Handbook for Sound Engineers

284 Chapter 11

tance, were zero, the voltage source—by definition zero
impedance—would effectively short out RC, resulting in
zero distortion! But in a real transformer design there is
a fixed relationship between signal level, distortion, and
source impedance. Since distortion is also a function of
magnetic flux density, which increases as frequency
decreases, a maximum operating level specification
must also specify a frequency. The specified maximum
operating level, maximum allowable distortion at a
specified low frequency, and maximum allowable
source impedance will usually dictate the type of core
material that must be used and its physical size. And, of
course, cost plays a role, too.

The most commonly used audio transformer core
materials are M 6 steel (a steel alloy containing 6%
silicon) and 49% nickel or 84% nickel (alloys
containing 49% or 84% nickel plus iron and molyb-
denum). Nickel alloys are substantially more expensive
than steel. Fig. 11-17 shows how the choice of core
material affects low-frequency distortion as signal level
changes. The increased distortion at low levels is due to
magnetic hysteresis and at high levels is due to
magnetic saturation. Fig. 11-18 shows how distortion
decreases rapidly with increasing frequency. Because of
differences in their hysteresis distortion, the falloff is
most rapid for the 84% nickel and least rapid for the
steel. Fig. 11-19 shows how distortion is strongly
affected by the impedance of the driving source. The
plots begin at 40ȍ because that is the resistance of the
primary winding. Therefore, maximum operating levels
predicated on higher frequencies, higher distortion, and
lower source impedance will always be higher than
those predicated on lower frequencies, lower distortion,
and lower source impedance.
As background, it should be said that THD, or total
harmonic distortion, is a remarkably inadequate way to
describe the perceived awfulness of distortion. Distor-
tion consisting of low-order harmonics, 2nd or 3rd for
example, is dramatically less audible than that
consisting of high order harmonics, 7th or 13th for
example. Consider that, at very low frequencies, even
the finest loudspeakers routinely exhibit harmonic
distortion in the range of several percent at normal
listening levels. Simple distortion tests whose results
correlate well with the human auditory experience
simply don’t exist. Clearly, such perceptions are far too
complex to quantify with a single figure.

One type of distortion that is particularly audible is
intermodulation or IM distortion. Test signals generally
combine a large low-frequency signal with a smaller
high-frequency signal and measure how much the
amplitude of the high frequency is modulated by the

lower frequency. Such intermodulation creates tones at

new, nonharmonic frequencies. The classic SMPTE

(Society of Motion Picture and Television Engineers)

Figure 11-17. Measured THD at 20 Hz and 40: source

versus signal level for three types of core materials.

Figure 11-18. Measured THD at 0 dBu and 40: source

versus frequency for the cores of Figure 11-17.

Figure 11-19. Measured THD at 0 dBu and 20 Hz versus

source impedance for the cores of Figs. 11-17 and 11-18.

1.0%

0.1%

0.01%

0.001%

49% Nickel

84% Nickel

M6 Steel

50 

40 

30 

20 10 0 +10 +20 +30

Signal level—dBu

THD

1.0%

0.1%

0.01%

0.001%20 Hz 200 Hz 2 kHz 20 kHz

M6 STEEL

49% NICKEL

84% NICKEL

Frequency—Hz

THD

10%

1.0%

0.1%

0.001%

0.001%

10 100 1 k 10 k

M6 Steel

49% Nickel

84% Nickel

THD

Source impedance—ohms