Handbook for Sound Engineers

780 Chapter 22

22.2.15 Mixers

A mixer is a device used to mix two or more signals into

one composite signal. Mixers may be adjustable or

nonadjustable and either active or passive.

A passive mixer uses only passive devices (i.e.,

resistors and potentiometers), Fig. 22-27.

The main disadvantage of passive mixing is that an

amplifier is required after mixing to boost the gain back

to the level at the input of the mixer. As the attenuator

controls are lowered, the signal on the mixing buzz is

reduced; however, the mixing buzz noise remains the

same, so the SNR is reduced, causing more apparent

noise at low levels where high signal-to-noise is most

important. This can be seen in the analysis of Fig.

22-28.

In Fig. 22-28A, the input signal of –110 dBm is not

Figure 22-25. FET attenuators. attenuated; therefore, the signal going into the booster

A. FET as the lower leg of a voltage divider.

B. Feedback required around the FET.

C. An op amp used in conjunction with an FET.

D. An FET used to control feedback.

Vin

Vin

Vin

Vin

Vin

Vout

Vout

Vout

Vout

Vout

Vcontrol

Vcontrol

Vcontrol

Vcontrol

Vcontrol

Rf

Rf

E. An FET as a T attenuator.

Figure 22-26. Functional block diagram of a automated

fader.

Figure 22-27. A passive mixer circuit.

1

n

n+1

master

Audio

input

Automated

fader modules Audio

out

1

n

Write

Safe

Local

Control in

Write

Read

1

n

Control

Write

Read

Control

out

Write

Read

Encoder

Decoder

To tape

machine data

track input

(record)

From tape

machine data

track output

n+1 (sync playback)

1

n

1

n

To all

faders

Programmer

()

... ...

...

...

Vin

Vin

R 1

R 3

Vout

R 2

R 4