Pioneer SPEC-2 - Current Limiter Circuit; Peak Level Meter Circuit

Current
Limiter
Circuit
The
current
limiter
circuit, shown in
Fig.
2,
consists
of
Q9
-
qLz.
If
an overcurrent
flows in
the
power
transistors,
due
to low load
(less
than
4S2)
or load
shorting,
this circuit utilizes
the
voltage
drop
produced
in the
power
transistor
emitter
resistance
to limit
the input voltage.
Q9
-
Q12
are normally
OFF,
but if for
some rea.son
an overcurrent
occurs
in
the
power
transistors,
the
voltage
drop compo-
nent
of
the
power
transistor emitter resistance
increases.
This voltage
is
divided
and
used
to bias
the
bases of
Q9
and
Q11,
switching
them
ON.
Q9
and
Ql1
make
Q10
and
Q12
conductive,
limiting
the
signal voltages
applied
to the basis of
Q13
and
Q14.
VR2
and VR3 in
this circuit set the current limit-
ing value,
while TH1
and TH2
are
for
temperature
compensation.
RLG is
a selector switch for
chang-
ing
the current limiting value
at
4O
load.
4.2 PEAK
LEVEL METER
CIRCUIT
Logarithmic
indication
is required in order
to
provide
a broad indicating
range in a
single meter,
without
the
need for
range
selection by the
user.
With
respect
to
an
8O
load,
approximately 50dB
logarithmic
compression
is
performed
to
allow
meter
indication
in the range
of 0.01
-
500W.
As
shown in Fig.
3,
the circuit
is
divided into
positive
and negative sides. Each
side
consists
of
logarithmic
compression,
peak
value
holding and
voltage
to current
converter circuits.
In
addition
there
is
a
cunent resultant
circuits, common to
both
sides. The
positive
side circuit operation is
described
here.
SPEC-p
A
portion
of
the
pov/er
amplifier
output signal
is
compressed
by
the logarithmic
compression
circuit,
which utilizes
the
exponential
function
properties
of
diodes. Figs.
4
and 5 illustrate
the operating
principle
and input-output
response.
The
com-
pressed
signal
is
then rectified
and retained
for
a
suitable
length
of
time by
the
peak
holding
circuit,
which
employs
a
simple
diode
and capacitor
con-
struction.
Holding
time is
determined
by
ttre time
constant
of C1
and R1.
The holding
circuit voltage
is
then applied
to the
voltage-current
converter,
where it is
converted
into
a current value
and
amplified. The
current
afterwards
passes
through
the current resultant
circuit
and drives
the meter.
The
current
resultant
circuit applies
the larger
current
of
the
positive
and negative sides
to the
.meter
for
operation.
Fig.
4 Equivalent
Circuit of the Logarithmic Compression
lC input
voltage
Fig.
5 Input-output Response
I\ f
EL
+
Inprt voltagc(lC)
o
E
o
e
I
Yoltage
current
Conv e
rter
e
4
=
e
e
€
o
E
PostTtvt
stDE
logarithmic
comp
ress
ion
Peak
r alue
ho ld ing
7
It0lltvt
st0t
Fig.
3
Peak
Level Meter
Circuit
-
Positive
peak
current
--'
llegatiYe
peal
current