Massive Audio DB8000 Manuel d'utilisateur télécharger pdf (Page 40)

GRN

GRN-rEL

RED

30v

RED

-

L -

0

RED

30V

IIYVAC

*42V- NO

LOAD

39V-FULL LOAD

F2

+42V

2.5AMP

BI

CB

4500PF

GND

4500yF

F

F3

-42V

2.5AMP

B2

Fig.

2.

Positive

and

negative

supply

voltages

are

balanced with respect

to

ground

through

a

60-volt center-tapped

winding. Try

using 2-ampere,

fast

-blow

fuses

for F2

and

F3, If

they

don't

stand

up.

use

the

21/2-

ampere size shown here.

Fig.

3.

Actual

-size photo

of

printed

circuit board. High power

and

high

gain

characteristics make

lead dress criti-

cal. Point -to

-point

wiring

on

a perfor-

ated board

may be used if proper lead

dress

and

parts layout is maintained.

of transistors (Q4 and Q5) which are

used

to direct -drive the two series

-con-

nected

power transistors.

Capacitor C7 performs two functions:

first, it

decouples

the power supply to

remove

ripple

voltage

from

the predriver

and

driver

stages; and second, it

pro-

vides a

bootstrap

voltage

to

increase

the drive voltage to Q4.

Bias voltage

adjustment

for

the com-

plementary

driver stages

is

provided by

diodes

Dl, D2

and

D3 and

by

Bias Con-

trol R10. The

diodes

are connected ther-

mally to the output transistor

heat sinks

to

establish a thermal feedback circuit.

This thermal

feedback

arrangement

sta-

bilizes the quiescent current of the

out-

put stages

at

its

preset

value for

all

case temperatures up

to

100

°C, thus pro-

tecting

the

driver and output transistors.

The

Bias

Control is

adjusted to ob-

tain

20 mA quiescent

current

in

the col-

lector circuit

of

Q6. An ammeter can

be plugged

into

J2 to

measure

this cur-

rent. The forward voltage

drop across

three

diodes

(Dl, D2, D3) and the volt-

age across R13 provide the bias

voltage

necessary to

maintain the output stages

in class AB operation.

The Bias

Con-

trol

permits

adjustment

for component

variations.

Another benefit of the high

-tempera-

44

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