Adcom GFA 565 Service Manual

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SERVICE MANUAL

ADCOM

POWER AMPLIFIER
h » GFA~565

TABLE OF CONTENTS
Introduction ........................... 1
_ Service Parts List ..................... 3
Audio lnpui/Driver PCB ............... 10
, 4 AC Input/Bias Time Delay PCB ....... 1O
; Left Output PCBs ...................... 11
Flight Output PCBs .................... 12
Filter Capacitor PCBs ................. 13
Options PCBs ......................... 13
Performance Data ..................... 14
Specifications .................... . ..... 16
Schematic Diagram ................... 18

n i meow
TECHNICAL same DEPARTIIENT
11 ELKINS mom
.EASI BHUNSWK. NJ 08516
raamous (905) 390-1130

FAX (908) 390-9152

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INTRODUCTION

This service manual is intended to assist trained and qualified technical personnel in verifying the performance of,
adjusting, and repairing the ADCOM GPA-565 power amplifier. The procedures described here are not intended for
persons unfamiliar with the appropriate satety 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 GPA-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-565 UNLESS YOU ARE TECHNICALLY QUALIFIED TO DO SO.

CIRCUIT DESCRIPTION

The ADCOM GPA-565 is a monaural power amplifier rated at <0.02% THD trorn 20Hz to 20kHz with 300 watts into 8
ohms, 450 watts into 4 ohms, and 850 watts into 2 ohms. The output stage is capabie at greater than 60 amps into
low-impedance loads. The amplitier employs a discrete dual-ditterentiat cascode Class/t 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 tripieADarlington-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 0101, (2102, R102, and R103 which
provides a 3dB bandwidth of0.7Hz to 700kHz to the input 01 the amplifier. C101 is an extremely high quality capacitor and
serves to protect the amplifier and the speakers connected to it from DC faults at the outputot the preampiitier. WE DO
NOT RECOMMEND THAT c101 BE SHORTED OUT. ()1 01, 0103, 0105, and 01 071mm one ditterential/cascode input
stage and 01 02, 0104, 0106.and Q1 08 form the other. R1 08, R109. R1 12, and R113 provide local teedbackto control the
input-stage open-loop gain. Open-loop gain is defined bth 06, R1 10, and C103 on one side,and R107, R11 Land C104
on the other. The next voltage-gain stage consists of0109 and Qt 11 on one side and 0110 and 0112 on the other. DC
bias is set by R125, D103 through 0103, R131. R132, and R126. Open-loop gain is defined by R125, R126. R133, and
R134, with R135 through R138, 0110, 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 Ct 05. C105
provides a high-frequency rollotf above BOkHz, 0101, D102, 0109 through 01 12, 01 1 3, 01 14, R1 22 through R124. and
(3106 provide an anti-saturation feedback path to the input stage. For example, when the cascode voltage gain stage
approaches saturationto the positive supply, D101 beginstoconduot,turning01130n.Currenttlowsthrough D803.the
INSTANTANEOUS DISTORTION ALERT LED, through D112 and R123. When the error voltage across D109/D110
becomes great enough, 0109 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, 0115 through 0119,0501,0113 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 O1 1 9. lithe heatsinks overheat. the breakers open and the
current flows through D804. the THERMAL PROTECTION LED, instead. When 0119 is carrying the current, 01 15 is
biased at 2.5V. This creates about 1 .8V across R149.Qt 18 then sources about 3mA through D1 14, which develops about
2.2V across it. About t.6V is developed across, R144 and R146. 0117 conducts about 1 .amA through D113 and 0115
conducts about 3mA to the PNP differential input stage. D113 develops about 2.2V across 11. causing about 1.6V across
R145. 01 16 conducts about SmA to the NPN diiferential input stage. lithe negative 80V supply fails or its luse opens,
01 1 9 turns oii, turning offall the amplifier bias circuitry. lithe positive 80V supply fails or itstuse opens, again Q1 1 9 turns
off and the bias circuitry is disabled. Using LEDs for the bias circuitry causes rough temperature compensation oi the
amplifier Clasth DC bias current.

Any DC imbalance in the amplifier is corrected by R117, R118, C107, C108, and |Ctot. Any DC error at the amplifier
output is servoed back through I61 01 to adiustthe DC current through the inputtransistors. DC bias is nominally 1.5mA
through 0101, 0102, 0105, and 010610101 can modiiy this by up to 0.3mA to bring the amplifier into balance.

The bias network of R1 Bathrough R143, 0201 and 0301tormsatemperature-compensated DC-bias voltageto the input
of the lripie-Darlington-Ioiiower output stage. Mid- and high-trequency bypassing is provided by C109.