Handbook for Sound Engineers

Audio Transformers 281

turns of heavier wire is needed for the secondary.
Compare this to the total of 240 turns of heavier wire
required in the transformer.
A step-down auto-transformer is shown in the
bottom diagram of Fig. 11-12. Operation is similar
except that the secondary is connected so that its instan-
taneous polarity subtracts from or bucks the input
voltage. For example, we could step down U.S. 120 Vac
power to Japanese 100 Vac power by configuring a
100 V to 20 V step-down transformer as an auto-trans-
former. Thus, a 100 W load can be driven using only a
20 W rated transformer.
The windings of low level audio transformers may
consist of hundreds or even many thousands of turns of
wire, sometimes as small as #46 gauge, whose 0.0015
inch diameter is comparable to a human hair. As a

result, each winding may have a dc resistance as high as

several thousand ohms. Transformer primary and

secondary winding resistances are represented by RP

and RS, respectively, in Fig. 11-8.

11.1.2.3 Leakage Inductance and Winding Techniques

In an ideal transformer, since all flux generated by the

primary is linked to the secondary, a short circuit on the

secondary would be reflected to the primary as a short

circuit. However, in real transformers, the unlinked flux

causes a residual or leakage inductance that can be mea-

sured at either winding. Therefore, the secondary would

appear to have residual inductance if the primary were

shorted and vice-versa. The leakage inductance is

shown as LL in the model of Fig. 11-13. Note that leak-

age inductance is reflected from one winding to another

as the square of turns ratio, just as other impedances are.

The degree of flux coupling between primary and

secondary windings depends on the physical spacing

between them and how they are placed with respect to

each other. The lowest leakage inductance is achieved

by winding the coils on a common axis and as close as

possible to each other. The ultimate form of this tech-

nique is called multi-filar winding where multiple wires

are wound simultaneously as if they were a single

strand. For example, if two windings—i.e. primary and

secondary—are wound as one, the transformer is said to

be bi-filar wound. Note in the cross-section view of Fig.

11-14 how the primary and secondary windings are

side-by-side throughout the entire winding. Another

technique to reduce leakage inductance is to use

layering, a technique in which portions or sections of the

primary and/or secondary are wound in sequence over

each other to interleave them. For example, Fig. 11-15

shows the cross-section of a three-layer transformer

where half the primary is wound, then the secondary,

followed by the other half of the primary. This results in

considerably less leakage inductance than just a

secondary over primary two-layer design. Leakage

Figure 11-12. Auto-transformers employ a buck/boost
principle.

EP

EP

EP

IP

IP IS RL

IP

IS

IS

RL

RL

Figure 11-13. Transformer high frequency parasitic

elements.

CW

RG RP IDEAL LL RS

CS SEC

RL

CL

CW

XFMR

RC

PRI CP^1 N