately 0
volts
and the
relay
turns on. Current which would have
gone to
the
STANDBY
LED
is shunted
by diode
CR12.
The relay is driven by IC2C
through
current
limiting
resist-
or R13. The circuits configured around
IC2C
perform several
functions.
A resistor to each
low
voltage power supply rail
in
combination with capacitor
Cl
3
provide turn-on
delay.
Resistors
R59 and R60 sense excess
DC
on the outputs of the
amplifiers and
resistor
R41 with thermistor R35 determine
overtemperature
operation of the heatsink.
All
of these circuits
are
diode
OR-ed
to the inputs of the
operational
amplifier which
is
configured
as
a
Schmitt trigger. Resistor R35 overcomes
any possible
input
offset and R34 is
positive
feedback hysteresis for all
circuits
except
overtemperature,
for which
resistor
R33 provides
hyster-
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24
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