Handbook for Sound Engineers

1212 Chapter 32

help. In stubborn cases, a 0.1μF or a 0.22μF capacitor
directly across the output terminals of the amplifier may
also be required.

32.6.3.5 Signal Quality

Audio transformer design involves a set of complex
tradeoffs. The vast majority of available audio trans-
formers, even when used as directed, fall short of
professional performance levels. As Cal Perkins once
wrote, “With transformers, you get what you pay for.
Cheap transformers create a host of interface problems,
most of which are clearly audible.”^45

The frequency response of a high-quality audio trans-
former is typically ruler flat, ±0.1 dB from 20 Hz to
20 kHz and –3 dB at 0.5 Hz and 100 kHz. The extended
low-frequency response is necessary to achieve low
phase distortion.^46 The high-frequency response is
tailored to fall gradually, following a Bessel function.
This, by definition, eliminates overshoot on square
waves and high-frequency response peaking. Dramatic
improvements in sonic clarity due to the Bessel filter
action are often reported by Jensen customers who add
transformers at power amplifier inputs. On the other
hand, cheap transformers often have huge ultrasonic
peaks in their response that are known to excite particu-
larly ugly intermodulation distortions in even the finest
downstream power amplifiers.^47

Accurate time domain performance, sometimes
called transient response, requires low phase distortion
to preserve musical timbre and maintain accurate stereo
imaging. Phase distortion not only alters sonic quality, it
can also have serious system head room effects. Even
though it may have a flat frequency response, a device
having high phase distortion can increase peak signal
amplitudes up to 15 dB. Phase distortion should never
be confused with phase shift. Linear phase shift with
frequency is simply a benign time delay: only devia-
tions from linear phase or DLP create true phase distor-
tion.^48 This DLP in a high-quality audio transformer is
typically under 2° across the entire audio spectrum.

Harmonic and intermodulation distortion in audio

transformers is unusually benign in character and cannot

fairly be compared to electronic distortion. By their

nature, transformers produce the most distortion when

driven at high levels at very low frequencies, where the

major distortion product is third harmonic. Transformer

distortion mechanisms are frequency selective in a way

that amplifiers, for example, are not. Electronic nonlin-

earities tend to produce harmonic distortions that are

constant with frequency while high-quality transformer

harmonic distortions drop to well under 0.001% at

frequencies over a few hundred Hz. Transformers also

tend to have remarkably low intermodulation distortion

or IMD, to which the ear is particularly sensitive.

Compared to an amplifier of comparable low-frequency

harmonic distortion, a transformer typically has only a

tenth the IMD. While cheap audio transformers use steel

cores producing 1% low-frequency harmonic distortion

at any signal level, high-quality transformers use cores

of special nickel-iron-molybdenum alloys for vanish-

ingly low distortion.

Of course, noise rejection or CMRR is often the

most important property of a ground isolator. As

discussed in Section 32.6.3.1 and Chapter 11, a trans-

former requires an internal Faraday shield (not a

magnetic or case shield) to maximize CMRR. Most

commercial isolators or hum eliminators consist of tiny

imported telephone-grade transformers that do not

contain such a shield. Beware of products with vague or

nonexistent specs! For example, distortion described as

under 0.1% is meaningless because frequency, signal

level, and source impedance are not specified. The most

common problems with inexpensive isolators are

marginal noise reduction, loss of deep bass, bass distor-

tion, and poor transient response. Of course, ad copy

and specifications of these transformers will put on their

best face, withholding the ugly truth! However, isolators

using well-designed and properly applied audio trans-

formers qualify as true high-fidelity devices. They are

passive, stable, reliable, and require neither trimming,

tweaking, nor excuses.

32.6.3.6 Tips for Balanced Interfaces

Be sure all balanced line pairs are twisted. Twisting

is what makes a balanced line immune to interference

from magnetic fields. This is especially important in

low-level microphone cabling. Wiring at terminal or

punch-down blocks and XLR connectors is vulnerable

because the twisting is opened up, effectively creating a

magnetic pickup loop. In very hostile environments,

consider starquad cable because it has less susceptibility

Figure 32-56. RF interference filter for solid-state light
dimmer.

L = 68 MH, 5 A, 0.054 7

J. W. Miller 5707

C = 0.1 MF, 250 Vac

Panasonic ECQ-E2A104MW

Line

C

L

Dimmer Load