B&K 2627 Guide de l'utilisateur

1 Inch Preamplifier for

Condenser Microphones

Type 2627

A field

effect

transistor Preamplifier

with insert voltage calibration

capability

for

1 inch condenser

microphones. The extremely high

input impedance ensures a flat

frequency response over a wide

range. The Preamplifier conforms

with the IEC Recommendation R

327,

and the American Standard

ASA 81.10-1966.

MICROPHONE PREAMPLIFl

TYPE 2627

September 1971

CONTENTS

INTRODUCTION

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

GeneraI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2. Preamplifier Requirements . . . . . . . . . . . . . . . . . . . . . 5

DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2. Characteristics

................

;

...........

Input

Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . .

Output

Characteristics

........................

Frequency

Response

..........................

Attenuation

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Distortion

..................................

Noise

..................

2.3. Environmental Sensitivity

......................

Temperature

................................

Hu!Jlidity

..................................

Vibration

..................................

Acoustic Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Magnetic Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Shock Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3. INSERT

VOLTAGE

CALIBRATION

......................

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.2. Principle

The Insert Voltage Method

...........

Use

with

8 & K equipment

with

Insert Voltage

Calibration

Facility

...........................

Insert Voltage Junction Box ZH 0007

............

3.3. System Calibration

...........................

USE

WITH OTHER EQUIPMENT

........................

4.1. Microphones

................................

4.2. Accelerometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.3. Microphone Power Supplies

....................

4.4. Extension Cables

............................

SPECIFICATIONS

....................................

5.1. 2627 Preamplifier

............................

5.2. Junction Box ZH 0007

........................

INTRODUCTION

1.1.

GENERAL

The

Type

2627 Preamplifier

especially designed

for

use

with

the 1 inch

B & K Condenser Microphone Cartridges

Type

4144, 4145,

and

4146.

features the capability

for

calibration using the insert voltage method. The

Preamplifier may also

used

when impedance matching

from

high

low

impedance

required. The Preamplifier may

powered

directly

from

the

B & K Measuring

Amplifiers

and Analyzers,

from

the B & K Microphone

Power Supplies.

The

input

configuration

the Preamplifier

in accordance

with

IEC

Recommendation R-237.

1.2.

PREAMPLIFIER

REQUIREMENTS

Transducers

such

condenser microphones generally

have

a small

capacitance associated

with

them. Therefore any preamplifier

for

use

with

these transducers must

special design.

Two

equivalent

circuit

diagrams

a transducer and preampIifier

are

shown in Fig.1.1.

The

transducer may

regarded

a constant current generator providing a current

jwCr

R R

Fig.1.1. Equivalent circuits

a transducer

and

preamplifier

input

circuit

The constant current

circuit

may

simplified

further

the

circuit

shown in Fig.1.2.

.,....7....-'£69

Fig.1.2. Simplifiedequivalent circuit

It can

seen

from

the

simplified equivalent circuit diagram

that:

jwRCI

. V

1 +

jwRC

where: V voltage across

the

preamplifier input

open

circuit voltage

the

transducer

preamplifier

input

resistance

transducer

capacitance

cable capacitance

and C preamplifier

input

capacitance

+ Cp

Hence

the

transmission ratio is:

jwRCr

1+jwRC

high frequencies

jwRC

1, so

the

transmission ratio

tends

to~

This

means

that

the

transmission ratio

independent

frequency,

these frequencies,

but

depends

the

total

capacitive loading

the

trans-

ducer.

Any

capacitance additional

the

transducer

capacitance, e.g.

that

the

preamplifier, will load

the

transducer

and lower

the

output

voltage.

Therefore,

the

preamplifier capacitance

should

be small

compared

with

the

transducer

capacitance.

The

cable capacitance,

Cc,

can be neglected provid-

ing

the

transducer

and

preamplifier are

proximity

each

other.

Note:

The

cable capacitance

avoided

altogether

with

the

Type

2627

Preamplifier

and

the

1 inch Microphone Cartridges, by screwing

the

Cart-

ridge

directly

the

Preamplifier.

Now, if

the

response

low frequencies is considered, it can be seen

that

jwRC~

the

transmission ratio

tends

jwRCr.

This

gives a

drop

gain

per

octave

(20

per

decade) as

the

frequency

decreases.

The

cut-off

frequency

(-3

point)

where

wRC

=1. Hence

the

cut-

off

frequency,

f, = 1/(27TRC).

A lower

cut-off

frequency

could

obtained

increasing

the

total

capacitance,

However, this

would

reduce

the

signaI

output

from

the

transducer.

Therefore

necessary for

the

input

resistance, R, of

the

preamplifier

be as high as possible, if a low

frequency

response

re-

quired.

DESCRIPTION

2.1. GENERAL

The

Type

2627 Preamplifier

supplied in a mahogany

case

which con-

tains the Preamplifier,

Input

Adapter

2612

for

a B & K plug

0101,

and

a small screwdriver

0001. Provision

made in

the

Preamplifier

case

for

storage

a B & K microphone cartridge in its

own

case,

see

Fig.2.1.

Fig.2.

The

Preamplifier Type

2627

andmahogany

case

The physical dimensions

the preamplifier alone, and when

fitted

with

a B & K 1 inch microphone cartridge,

are

shown in Fig.2.2. The preamplifier

fitted

with

a 2 metre long cable, which

terminated in a B & K 7

Microphone plug

0701,

see

Figs.2.2

and

2.3.

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