Handbook for Sound Engineers

776 Chapter 22

(22-42)

where,

,

The value of K is taken from Table 22-1.

Any U pad may be balanced to ground by connecting
a ground to the electrical center of the shunt resistor.

22.2.7 Ladder Attenuators

Ladder-type pads, Fig. 22-17, are so named because
they look like a ladder laying on its side. The ladder pad
is actually a group of pi attenuators in tandem, R 2 being
common to each section. Because of resistor R 4 , this
type of attenuator has a fixed 6 dB loss, exclusive of the
attenuator setting, which must be taken into account
when designing a ladder attenuator. The ladder attenu-
ator does not have a constant input and output imped-
ance throughout its range of attenuation. However it
does reflect a stable impedance into its source.

Ladder potentiometers for mixercontrol use may be
obtained in two types of construction—slide-wire and
contact types.
For mixers, the slide-wire type control is generally
employed because it permits a smooth, even attenuation
over a wide range. The contact type, although not quite
as smooth in operation as the slide-wire, has only one
row of contacts, which reduces the noise and mainte-
nance.
Ladder networks may also be designed for balanced
operation. This is accomplished by connecting two
unbalanced networks side by side, Fig. 22-18. The
circuit elements are not divided in the same manner as
for other types balanced networks. If an unbalanced
ladder network is compared with a balanced ladder
network, resistors R 1 are divided by two, resistors R 4
are also divided by two, and at the output R 2 is now

twice the value for the unbalanced configuration.

Resistor R 3 remains at its original value on each side of

ground.

The equations used to calculate the resistor values

are:

(22-43)

(22-44)

(22-45)

(22-46)

where,

R 1 is the series resistance in ohms,

R 2 is the shunt resistance in ohms,

R 3 is the input shunt resistor in ohms,

R 4 is the series resistance in the contact arm circuit in

ohms,

The values of K and K^2 are taken from Table 22-1.

The value of K is dependent on the loss per step, not

the total loss.

The noise level for a ladder attenuator is on the order

of –120 dB, and as the attenuation increases, the SNR

increases. This type of attenuator will show impedance

variations at both the input and output and between

steps. However, when used in a combining network

with the proper building-out resistors, these variations

are of little consequence. A typical impedance curve is

shown in Fig. 22-19.

22.2.8 Simple Volume and Loudness Controls

A simple volume control consists of a potentiometer

with the two ends connected to the source and the wiper

Figure 22-17. Unbalanced ladder attenuator with five fixed
steps of loss.

R 2

Z 1

S

----- K

KS 1–

= ©¹§·----------------

S is

Z 1

Z 2

-----

R 1

Input Output

p p

   

R 1 R 1 R 1 R 1

R 3 R^2 R^2 R^2

R 2

R 2

R 4

Figure 22-18. Balanced ladder attenuator.

R 1

2

R 1

2

R 1

R 24 2

Input

p

p R^3 R 2 R 2 R 2 R (^2) R^2
× 2Output
R 4
2
R 1
2
R 1
2
R 1
2
R 1
2
R 1
2
R 1
2
R 1
2
p
p
R 1
K^2 1–
2 K
=--------------- Z
R 2 =Ze
R 3
R 2 Z
R 2 +Z
=---------------
R 4 Z
2
---=