350 Chapter 12
where,
Gain is the voltage gain in dB of the transformer,
Zp is the primary transformer impedance in ohms,
Zs is the secondary transformer impedance in ohms.
A properly designed transformer with a 150:
primary and 15 k: secondary produces 20 dB of free
voltage gain without adding noise. Well-made trans-
formers also provide high common-mode rejection,
which helps avoid hum and noise pickup. This is espe-
cially important with the low output voltages and long
cable runs common with professional microphones. In
addition, transformers provide galvanic isolation by
electrically insulating the primary circuit from the
secondary while allowing signal to pass. While usually
unnecessary in microphone applications, this provides a
true ground lift, which can eliminate ground loops in
certain difficult circumstances.
Transformer isolation is also useful when feeding
phantom power (a +48 Vdc current-limited voltage to
power internal circuitry in the microphone) down the
mic cable from the preamp input terminals. Phantom
power may be connected through a center tap on the
primary to energize the entire primary to +48 Vdc, or
supplied through resistors (usually 6.8 k:) to each end
of the primary of the transformer. (The latter connection
avoids dc currents in the coils, which can lead to prema-
ture saturation of the core magnetics.) The galvanic
isolation of the transformer avoids any possibility of the
48 Vdc signal from reaching the secondary windings.
12.3.5.2 Active Microphone Preamplifiers Eliminate
Input Transformers
As is common in electronic design, transformers do
have drawbacks. Perhaps the most prominent one is
cost: a Jensen Transformer, Inc. JT-115K-E costs
approximately $75 US or $3.75 per dB of gain.^24 From
the point of view of signal, transformers add distortion
due to core saturation. Transformer distortion has a
unique sonic signature that is considered an asset or a
liability—depending on the transformer and whom you
ask. Transformers also limit frequency response at both
ends of the audio spectrum. Furthermore, they are
susceptible to picking up hum from stray electromag-
netic fields.
Well-designed active transformerless preamplifiers
can avoid these problems, lowering cost, reducing
distortion, and increasing bandwidth. However, trans-
formerless designs require far better noise performance
from the active circuitry than transformer-based
preamps do. Active mic preamps usually require capaci-
tors (and other protection devices) to block potentially
damaging effects of phantom power.^2512.3.5.3 The Evolution of Active Microphone
Preamplifier ICsActive balanced-input microphone preamplifier ICs
were not developed until the early 1980s. Early IC
fabrication processes did not permit high-quality
low-noise devices, and semiconductor makers were
uncertain of the demand for such products.
Active transformerless microphone preamplifiers
must have fully differential inputs because they inter-
face to balanced microphones. The amplifiers described
here, both discrete and IC, use a current feedback CFB
topology with feedback returned to one (or both) of the
differential input transistor pair’s emitters. Among its
many attributes, current feedback permits differential
gain to be set by a single resistor.
Current feedback amplifiers have a history rooted in
instrumentation amplifiers. The challenges of ampli-
fying low-level instrumentation signals are very similar
to microphones. The current feedback instrumentation
amplifier topology, known at least since Demrow’s
1968 paper,^26 was integrated as early as 1982 as the
Analog Devices AD524 developed by Scott Wurcer.^27 A
simplified diagram of the AD524 is shown in Fig.
12-55. Although the AD524 was not designed as an
audio preamp, the topology it used later became a de
facto standard for IC microphone preamps. Demrow
and Wurcer both used a bias scheme and fully balanced
topology in which they wrapped op-amps around each
of the two input transistors to provide both ac and dc
feedback. Gain is set by a single resistor connected
between the emitters (shown as 40:, 404:and
4.44 k:), and feedback is provided by two resistors (R 56
and R 57 ). The input stage is fully symmetrical and
followed by a precision differential amplifier to convert
the balanced output to single ended. Wurcer’s AD524
required laser-trimmed thin film resistors with matching
to 0.01% for an 80 dB common mode rejection ratio at
unity gain.
Audio manufacturers, using variations on current
feedback and the Demrow/Wurcer instrumentation amp,
produced microphone preamps based on discrete
low-noise transistor front ends as early as 1978; an
example is the Harrison PC1041 module.^28 In
December of 1984, Graeme Cohen also published his
discrete transistor topology; it was remarkably similar
to the work of Demrow, Wurcer, and the Harrison
preamps.^29