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
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
1.8 volts, 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.
FUNCTIONAL BLOCK DIAGRAM
AMPLIFIER CIRCUIT
INPUT
DIFFERENTIAL
CLASS AB
AMPLIFIER
BIAS
REGULATOR
Q4
l
.
PASSIVE
FEEDBACK
OUTPUT
. .
NETWORK
SIGNAL
AMPLIFIER
I
1
NEGATIVE
1
NEGATIVE
OUT PUT
CURRENT
L
D R I V E R
SOURCE
B -
& .
R E L A Y C I R C U I T
F I L T E R
,
, ,
NEGATIVE
DELAY
I
RELAY
I I
SPEAKER
O F F S E T
ETECTOR
T I M E R
DRIVER
R E L A Y
, .
l
15
