Handbook for Sound Engineers

740 Chapter 21

20 Hz–20 kHz, giving it truly excellent waveform
fidelity and square-wave response.
As discussed in detail in Chapter 11, Audio Trans-
formers, audio transformer distortion is quite different
from electronicdistortion in ways that make it unusually
benign. First, transformer distortion is frequency and
level dependent. Significant distortion occurs only at low
frequencies and high signal levels, typically dropping to
under 0.001% above a few hundred hertz. Second, the
distortion is nearly pure third harmonic and is not
accompanied by the high levels of much more irritating
intermodulation distortion that occurs in electronics.

A high degree of RF attenuation, both normal mode
and commonmode, is also inherent in transformers that
contain Faraday shields. For example, in Jensen
designs, common-mode attenuation is typically over
30 dB from 200 kHz–10 MHz. And, as discussed in
Chapter 16, transformers enjoy a great CMRR advan-
tage over most electronically balanced input stages
because they are relatively insensitive to the impedance
imbalances that normally exist in real-world signal
sources. If well designed and properly applied, audio
transformers qualify as true high-fidelity devices. They
are passive, robust, and stable and have significant
advantages, especially in electrically hostile environ-
ments.

21.2.2 Class A Circuitry

Another divisive issue among preamplifier manufactur-
ers involves class A circuitry. Although it has certain
advantages, it is not necessarily inherently superior to
much more widely used class AB designs. Class A
operation occurs when the active device (or devices in
the case of a push-pull output stage) conducts current
during the entire 360° signal cycle. Class AB occurs
when each device conducts for more than 180° but less
than 360°. In class B operation, each device conducts
for exactly 180°. In class C, conduction is less than 180°
and this is generally done only in RF circuits or where
intentional distortion is desired.
Most op-amp output stages operate class AB to
avoid crossover distortion of small signals. Practical
active devices are generally unable to behave linearly
near zero current (cutoff) as is required for low-distor-
tion pure class B operation. A small idling or quiescent
current flows in both devices at zero signal and opera-
tion remains class A (both devices conducting for full
signal cycle) up to some signal level, at which point one
device begins to be cut off for part of the cycle,
producing class AB operation.

For example, in the Jensen-Hardy 990 amplifier

module used in the circuit of Fig. 21-13, this output

stage quiescent current is about 15 mA. Therefore,

amplifier operation is class A until peak output current

(plus or minus) reaches about 15 mA. Peak output

current, of course, depends on peak signal level and

load impedance. For example, the output voltage clips

at about 24 Vpeak, so any load impedance higher than

about 1.6 k: results in class A operation at all times.

Likewise, with a 600: load, operation is class A until

output signal level reaches ±9 Vpeak. Above that peak

level, operation becomes class AB. The “front end”

circuitry of the 990, like most operational amplifiers,

always operates class A unless the output is clipped.

The line between class A and AB operation is very

distinct: operation is no longer class A as soon as

current in any active device (vacuum tube or transistor)

becomes zero. The main advantage of class A circuit

designs is that the curvature of the nonlinearity plot is

likely to be smoother (i.e., free of a sharp discontinuity

at crossover) so that there will be fewer problems

related to negative feedback, slew rate, and gain-band-

width limitations.

21.2.3 Shure SCM268 Four-Channel Mixer

The Shure SCM268 is an example of a compact, simple

mixer with basic features. Notable features include

transformers on balanced inputs and outputs, mic or line

level output, phantom power, and optional pads allow-

ing for balanced line level inputs, Fig. 21-8. A func-

tional block diagram is shown in Fig. 21-9.

21.2.4 Cooper Sound CS 104 Four-Channel ENG

Mixer

The Cooper Sound Systems CS 104 is an example of a

portable, battery-powered mixer with a number of

sophisticated features, Fig. 21-10. Notable features

include stereo mixing, pan pots and channel linking,

Figure 21-8. Shure SCM268 four-channel microphone

mixer. Courtesy Shure Incorporated.