Handbook for Sound Engineers

Amplifier Design 719

Fig. 20-22 is a complete though modest amplifier. Q 1
and Q 2 are complementary symmetry monolithic
darlington power transistors. Q 3 is employed to adjust
the forward bias on the output stage. Q 4 is a constant
current source that insures that Q 2 receives adequate
voltage drive under large signal conditions. Most of the
open loop voltage gain is provided by Q 5 while C 1 deter-
mines the dominant pole, which was discussed in
connection with operational amplifiers. R 12 and Z 1 deter-
mine the total current in the matched differential pair Q 6.
The ac voltage gain of the amplifier is set by R 9 and R 10
which determine the feedback fraction above a
frequency of a few hertz. There is complete negative
feedback at dc as brought about by the presence of C 2 in
series with R 10. This in collaboration with Q 6 insures that
the output voltage of the amplifier is zero when no signal
is applied at the input. By monitoring the current in R 1
and R 2 it is possible to provide protection against load
short circuits by means of a relatively simple additional
circuit. If C 2 , R 10 , and R 9 were removed from the circuit
of Fig. 20-22 altogether and further if the right- hand
base of Q 6 were connected through a resistor equal to
R 14 , the resulting circuit would be a power operational
amplifier built from discrete components. The present
input would become the noninverting input while the
right-hand base of Q 6 would be the inverting input.

20.3.1 Protection Mechanisms

Power amplifier protection mechanisms fall roughly

into two categories, the protection of the amplifier

against faults in the load and protection of the load

against faults in the amplifier. The amplifier designer

must, unfortunately, shoulder the burdens of both cate-

gories. The load must be protected against turn-on and

turn-off transients, against dc appearing at the amplifier

output terminals unless that is the intended purpose of

the amplifier, and against unwarranted oscillation in the

amplifier caused by the load if the load itself presents a

reasonable impedance to the amplifier. The amplifier

must be protected against short-circuited or very low-

impedance loads, excessive temperature within the

amplifier, wide variations in ambient temperature,

radio-frequency signals induced in the loudspeaker

lines, radio-frequency signals induced in the input

signal lines, dc on the input signal lines if such is not the

intended use, and other types of reasonable abuse. All

of the items above can be dealt with in practice but to do

so involves an enormous additional expense in design,

manufacture, and maintenance. Inferior as well as less

costly products treat these features minimally if at all.

The most common of all load protection schemes is a

fuse in series with the load. It may be a single fuse,

fusing the overall system, or in a case of a multiway

loudspeaker system, it may be one fuse on each loud-

speaker.

Fuses help to prevent damage due to prolonged over-

load but provide essentially no protection against

damage that may be done by large transients and such.

To minimize this problem, high-speed instrument fuses

such as the Littelfuse 361000 series should be used. Fig.

20-23 shows the fuse size versus loudspeaker power and

impedance ratings.

The load protection mechanism against turn-on,

turn-off transients and against dc usually involves a pair

Figure 20-22. Complete power amplifier.

R 11

R 10 R 5

R 9 R^1

R 2

R 8

R 14 R 12

R 13

R 6

R 7

R 3

R 4

Load

Input

Z 1

C 1

Q 6

Q 1

Q 2

Q 4

Q 3

Q 5 +

+

D 1

D 2

C 2

Figure 20-23. Fuse selector nomograph for loudspeaker

protection. Courtesy Crown.

4 5 6 7 8 9

10

12

14

16

20

25

30

40

5

4

3

2

1.5

1

0.8

0.6

0.5

0.4

0.3

0.2

0.15

0.1

0.08

400

300

200

150

100

80

40

30

20

15

10

8

4

6

3

2

1.5

1

60

Fuse–A

Loudspeaker Z =

7

Example Z = 8 7

Power peak = 37

Answer: Fuse =!

Peak music power at loudspeaker–W