Circuit Description - Adcom GFA-565 Service Manual

INTRODUCTION
This service manual is intended to assist trained and qualified technical personnel in verifying the performance of,
adjusting, and repairing the ADCOM GFA-565 power amplifier. The procedures described here are not intended lor
persons unfamiliar with the appropriate safety and test procedures.
A
WARNING A
THERE ARE POTENTIALLY LETHAL VOLTAGES WITHIN THE GFA-565 AMPLIFIER WHICH WILL BE
ACCESSIBLE ONCE ITS TOP COVER IS REMOVED. DO NOT ATTEMPT FAMILIARIZATION, INSPECTION
OR ANY PROCEDURE WHATSOEVER UNLESS YOU HAVE DISCONNECTED THE GFA-565 FROM THE
WALL AC OUTLET OR OTHER SOURCE OF AC POWER AND THE POWER-SUPPLY CAPACITORS ARE
COMPLETELY DISCHARGED. PLEASE TAKE NOTE THAT THE POWER-SUPPLY CAPACITORS TAKE AS
LONG AS 5 MINUTES TO DISCHARGE. THESE INSTRUCTIONS ARE PROVIDED FOR USE ONLY BY
COMPETENT TECHNICAL PERSONNEL DO NOT UNDERTAKE ANY SERVICE PROCEDURES IN THE
GFA-56S UNLESS YOU ARE TECHNICALLY QUALIFIED TO DO SO.
CIRCUIT DESCRIPTION
The ADCOM GFA-565 1$ a monaural power amplifier rated at <0.02% THD from 20Hz to 20kHz with 300 watts into 8
ohms, 450 watts into 4 ohms, and 850 watts into 2 ohms. The output stage is capable of greater than 60 amps Into
low-impedance loads. The amplifier employs a discrete dual-differential cascode Class-A front-end followed by a dual
cascode Class-A voltage gain stage which amplify the input signal to the voltage required at the output of the amplifier.
This high-voltage signal drives the high-current triple-Oarlington-follower output stage which amplifies the current by a
factor of about 50,000.
Referring to the accompanying schematic, the input signal passes through network Cl 01, Cl 02, R102, and R103 which
provides a 3dB bandwidth of 0.7Hz to 700kHz to the input of the amplifier. C101 is an extremely high quality capacitor and
serves to protect the amplifier and the speakers connected to it from DC faults ai the output of the preamplifier. WE DO
NOTRECOMMENOTHATC101 BESHORTEOOUT.OiOl.Ol03.QiOS,andOi07form onedifferential/cascode input
stage andO102,0104, Qi06,andQi08formtheother. R106. R109. R112. and R113 provide local feedback to control the
input-stage open-loop gain. Open-loop gain is defined by Rt 06, Ri 10. and Cl 03 on one side, and R107, R111, and Cl 04
on the other. The next voltage-gain stage consists of 0109 and Oi 11 on one side and 0110 and 0112 on the other. DC
bias is set by R12S, 0103 through 0108, R131. R132, and R126. Open-loop gain is defined by R125, R126, R133, and
R134, with R13S through Ri36, Cl 10, and C111 providing high-frequency compensation.
Feedback is provided from the output to the bases of 0105 and 0106 by the network R119, R120, and C105. CIOS
provides a high-frequency rolloff above 80kHz. OiOi. 0102.0i09 through Di 12, Oi 13,0114, R122 through RI 24, and
C106 provide an anti-saturation feedback path to the input stage. For example, when the cascode voltage gain stage
approaches saturation to the positive supply, 0101 beginsto conduct, turning Q113 on. Current flows through D603,the
INSTANTANEOUS DISTORTION ALERT LED. through Dll2 and R123. When the error voltage across D109/D110
becomes great enough, DIOS begins to conduct, forcing the input stage to reduce the drive to the voltage-gain stage.
The input stage is biased by R144 through R154, Q115 through Q119,0501 ,D113 through D115, and D804. 0501 is
turned on by the amplifier bias delay circuitry on the AC input power supply board. A current of about 2mA flows through
the thermal breakers on the heatsinks and into the emitter of 0119. if the heatsinks overheat, the breakers open and the
current flows through D804, the THERMAL PROTECTION LED. instead. When 0119 is carrying the current, D115 is
biased at 2.5V. This creates about 1.8V across R149. 011 6 then sources about 3mA through D114, which develops about
2.2V across it. About 1.6V is developed across. R144 and R146.0117 conducts about 1.3mA through D113 and 0115
conducts about 3mA to the PNP differential input stage. D113 develops about 2.2V across it, causing about 1.6V across
RI 45. 0116 conducts about 3mA to the NPN differential input stage. If the negative 80V supply fails or its fuse opens,
0119 turns off, turning off all the amplifier bias circuitry, lithe positive 80V supply fails or its fuse opens, again 0119 turns
off and the bias circuitry is disabled. Using LEDs for the bias circuitry causes rough temperature compensation of the
amplifier Class-A DC bias current.
Any DC imbalance in the amplifier is corrected by R117. R118, Cl 07. CT 08, and IC101. Any DC error at the amplifier
output is servoed back through IC101 to adjust the DC current through the Input transistors. DC bias is nominally 1.5mA
through 0101, Ql 02, Q105, and 0106. IC101 can modify this by up to 0.3mA to bring the amplifier into balance.
The bias network of Ri 39 through R143,0201 and 0301 forms a temperature-compensated DC-bias voltage to the input
of the triple-Darlington-foliower output stage. Mid- and high-frequency bypassing is provided by C109.
1