SEA 222 - Audio Power Amplifier

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5.3.5

S.3.6

S.3.7

5.3.8

The

second

stage

{U3B)

is

configured

as

an

inverting

amplifier

with

a

stage

gain

of

approximately

9.

The

inverting

configuration

and

lower

gain

assure

maximum

dynamic

range.

Overall

gain

of

the

preamplifier

stage

is

approximately

315

with

a

noise

contribution_

equivalent

~o

approximately

0.5

uV.

Since

the

minimu·m

product

detector

output

is

normally

approximately

20

uV

or

better,

the

AUDIO

signal

to

noise

floor

exceeds

30 dB.

AUDIO

PREAMPLIFIER/FILTER

USA

is

connected

as a

moderate

gain

active

low

pass

filter

with

a

cutoff

frequency

of

approximately

3000

Hz

and a

slope

of

12

dB/octave.

Some

additional

rolloff

is

provided

in

the

low

noise

preamplifier.

The

effect

of

these

filters

is

to

remove

the

high

frequency

11

hiss

11

present

at

the

output

of

the

product

detector

and

to

enhance

the

speech

signal

to

noise

ratio.

AUDIO

POWER

AMPLIFIER

Power

integrated

circuit

U6, a

type

TOA2002,

is

used

to

raise

the

audio

power

level

to

about

4

Watts,

which

drives

either

the

internal

3.2

Ohm

loudspeaker

or

may

be used

to

drive

remote-

speaker

systems

through

use

of

the

rear

panel

accessory

connector.

SQUELCH

LIMITER/AMPLIFIER

Dual

operational

amplifier

U7

is

configured

as

a

cascaded

two

stage

audio

amplifier.

Gain

per

stage

is

about

22,

providing

a

total

gain

of

about

480.

Since

the

amplifier

is

powered from

the

+5

Volt

CPU

power

buss,

the

maximum

output

voltage

swing

is

5

Volts

P-P

•.

Thus

the

normal

output

from

U7

consists

of

heavily

limited

noise

and (when

present)

voice

audio.

This

signal

is

sent

to

the

control

computer where a

software

analogue

of

SEAs

normal

"voice

operated"

squelch

system

resides.

If

a

voice

is

present

in

the

signal,

the

computer program

will

unsquelch

the

receiver

if

the

squelch

has been

activated.

Like

the

standard

SEA

"hardware

squelch"

system,

the

squelch

requires

a

moving

audio

tone

to

remain

open.

A

single

tone

will

momentarily

open

the

squelch

but

will

not

hold

it

open.

THE

AUDIO

DERIVEO

AGC

SYSTEM

A

sample

of

the

low

noise

audio

preamplified

output

is

routed

to

USB

which

is

configured

as

a low

gain

inverting

amplifier/low

pass

filter.

The

amplified,

band

limited

output

'is

then

applied

to

the

input

of

USC,

which

is

configured

as a

inverting

amplifier

with

a

gain

of

approximately

8.

The

non-inverting

input

of

USC

is

connected

to

approximately

+3.4

volts

from

voltage

divider

RS9,

R60

and R61. The

28

I