Boss Audio Systems BV8.5GA User Manual download pdf (Page 51)

PARTS

LIST

B1,

-9

-volt battery

C1 -3 -pF capacitor

C2, C4

-1 -p.F,

15 -volt electrolytic

capacitor

C3-

100

-pF

capacitor

ICI

-709 operational

amplifier

1C2 -741 operational

amplifier

J l , J2

-Phono

jack

-3900 -ohm,

1/4-watt

resistor

-37 -ohm, 'h -watt

resistor

R3-

1500 -ohm, 1 -watt

resistor

R4, R11-

1- megohm,1/4-watt

resistor

R5, R6-

10,000 -ohm, 1/4-watt

resistor

R7- 10,000

-ohm trimmer

potentiometer

R8, R9- 25,000

-ohm trimmer

potentiome-

ter

R10- 100,000 -ohm

potentiometer

R12- 100,000 -ohm

linear

potentiometer

l -Spdt

toggle or slide

switch

-Dpdt slide or toggle

switch

S3 -Spst

toggle or slide

switch

S4-4-

position,

double -throw

slide switch

Misc.-

Chassis

box, 9 -volt transistor

bat-

tery clips,

shielded cable

and audio

phono

plugs

(for test cables).

IC sock-

ets or Molex Soldercons

board or

perforated phenolic

board

and solder

clips,

hookup wire,

solder,

hardware,

control knob, etc.

ICI

10K

-9V

.41..le

RI I

29K

51.1.-

+9V

ZC2

5110.0-

-9V

..11;

id-

II.

T9"

S2 AC

DIR

EXP

R12

100K

Two operational

amplifiers

(ICI and IC2) forra heart

expande,

TPU

INPUT

meter.

You

can even

use

the range

expander

measure

the

output

volt-

age of

a phono cartridge.

Try

that with

an ordinary

multimeter.

About the Circuit.

The

range

ex-

pander

makes use

two

operational

amplifier

IC's (IC1 and

IC2

in the

schematic),

exploiting

the particular

advantages

the types

709 and

741

op amps.

monolithic amplifier

using

bipolar

transistors

appears

the

signal

being

processed as a series of

resistances and

shunting capaci-

tances.

An RC system

this forms a

phase

-shift

network

that

at some

fre-

quency will cause

the amplifier

os-

cillate.

Compensation

is required to insure

low

gain

at the

frequency at which os-

cillation

occurs.

The 741

amp

unconditionally compensated.

(Gain

is reduced to unity at

the

point

where

oscillation

possible.)

The

709

amp

is not internally compensated,

requiring

external

components

to ob-

tain the necessary

compensation.

However, it

can

be compensated

for

frequencies up

to 1 MHz, while the

741

is restricted to a top

-end

frequency

about

1 kHz by its

internal

compensa-

tion.

The

741

amp

has

provisions

for

input

offset

nulling, which

makes

operate well as a

dc amplifier.

In the

range

expander,

the

741 (IC2) is used

as a dc amplifier with output

nulling

NOVEMBER 1974

and a

feedback

network

that

mini-

mizes

drift.

The

741 has

input over

voltage protection

and output

short

circuit

protection,

while

the

709 has

neither.

próvide

input overvoltage

and output

short

-circuit

protection

for

the

709, R1 and

R2 are

used.

The

709

(IC1)

in the range expander

is compensated

for a

-dB

gain

about

200 kHz by Cl

C3,

and R3.

It has

feedback

network

consisting

and R5.

Both ac and

dc amplifiers

(IC1

and

IC2)

have a common

vernier con-

trol (R12)

that can

be used

where

exact

values of

gain are

not required.

The incoming signal

(or

voltage) is

applied

viaJl , while

the

mode

of oper-

ation

(ac

dc)

selected

with

S2.

Switch S1 permits

selection

X10

X100 in the dc

mode, while switch S3

applies power

to either

the

IC1 or

the

IC2 circuit.

The final

switch,

S3, per-

mits the

range

expander

to be

by-

passed when

the

DIR

position. In this

position,

it routes the

incoming

signal

at J1

directly

output

jack J2.

(Note:

When

is in the

DIR position, S4

can

be switched

off

to conserve

battery

life.)

Construction.Assemblingthe

range

expander

is relatively easy, owing

the simplicity of

the circuit.

The

entire

circuit can

be easily

accommodated

inside

4 in. by 23/4

in.

metal

utility

box, with

the

four

switches

and

vernier control R12

mounted

the

top

the box

for

convenience.

You

can

use a printed

circuit board

your

own

design

perforated

phenolic

board

and

solder

clips

for

mounting

the ICI and

IC2 amplifier

circuits

inside

thebox.JacksJl

and J2

can

mounted at one

end of

the

box.

When the circuit

has been

fully as-

sembled

and

all

parts are

mounted

place, use

dry- transfer

letters to

label

the control,

switches,

and

jacks.

Calibration.

With the

range expand-

switched

Dc (both S2

and S4

must

be set

to this position)

and

R12 set

for

maximum sensitivity,

connect

mul-

timeter set

to a

low- voltage

range ac-

rossJ2.

Adjust

fora zero

indication

the

multimeter's

scale.

Connect

variable-

output

power

supply

a potentiometer

in parallel

with a

1.5

-volt

battery

and adjust

the supply

pot

for a 0.1-

to 0.5 -volt

indication on

the

multimeter's scale.

Adjust R8

for

indication of

times

the

reading of

the

input

voltage

level.

(Use the

multimeter

monitor both

the

input and output

voltage

levels.)

Now,

decrease

the

output

voltage of

the

power

supply

(or battery

/pot

setup)

again

for a

meter

reading of 0.1

to 0.5

volt and switch

to the

X100

position.

Adjust

for

reading of

times

the previous

meter

reading.

With the

input disconnected,

recheck

the

null produced

by adjustment

R7.

If necessary,

readjust

the null.

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Boss Audio Systems BV8.5GA User Manual Page 51

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