Handbook for Sound Engineers

Preamplifiers and Mixers 741

transformers on main inputs and outputs, built-in stereo

limiter, input overload indicators, selectable high-pass

filters, prefade listen, tape monitor, and built-in tone and

slate functions. Fig. 21-12 is its functional block

diagram.

21.2.5 Jensen-Hardy Twin-Servo® 990 Microphone
Preamplifier

The Jensen-Hardy Twin-Servo® 990 Microphone Pre-
amplifier is an example of a high-performance design,
Fig. 21-11. It features patented discrete component 990
amplifier modules that combine low-input noise,
high-output voltage and current, low distortion, and
high gain-bandwidth performance that is unavailable
with integrated circuits. It uses two cascaded variable
gain stages per channel to maintain high bandwidth and
low distortion overall. Unlike most designs, this topol-
ogy also keeps EIN very low at the lowest gain settings.
Extended low-frequency response is preserved by using
dc servo feedback circuitry to eliminate coupling capac-
itors and their attendant problems, Fig. 21-13.

21.3 21.3 Automatic Microphone Mixers

Automatic microphone mixers, also known as voice-acti-
vated mixers or sound-activated mixers, have become a

necessary part of sound systems designed for speech.

All automatic microphone mixers have a fundamental

function: to attenuate (reduce in level) any microphone

that is not being spoken into by a talker, and conversely,

to rapidly activate any microphone that is being spoken

into by a talker. An automatic microphone mixer should

Figure 21-9. Block diagram of a Shure SCM268.

Option50 dB

Mic 1 Pad

aux

Mic

2

aux

Mic

3

aux

Mic

4

aux

Aux

Option50 dB

Pad

Option50 dB

Pad

Option50 dB

Pad

Option80 Hz

HPF

Option80 Hz

HPF

Option80 Hz

HPF

Option80 Hz

HPF

XFMR

XFMR

XFMR

XFMR

Phantom

power

Gain

1

2

3

4

AUX

Gain

Amp

+12

Amp

Amp

Amp

Amp

Amp

Amp

Amp

Amp

Gain

Gain

Gain

MASTER

Gain

Amp Buf

XFMR

Line

Mic

Mic/Line

12 dBpad Aux

LED

peak

level

meter

Figure 21-10. Cooper Sound CS 104 battery-powered

portable ENG/EFP four channel audio mixer.

Figure 21-11. Jensen Twin-Servo® 990 four channel micro-

phone preamplifier. Courtesy Jensen Transformers, Inc.