GFA 5002 is a series amplifier of ADCOM at 90s. Its able to producing up to 50 Watts of output at 8 Ohm stereo load. While bridging or BTL is perform, it will be able to produce up to 175 watts of audio output at 8 Ohm loads.
To inform the owner that there are problems such as distortion and thermal protect for distortion, The GFA power circuit is available with two LEDs.
SCHEMATIC DIAGRAM
Below are editing schematic from the original service manual of the ADCOM GFA:
This circuit also has a circuit breaker some kind like a thermswitch to prevent damage to the circuit breaker ( S602 ).
This circuit comes from a temperature detector or commonly called a thermostat.
By default, thermostat is connecting at normally close. But if there is a problem with the temperature at the surface of heatsink, then it will release.
Its affecting to the transistor Q6 or Q5 cannot deliver current, and of course the differential amplifier circuit will stop.
And the audio signal will not will arrive at the voltage amplifier circuit.
Transistor Q617 serves as a servo driver transistor by setting the trimpot VR 63 it will be possible for this circuit to limit the idle current for power transistors Q627 Q631 Q629 and Q633.
At the output of this power circuit there are two diodes that are installed in parallel with each power transistor which aims to prevent backflow from the speaker coil reflection so that it can secure the transistor power.
In the amplifier driver section, which uses two levels of amplification, namely the next Q619 with Q623.
If you look inside the circuit, position on the Darlington transistor, it’s just that the gain is reduced so that it doesn’t get too big starting from Q619 then goes to Q623 with the hope that the current is quite small from the output of q613 is enlarged the current so that it can be further amplified to the final power transistor.
NOTE
LED DISTORTION & LED OVERHEAT TEMP
The principle is that the signal that enters the free amplifier circuit is then taken the potential difference across resistor R615 to be amplified by transistor Q607 and this current then enter an LED which means that when there is a signal that is more than gold or exceeds the threshold of the q607 emitter base, the q607 will deliver and turn on the LED light.
POWER TRANSISTOR
The power transistor in this circuit uses two pairs of transistors with the number 2SD1047 paired with 2SB617.
Transistor maximum Power Dissipation is 100 Watts each ( DATASHEETS ).
