Handbook for Sound Engineers

Preamplifiers and Mixers 735

21.1 Microphone Preamplifier Fundamentals

Microphones are transducers that typically have very
low output signal levels. A voltage gain of 1000 (60 dB)
or more may be required to bring the signals up to stan-
dard line levels, hence the name preamplifier. Amplifi-
ers for such low-level signals are prone to problems
unique to high-gain, low-noise electronic circuits.
Microphone preamplifiers are available as stand-alone
devices or as part of simple mixers or complex record-
ing consoles. In this section, we will limit our discus-
sion to preamplifiers and mixers intended for use with
professional microphones that have balanced,
low-impedance outputs.

21.1.1 The Microphone as a Signal Source

As discussed in Chapter 16, microphones may vary con-
siderably in output impedance, output level or sensitiv-
ity, and self-noise. For professional microphones,
impedance has a rated or nominal value of 150: (U.S.
standard) or 200: (European standard). Dynamic
microphones, like loudspeakers, have an actual imped-
ance that varies with frequency. Note the similarity of
the impedance plot of Fig. 21-2 to that of a loudspeaker.
A single figure representing the impedance of such
devices is usually taken as the first minimum that occurs
after the first maximum as frequency is increased from a
low-frequency limit. The first maximum is usually cone
or diaphragm resonance. For microphones, this imped-
ance would be measured between the signal the output
pins (2 and 3 for an XLR) and is variously referred to as
output, source, internal, signal, or differential imped-
ance. Such microphones are broadly classified as float-
ing balanced sources. Floating refers to the fact that the
common-mode impedances—i.e., those from output
(pins 2 and 3) to case and shield (pin 1)—are very much
higher than the signal or differential impedance. As
emphasized in Chapter 32, Grounding and Interfacing,
balanced refers to the matching of these common-mode
impedances.

21.1.1.1 Electrical Model of the Microphone

Since the Shure SM57 dynamic microphone is so popu-
lar, it will be used as an example. Fig. 21-1 shows its
internal schematic, the electrical equivalent circuit of the
capsule and transformer, and the combined equivalent
circuit. The equivalent circuits do not model the dia-
phragm resonance at approximately 150 Hz. Note the
pair of 17 pF capacitances to the case. These determine
the common-mode output impedances that play a role in

noise rejection or CMRR when the microphone is con-

nected to a preamplifier and cable. The actual measured

output impedance of an SM57 is shown in Fig. 21-2. The

Shure data sheet accurately specifies the actual imped-

ance as 310:. The equivalent circuit of Fig. 21-1 mod-

els the impedance behavior above 1 kHz as well as the

equivalent noise resistance. Condenser microphones

generally have both lower output impedances and less

variation with frequency than dynamic types.

21.1.1.2 Interactions with Preamplifier and Cable

A microphone preamplifier is normally designed to

recover as much of the available microphone output

voltage as possible. Since the noise floor of the pre

amplifier is nearly constant, signal-to-noise perfor-

mance is improved by making its input voltage as large

as possible. It is very important to understand that the

fraction of available microphone voltage actually deliv-

ered to the preamplifier depends on both the output

impedance of the microphone and the input impedance

of the preamplifier.

As shown in Fig. 21-3, these two impedances effec-

tively form a voltage divider. The voltage lost in the

output impedance ZS of the microphone depends on the

input impedance ZL of the preamp. Loading loss,

usually expressed in dB, compares the output voltage

with some specified load to the output voltage under

open circuit or unloaded conditions. For example, a

150 : impedance (actual) microphone will deliver 91%

of its unloaded voltage when loaded by a preamplifier

having a 1.5 k: input impedance. The loading loss is

Figure 21-1. Shure SM57 schematic and equivalent circuits.

Capsule

3

2

TransformerCase 1

Capsule Transformer Parasitics

12

1.5 7 24 7

(^7) 2.75
1:4.5
mH^17 pF
17 pF
2
1
3
300
2
3
6 mH
7
157 MH