Nagra III - Internal Noise Precautions

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14

A

misplacement

cannot

happen

except

by

a

previous

careless

service

work

or

by

unscrewing

or

dismantling

of

the

magnet.

The

left

end

thread

on

the

magnet

ensures

that

this

cannot

unscrew

it

self

.

On

the

other

hand,

it

can

happen

when

the

adjustment

mentioned

above

has

been

made

and

the

magnet

insufficiently

well

secured.

In

practice,

it

should

be

tightened

up

securely

before

re-adjustment.

If

the

difference

is

small,

replace

the

brushes

where

they

were

because

the

difference

is

probably

an

error

in

measurement.

Our

factory-

ad

justment

made

by

the

aid

of

a

special

machine

is

much

more

accurate

than

that

made

by

adjusting

to

minimum

curr

e

nt.

PRECA

UTIONS

AGAINST

INTERNAL

NOISE

As

with

all

tape

recorders,

the

Nagra

III

has

some

residual

noise.

We

must

distinguish:

1 ) NOLse

in

the

microphone

preamplifi

er

2) N

oise

in

the

record

amplifier

(output

amplifier)

3)

Noise

in

recording

on

the

magnetic

track

4)

Noise

in

the

play

-

back

amplifier

5)

Noise

in

the

switching

dlOdes

of

the

speed

standard

selector

6)

Noise

in

the

play-back

am

plifier

7)

For

the

second

time

noise

in

the

output

amplifier

Discussing

the

nature

of

these

noises,

we

have:

a)

Hiss

This

is

approx

imatel

y

white

noise,

that

is

to

say,

composed

of

all

frequencies

in

th

e

spectrum.

It

resembles

the

noise

of

a

jet

of

air

or

dry

steam.

It

is

quite

clear

but

a

little

less

than

the

pronounciation

of

the

letter

"S".

Hiss

is

essentially

produced

by

thermal

agitatio

n

and

has

a

limiting

minimum

value

fixed

for

a

given

temperature

and

a

given

impedance.

Similarly,

the

noise

following

the

breakage

of

a

connection

on

the

input

circuit

is

of

this

type.

b)

Semiconductor

Noise

(Flicker

Noise)

This

is

similar

to

the

preceding

noise,

but

includes

more

low

frequencies.

Our

ears

are

always

less

sensitive

to

low

frequencies

at

low

level,

and

semiconductor

noise

is

distinguished

by

the

fact

that

it

is

modulated

by

these

loyv

frequencies.

The

sound

which

most

nearl

y

describes

this

type

of

noise

is

that

of

a

jet

of

vapour

containing

drops

of

liquid,

for

example,

an

expresso

coffee

machine.

The

noise

is

produced

both

by

defective

resistances

carrying

continuous

current.

In

cases

1,2,

4,6

and

7

the

transistors

should

be

first

suspect,

then

the

base

dividing

resistances,

and

the

collector

resistances.

SPECIAL

WARNING

Do

n

ot

dismantle

your

Nagra

because

of

the

results

obtained

with

a

doubtful

tape.

A

bad

tape

can

produce

such

noise

on

a

perfect

machine.

c)

Residual

Noise

at

1000,

2000

and

4000

c.

p.

s.

The

speed

stabilisation

system

of

the

Nagra

uses

these

frequencies

and

it

can

happen

that

they

may

break

through

and

become

audible

at

the

speed

of

3

3/4"

per

second

which

we

do

not

consider"

professional".

A

perc

eptible

residual

signal

is

a

llowed.

On

the

other

hand,

it

should

not

be

possible

to

hear

such

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