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