Adcom GFA 585 Service Manual

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SERVICE MANUAL

POWER AMPLIFIER
' GFAm585

TABLE OF CONTENTS
Introduction ........................... 1
Test Procedures ...................... 2
- Service Parts List ..................... 3
Typical Performance Data ............. 9
Specifications ......................... 1 1
3 Audio Input/Driver PCBs .............. 12
AC Input/Bias Time Delay PCB ....... 13
Fitter Capacitor PCBs ................. 13
Optional Fan Motor PCB .............. 13
Output PCBs .......................... 14
Schematic Diagram ................... 15
DOOM
rEcnchi. same: DEPAHTIIENT
A 11 Emma ROAD
EAST BRUNSWICK, m onus
TELEPHONE (my 390-1130

FAX (908) 390-91 52

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INTRODUCTION

This service manual is intended to assist trained and qualified technical personnel in verifying the perlormance of,
adjusting, and repairing the ADCOM GFA~585 power amplifier. The procedures described here are not intended for
persons unfamiliar with the appropriate safety and test procedures.

A WARNING A

THERE ARE POTENTIALLY LETHAL VOLTAGES WITHIN THE GPA-585 AMPLIFIER WHICH WILL BE
ACCESSIBLE ONCE ITS TOP COVER IS REMOVED. DO NOT ATTEMPT FAMILIARIZATION WITH.
INSPECTION OF. 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~585 UNLESS YOU ARE TECHNICALLY QUALIFIED TO DO SO.

CIRCUIT DESCRIPTION

The ADCOM GPA-585 is a stereo poweramplifier rated at 0.02% THDfrom 20Hz to 20kHz with 250 watts into 8 ohms, and
400 watts into 4 ohms. The output stage is capabie of greater than 60 amps into low-impedance loads. The amplifier
employs a discrete dual-differential cascade Class-A front-end followed by a dual-cascade Ciass-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-Dariington-follower output stage which amplifies the current by a factor of about 50.000.

Referring to the accompanying schematic, the input signal passesthrough network 0101, C102,R102. and R103 which
provide a 3dB bandwidth at 0.7Hz to 700kHzto the input of the amplifier. 0101 is an extremely high quality capacitor and
serves to protect the amplifier and the speakers connected to it from DC faults at the output ofthe preamplifier. WE DO
NOT RECOMMEND THAT 0101 BE SHORTEDOUT. 0101 . 01 02. O1 03. and Q1 04 form one differentiallcascode input
stage and 0109.01 10, Q1 1 1, and O1 12torm the other, R106. R108. R141. and R144 provide local feedback tocontrol the
input-stage open-loop gain. Open-loop gain is defined by R105, R107. and C103 on one side. and R142, R143.and 0117
on the other. The next voltage-gain stage consists of Q1 05 and 0106 on one side and 0116 and 0119 on the other. DC
bias is set by Rita, D105. 0106, D108, DI10,D118. D119. R123. R130. and R210. Open-loop gain is defined by R118.
R124, R131, and R210, with R120. R135. R313, R413, 0301, and C403 providing high-Iroquency compensation.

Feedback is provided from the output to the bases 010104 and 0112 by the network R112. R114. and C106. C106
provides a high-trequency rolloft above SOkHzt 0101 through D104, 0107, D1 09,0120,0121,0107.Q108.R113.R115.
R1 17, R128. R129 and C107 provide an anti-saturation feedback path to the input stage. For example, when the cascade
voltage-gain stage approaches saturation to the positive supply. D107 begins to conducl,turning 0107 on. Current flows
through D802. the lNSTANTANEOUS DISTORTION ALERT LED. through D104and R1 1 7. When the error voltage across
D101 /D102 becomes great enough, 01 01 begins to conduct. forcing the input stage to reduce the drive to the voltage-
galn stage.

The input stage is biased by R146 through R149. R201 through R207, 01 1 3 through 0117. 0502, D1 1 5 through D117,
and 0801 . 0502 is turned on by the amplifier bias-delay circuitryon the power supply board. A current of about 2mA flows
through the thermal breakers on the heatsinks and into the emitter at 0117 and 0167 (on the other channel). If the
heatsinks overheat, the breakers open and the current1lowsthrough 0801,the THERMAL PROTECTION LED instead.
When 0117 is carrying the current, Di 17 is biased at 2.5V. This creates about 1.8V across R202. 0116 then sources
about 2mA to D115. which develops about 2.2V across it. About 1.6V is developed across R146 and R148. 0115
conducts about 1 .3mA to D116 and 0113 conducts about 3mA to the PNP difterential input stage. D1 16 develops about
2.2V across it. causing about 1.6V across R147. 0114 conducts about 3mA to the NPN differentiat input stage. It the
negative 80V supply fails or its fuse opens, 01 17 turns off, turning off all the amplifier bias circuitry. If the positive 80V
supply tails or its luse opens. again 01 17 turns 011 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 R110. R111. C104. C105, and l0101.Any DC error at the amplifier
output is servoed back through lCt01 to adjust the DC current through the input transistors. DC bias is nominally 1 .SmA
through 0103. 0104. 0109. and @112. IC101 can modify this by up to 0.3mA to bring the amplifier into balance.

The bias network ofR1 22. R125, R126, R127.R132.Q1 20. and Q401torm a temperature-compensated DC-bias voltage
to the input of the triple-Dariington-tollower output stage. Mid» and high-irequency bypassrng is provided by C111.

R134 and 0113 provide a load for the amplifier at high frequencies. stabilizing the amplifier under varying load
conditions. 0301 and 0401 provide a high current return to the power supply for backlash current from the load.