Handbook for Sound Engineers

Audio Transformers 289

ments in signal-to-noise ratio are not solely due to this
gain. Because most amplifying devices generate current
noise as well as voltage noise at their inputs, their noise
performance will suffer when turns ratio is not the opti-
mum for that particular amplifier (see 21.1.2.3 Micro-
phone Preamp Noise). Noise figure measures, in dB,
how much the output signal-to-noise ratio of a system is
degraded by a given system component. All resis-
tances, including the winding resistances of transform-
ers, generate thermal noise. Therefore, the noise figure
of a transformer indicates the increase in thermal noise
or hiss when it replaces an ideal noiseless transformer
having the same turns ratio—i.e., voltage gain. The

noise figure of a transformer is calculated as shown in

Fig. 11-28.

11.1.3.7 Basic Classification by Application

Many aspects of transformer performance, such as level

handling, distortion, and bandwidth, depend critically

on the impedance of the driving source and, in most

cases, the resistance and capacitance of the load. These

impedances play such an important role that they essen-

tially classify audio transformers into two basic types.

Most simply stated, output transformers are used when

load impedances are low, as in line drivers, while input

transformers are used when load impedances are high,

as in line receivers. The load for a line driver is not just

the high-impedance equipment input it drives—it also

includes the cable capacitance, whose impedance can

become quite low at 20 kHz. The conflicting technical

requirements for output and input types make their

Figure 11-27. Multiple loads are effectively paralleled.

600 7

300 7

600 7

1:1:1

Figure 11-28. Finding the noise figure of a transformer.

Turns ratio = 1:10

JT - 115k E

Mic R^1

E

Red

Brn

Blk

Yel

Org

Eout

R 2 150 7

150 k

RP = 19.7 7 RS = 2465 7

Example transformer circuit

7

Real Transformer

RP

1970

RS

2465

Ideal Transformer

R 1

15 k

E

Redrawn circuit with all impedances

reflected to secondary

150 k 7

Eout

Whi

150 7

7 77

(0) (0)

R 1 = 150 7 × 10^2 = 15 k 7

RP = 19.7 7× 10^2 7 = 1970 7

RS = Secondary DCR = 2465 7

R 2 = 150 k 7 (load)

The transformer noise figure is calculated by comparing

a real transformer with its winding resistances to an

ideal transformer with no winding resistances.

First, transform all impedances to the secondary as

shown to the left.

There are two components to the calculation.

1. The additional noise due to the increased
   output impedance.

REAL Zout 150 k × (15 k + 1970 + 2465)

150 k + 15 k + 1970 + 2465 ^ 17.205 k^7

IDEAL Zout 1150 k + 15 k

50 k × 15 k

= 13.636 k 7

NF = 20Log 17,206 (REAL)

13,636 (IDEAL)

= 1.01 dB

2. The decrease in signal level at the output
   due to the increased series losses.

IDEAL Eout 150 k

150 k + 15 k

= 0.909

REALout

150 k

150 k + 15 k + 1970 + 2465

= 0.885

NF = 20Log

0.909 (IDEAL)

0.885 (REAL)

= 0.232 dB

3. Total NF = 1.01 dB + 0.232 dB = 1.23 dB

R 2

Primary side impedances get multiplied

by the square of the turns ratio.