Hafler DH-500 manual Circuit Description, Functional Block Diagram

CIRCUIT DESCRIPTION

The DH-500 amplifier components have been selected to pro- tect against failure, and all parts are operated conservatively to assure unaltered performance and proper operation for many years. The semiconductors have been thoroughly researched and selected for minimum noise and distortion in sustained use, and the printed circuit board associated with the audio signals has been pretested in the circuit to insure that each kit amplifier will meet or exceed all specifications.

The amplifier stages are fully DC coupled, with a dual differ- ential pair for the input, and a fully complementary MOSFET output stage. Its power supply is a full wave bridge with 20,000 mfd capacitors on both the plus “+” and minus “-” supplies to provide excellent filtering and dynamic load stability. Protection circuitry includes two separate power supply fuses for each channel, individual thermal sensing devices for each channel to turn the amplifier off if the heat sinks should reach excessive temperature, a relay to disengage the output if excessive DC offset is detected, and output fuses to protect the speakers, in ad- dition to an AC line fuse.

Transistors Q3, Q4, Q5 and Q6, the input differential pairs, are matched to control the DC offset of the amplifier. T o optimize the input pair parameters, they are driven from constant current sources - Ql and Q2.

An adjustable voltage reference is formed by transistor Q9 and

potentiometer PI. Thus the signal sees the bases of transistors Q12 and Q13 tied together. Pl adjusts the voltage to bias the amplifier for Class AB operation, and therefore determines the quiescent (idle) current in the driver and output stages. Complete circuit stability is assured by the positive temperature coefficient of the power MOSFETs.

The driver and output stages of the DH-500 are of the full com- plementary configuration. Q14, Ql5 and Q16 are N channel MOSFETs in parallel for the positive half of the signal, and Q17, Q18 and Q19 are P channel MOSFETs for the negative signal half.

There is none of the usual volt amp limiting in the DH-500. D7, D8, D9 and D10 prevent the gates of the MOSFETs from being overdriven in the event of abnormally high signal levels. The B +, B- and speaker fuses limit the maximum current in the outputs. The thermal protection, unlike most circuits, does not interrupt the output of the amplifier. If the heat sink temperature rises to 7X, the amplifier is turned off until it cools.

The relay circuit has a 3 section low pass filter which prevents signals above 10 Hz from opening the relay. This circuit allows DC to pass to IClOlA and IClOlB, which sense any offset above

1.8volts, opening the relay to protect the speakers. DlOl, RlO6 and R107 form the reference for the positive offset detector IClOlA. D102, R109 and RllO form the reference for the negative offset detector IClOlB. C104’s charging time is the time delay at turn on, and after any relay actuation.

INPUT

-zz

FUNCTIONAL BLOCK DIAGRAM

AMPLIFIER

CIRCUIT

rmunlvtn

DIFFERENTIAL

0

CLASS AB

BIAS

AMPLIFIER

REGULATOR

93

Q4

= •

r

.

Pig Q9

PASSIVE

4

OUTPUT

c

FEEDBACK

.

.

a

NETWORK

SIGNAL

I

I

4

DIFFERENTlAL

ih

AMPLIFIER

Q5vQ6

’

1

I

1

r

1

L

?

PREDRIVER

NEGATIVE 1

1 NEGATIVE

1

CURRENT

t

,,

L QlO* Qll

L

!

+

D R I V E R

OUT PUT

913

Q171Q11

SOURCE

Qlq

42

B -

&.

R E L A Y C I R C U I T

a, F I L T E R ] k ,, + ~

t

I

r

1

I I

NEGATIVE

1 1 1

DELAY

f

I

RELAY

I

I

SPEAKER

I

I

O F F S E T

1

+

T I M E R

DRIVER

R E L A Y

0 ETECTOR

’

+

r

D

lC[B

,

ICl D

.

Ql

•

15