Handbook for Sound Engineers

Tubes, Discrete Solid State Devices, and Integrated Circuits 349

12.3.5 Integrated Circuit Preamplifiers

The primary applications of preamplifiers for profes-
sional audio in the post-tape era are for use with micro-
phones. Before the development of monolithic ICs
dedicated to the purpose, vacuum tubes, discrete bipolar
or field-effect transistors,^22 or general-purpose audio
op-amps were used as preamplifiers.^23 Dynamic micro-
phones generally produce very small signal levels and
have low output impedance. Ribbon microphones are
notorious for low output levels. For many audio appli-
cations, significant gain (40–60 dB) is required to bring
these mic level signals up to pro audio levels.
Condenser microphones, powered by phantom power,
external power supplies, or batteries, often produce
higher signal levels requiring less gain.

To avoid adding significant noise to the micro-
phone’s output, professional audio preamplifiers must
have very low input noise. Transformer-coupled
preamps ease the requirement for very low noise ampli-

fication, since they take advantage of the voltage

step-up possible within the input transformer. Early

transformerless, or active, designs required performance

that eluded integration until the early 1980s. Until semi-

conductor process and design improvements permitted

it and the market developed to generate sufficient

demand, most microphone preamplifiers were based on

discrete transistors or discrete transistors augmented

with commercially available op-amps.

Virtually all professional microphones use two signal

lines to produce a balanced output. This allows a pream-

plifier to distinguish the desired differential audio

signal—which appears as a voltage difference between

the two signal lines—from hum and noise

pickup—which appears as a “common-mode” signal

with the same amplitude and polarity on both signal

lines. Common mode rejection quantifies the ability of

the preamplifier to reject common mode interference

while accepting differential signals.

Therefore, one goal of a pro-audio mic preamp is to

amplify differential signals in the presence of

common-mode hum. As well, the preamp should ideally

add no more noise than the thermal noise of the source

impedance—well below the self-noise of the micro-

phone and ambient acoustic noise.

Phantom power is required for many microphones,

especially professional condenser types. This is usually

a +48 Vdc power supply applied to both polarities of the

differential input through 6.8 k: resistors (one for each

input polarity). Dc supply current from the microphone

returns through the ground conductor. Phantom power

appears in common mode essentially equal on both

inputs. The voltage is used to provide power to the

circuitry inside the microphone.

12.3.5.1 transformer input microphone preamplifiers

Many microphone preamplifiers use transformers at

their inputs. Transformers, although costly, provide

voltage gain that can ease the requirements for low

noise in the subsequent amplifier. The transformer’s

voltage gain is determined by the turns ratio of the

secondary versus the primary. This ratio also trans-

forms impedance, making it possible to match a

low-impedance microphone to a high-impedance ampli-

fier without compromising noise performance.

A transformer’s voltage gain is related to its imped-

ance ratio by the following equation:

(12-64)

Figure 12-53. THAT2252 tone burst response. Courtesy

THAT Corporation

Figure 12-54. THAT2252 input versus output. Courtesy

THAT Corporation.

Audio input

Detector output

50 0 +50

+300

300

0

Input–dBr

Output–mV

Gain 20

Zs

Zp

= log -----