Handbook for Sound Engineers

Attenuators 773

(22-24)

where,
R 1 is the terminating resistor in ohms,
N is the number of branch circuits,
Z is the circuit impedance in ohms.

The insertion loss may be calculated:

(22-25)

where,

N is the number of branch circuits.

A series configuration can only be used in an

ungrounded circuit. The insertion loss of a combining

network may be avoided by the use of an active

combining network (see Sections 22.2.15 and 22.2.16).

22.2.3 T Attenuators

A T-type attenuator is an attenuator network consisting

of three resistors connected in the form of a T, Fig. 22-1.

The network may be designed to supply an impedance

match between circuits of equal or unequal impedance.

When designed for use between circuits of unequal

impedance, it is often referred to as a taper pad.

If a T pad is to work between equal impedances, the

resistor values will be

(22-26)

(22-27)

where,

Z is the input and output impedance in ohms,

R 1 and R 2 are the series resistors in ohms,

R 3 is the shunt arm in ohms,

The values of d and f are taken from Table 22-1.

A T type attenuator may be designed for any value of

loss if designed to operate between equal impedances.

The resistors for a T pad of unequal impedances are

calculated with the following equations:

(22-28)

(22-29)

(22-30)

where,

Z 1 is the larger of the two impedances.

The values of f and h are taken from Table 22-1.

Thus, for a network to match 600: to a circuit of

250 : with a loss of 20 dB, the resistor values are

A balanced T pad is called an H pad. The pad is first

calculated as an unbalanced T configuration. The series

resistance elements are then divided and one-half

connected in each side of the line, Fig. 22-2. The shunt

resistor remains the same value as for the unbalanced

configuration. A tap is placed at the exact electrical

center of the shunt resistor for connection to ground.

The average noise level for a T pad is 100 dB and

constant. Therefore, the signal-to-noise level varies with

the amount of attenuation.

22.2.4 Bridged T Attenuators

A bridged T pad is an attenuator network containing

four resistive elements, Fig. 22-11. The resistors are

equal in value to the line impedance; therefore, they

Figure 22-10. A combining or dividing network for

matching a single circuit to three circuits.

R 1

R 1

R 1

R 1

Z 1

Z 1

Z 1

Z 1

RB

RB

RB RB

Z 1

Z 1

Z 1

SourceZ 1

Load

Load

Load

A. Combining or dividing network for matching

a single circuit to three others.

B. Series combining network for combining

one circuit to three others.

R 1 N 1–

N 1+

= -------------Z

dBloss= 20 log N 1–

R 1 =R 2

=Zd

R 3 = 2 Zf

R 1 =Z 1 h– 2 Z 1 Z 2 f

R 2 =Z 2 h– 2 Z 1 Z 2 f

R 3 = 2 Z 1 Z 2 f

R 1 =600 1.0202 – 2 150 000 0.10101

=533.88 :

R 2 =250 1.0202 – 2 150 000 0.10101

=176.81 :

R 3 =2 150 000 0.10101

78.7 :.=