Handbook for Sound Engineers

716 Chapter 20

(20-25)

Substitution into Eq. 20-23 while invoking the neces-
sary value of Z 0 leads to

or

(20-26)

If C 2 is taken to be 0.05 μF, then Eq. 20-26 requires
R 1 and hence R 2 to be 5 k:. Eq. 20-25 would then
require

which is quite close to a readily available value of
0.068μF. These are all reasonable values; hence, the
example concludes with the circuit of Fig. 20-18.

In fairness it is necessary to state that the solutions

given to the two examples are not unique. In fact, even

more elegant solutions than the ones given are possible

though not as straightforward. These more elegant solu-

tions will no doubt occur to the reader after further study.

20.3 Power Amplifiers

Power amplifiers for professional applications, unlike

those intended for home entertainment use, must

usually be capable of providing a multiplicity of voltage

values at their output terminals. Furthermore, for

reasons of safety and in order to avoid inadvertent

mishaps in wiring or handling, it is often required that

neither side of the output distribution lines be refer-

enced to ground except in a balanced way through a

high impedance in order to provide a static discharge

path. These requirements are usually met by feeding the

distribution lines from an isolated transformer

secondary even though the transformer itself presents a

source of distortion and bandwidth limitation.

Power amplifiers, when operated within their

inherent limitations, are essentially constant-voltage

sources. The sinusoidal rms voltage at the output termi-

nals at rated power that is required in professional appli-

cations is commonly in voice coil values of 25 V,

70.7 V, or, in recent times, 200 V. The loudspeakers or

other loads are, in the case of 25, 70.7, or 200 V lines,

fed from the secondary of a stepdown transformer

which has several primary taps for determining the

actual average sinusoidal power supplied to an indi-

vidual device. When feeding several devices from a

common constant voltage distribution line it is only

necessary to insure that the sum of the power taps to the

individual devices does not exceed the output capa-

bility of the driving amplifier. High values, such as

70.7 V or 200 V, for the constant-voltage distribution

system will minimize the I^2 R loss in the distribution

lines themselves (see Chapter 14). It is an absolute

necessity, however, that the transformer at the amplifier,

when such is employed, and the step-down transformers

20-16t 1

20-16G

Table 20-1. Transfer Functions of Various Circuits of

Figure 20-16 (Continued)

Figure K Transfer function

where

A 0 is the gain at pass band center

Ao

R 2

1

K

---- 3 R 2 R 1 –+ KR 1

=------------------------------------------------

Q is the quality factor

Q

R 2 1

R 1

R----- -+ 2

3 R 2 R 1 –+ KR 1

=--------------------------------------

Zo

1

R 1

R 2

------+

=-------------------R 1 C

Vo

Vi

----- -

S^2

Rv–R

Rv+R

©¹§·---------------

Zo

Q

++©¹§·------S Zo^2

S^2

Zo

Q

++©¹§·------ S Zo^2

–= --------------------------------------------------------------

C 1

4

3

=---C 2

2000 rad/s^1

3 R 12 C 22 4

3

u---

=----------------------------------

1

2 R 1 C 2

----------------=

R 1 C 2 = 250 u 10 6– s

C 1 4

3

=---C 2

4

3

=---u 510 u 8– F

0.066PF=

Figure 20-18. Second-order unity gain Bessel low-pass with

a zero frequency group delay of 500 “s.

k 7 k 7

M

MF

F

+

• Vin Vo