Microamplifier Series
Circuit Description of Output Disable
GK Document # 450-0062-01
A. Mihalka, 2-13-91, Rev. 9/10/99-SW
Scope
This document describes the shutoff feature of the microamplifier series. Some boards may be stereo, but all boards work on the same
principle. There are 4 conditions whereby the output is disabled, each of which are described below (see Circuit Descriptions).
Disable Modes
Pop Elimination with Power Switch
The first type of output disabling is for the elimination of speaker pops when the unit is either turned on or off. In the case of turn-on, the
output is clamped for a couple of seconds and then released. At turn-off, the output is disabled and clamped until the entire power supply
decays. This takes about 10 seconds, after which a low thump may be heard if listening close to the speakers.
Output Overload
The second type of output disabling occurs when an unsafe output operating condition exists. A circuit senses overload conditions by
comparing output voltages and current. The unit is able to drive 4ohms per side stereo or 2ohms mono without tripping off. Some conditions
that will cause output disable are shorted cords, blown speakers, heavy loads, and plugging into the output jack while the unit is on. If an
overload condition is detected, the outputs will be clamped indefinitely. In this condition, the LEDs on the front panel will remain on, and the
preamp and its outputs will function, but no output is heard. The power switch must be turned off for 3 seconds before the unit can be turned
back on and again produce output power. Both outputs of a stereo unit are clamped if the condition is detected in one side.
Thermal Shutdown
Finally, if the heat sink becomes too hot the unit will shut off. This is also considered an unsafe operating condition and will disable the
outputs indefinitely. A circuit senses the temperature of the case of the output transistors. Both sides of a stereo unit will be disabled if one
side detects a problem. The power switch must be turned off for 3 seconds before output power is again enabled.
Circuit Descriptions
Output Disabling
The outputs are disabled by Q648 and Q647, an A06 and A56 respectively, which are either both on (disabled) or on (enabled). When on,
these transistors effectively clamp the output drivers. When off during normal operation, the base of Q648 should be low and that of Q647
should be high. The bases are driven by U605 (LM324), a low speed quad op amp. If the unit has no output, U605/7 will be low and U605/14
will be high. If these points are not OK, check the power supplies for U605. Pin 4 should be should be about + 5.3V and pin 11 –5.3V. Q722
and Q723 clamp the other side of a stereo unit.
Turn-On Pop
U605 is powered from the rails by 6.8K resistors and 5V zeners. This IC starts operating at about q3 volts. When the unit is powered up, pin 5
follows the + 15V on its way up to 3.14V. Meanwhile, pin 6 is climbing up to 4.8V, but is slowed by C821. Therefore, the output of the op
amp, pin 7, is initially high, disabling the outputs. Note that pin 14 is an inversion of pin 7 and drives the opposite polarity transistor. After a
delay the voltage on pin 6 exceed 3.14V, forcing pin 7 low and enabling the outputs.
Turn-Off Pop
Pin 5 is connected to pin 3 and is 3.14V. Pin 2 is 3.05V. Note that these voltages are obtained with 1% resistors, plus a 470j to give a higher
voltage on pin 3. The output pin 1 is always high when the outputs are enabled, and D802 is back-biased to enable pin 6 to function. When
the unit is turned off, the + 15V supply drops and pin 3 follows, but pin 2 lags because of C803, a 10uF cap. This causes pin 1 to go low,
forward-biasing D802 and pulling down pin 6. Also, R801 provides feedback to clamp pin 3 low and keep
