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Note that the circuit is sensitive to the actual power
supply voltages; if the ac line input voltage is less than
120V, clipping and overload indication will take place at
a lower output level. Note too that the LEDs remain on
during shutdown; this is a normal condition because the
power amplifiers are still partially powered during shut-
down.
DB PEAK/FEEDBACK FINDERTM LEDS
With the FEEDBACK FINDER switch (S903) in the OFF
position, LED string D1101-Dl108 indicates peak power
amplifier output voltage values corresponding to indica-
tions of -36 to +6 dB at the SPEAKER OUTPUTS. A +6 dB
indication is calibrated through R919 (or R916, Model 700)
for a 20 Vrms sinusoidal output (100 watts to a 4-ohm load).
The power amp assembly (A6) has a resistive voltage
divider at the power amp output that feeds the DB CAL
potentiometer R919. The DB CAL potentiometer sets the
point at which the $6 dB LED turns on. The ac signal is
then forwarded to a full-wave rectifier (U920).
The rectifier is composed of two sections: a unity gain,
inverting precision half-wave rectifier formed by U920A*
and its associated components, and an inverting summing
amplifier formed by U920B, which has two inputs. The first
is via R1045 and R1040, and exhibits a voltage gain of
approximately 1.5 times. The second input is from the out-
put of the precision half-wave rectifier via R1041. The gain
of this input is approximately 3 times.
When the input signal from the power amp is negative,
the output of the precision half-wave rectifier is zero; when
the input signal is positive, the rectifier output is negative.
When the summing amplifier adds the rectifier output and
the non-rectified signal together, the result is an output
voltage that is always positive 1.5 VIN.
The FULL WAVE BAL adjustment (R1045) is used to trim
the gain in one of the summing amplifier paths so that it
is exactly one-half the gain of the half-wave rectifier path.
The output of the full-wave rectifier is fed to the network
comprised of D940, C925 and R1037, which produces the
desired fast attack, slower decay operation. The dc voltage
developed at C925 is fed to the LED comparator string via
the FEEDBACK FINDER switch (S903).
The dc output voltages from U920 drive the comparators
U1101-U1102, which are referenced to a resistive voltage
divider. Each doubling of voltage on the input to the com-
parators turns on the next higher comparator, causing it
to light its associated LED, as well as keeping the LEDs
below it lit. For example, an input voltage of 1.OV to the
comparator lights the -18 dB LED, as well as the -24,
-30 and -36 LEDS.
Model 700: Note that when S903 is turned to A or B for
feedback determination, the corresponding LED string be-
comes a readout of frequency for that channel, and the
other channel retains its dB peak indicating function.
FEEDBACK FINDERTM
The Equalizer outputs are fed to the FEEDBACK FINDER
switch (S903) on the Output Controls board (A9), which
selects the desired output channel (A or B in Model 700)
for display. The FEEDBACK FINDER circuit senses the
frequency of the Equalizer signal and converts it to a dc
voltage for display on the selected channel LEDs.
'Channel
only: channel A components are shown In the Model 700 A9 Circuit
8
Diagram.
Amplifier U916B clips the input signal to establish a
square wave of approximately 1.2 Vpp. U916B also con-
tains the FEEDBACK FINDER THRESHOLD control (R999)
which is used to adjust the sensitivity of the circuit. U916B's
output is capacitively coupled to comparator U903A via an
RC filter (R915, C909) that attenuates any ultrasonic noise
present in the signal. The first comparator (U903A) is a
zero-crossing detector, with the operating threshold set
by the amount of hysteresis in the first stage.
The second comparator (U903B) holds capacitor C910
discharged until the output of U903A goes high. The out-
put of the third comparator (U903C) goes high at the same
time as U903A, and stays high until C910 charges to the
reference voltage of 7.5V. This time constant is adjustable
(through R972, FBF CAL) about the nominal 20 ,&ec period
to calibrate the frequency-to-voltage converter. The resul-
tant 20 @ec pulse from U903D is inverted by U903C and
switches a constant current source (Q901) on and off.
Thus, the output of U903D is a pulse train with a constant
pulse width of 20 ~ s e c and a repetition rate equal to the
input frequency. The current pulses are integrated by
capacitor C911 into a dc voltage which is directly propor-
tional to the input frequency (approximately 12 Vdc output
with an input frequency of 11 kHz). The dc output voltage
is fed through switch S903 to the LED comparator strings
to indicate the frequency band. For example, a 1 kHz signal
will produce approximately 1 Vdc at C911 and light the
LED above the 1 kHz graphic equalizer control (and all
lower frequency LEDs: 125, 250 and 500 Hz).
POWER AMPLIFIER PROTECTION CIRCUITS
Each power amplifier is protected by a voltage-current
(V-I) sensing circuit which limits the V-l conditions in the
output transistors to a safe level under overload conditions.
Transistors Q610 and Q611 conduct under overload condi-
tions and remove the input signal from the driver transistors
(Q608 and Q609), thus limiting the dissipation of the output
transistors (Q612-Q615). A single-slope boundary is de-
fined by the values of the resistor network on the bases of
Q610 and Q611.
Thermistors RT602 and RT603 (mounted on transistor
Q614 and Q615 heat sinks) monitor the transistor case tem-
peratures. RT602 and RT603 decrease in resistance as the
case temperatures go up, causing the boundary to "pivot"
to a lower value. The benefit of this arrangement is that
under severe overload conditions, such as operating into
a short circuit, dissipation is limited to a safe value. How-
ever, under normal conditions the protection boundary
moves out to allow operation with more severe "normal"
loads than would otherwise be possible. Capacitors C609
and C610 insure stability during limiting.
Q603 provides pre-driver (Q601) protection as follows.
When the current through Q601 reaches a sufficiently high
level during overload, the increased voltage drop through
resistor R614 is sensed by Q603, which then shorts out the
signal to (2601, limiting its collector current to a safe value.
Diodes D609 and D608 are normally reverse-biased and
do not conduct. But with highly reactive amplifier loads, the
diodes will conduct to prevent reverse-voltage breakdown
of the output transistors.
Thermistor RT601 is mounted on output transistor Q613.
Comparators U503A and U503B compare a constant refer-
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