Marantz 5220 User Manual download pdf (Page 33)

causing

possible

damage. The

diode

also

rectifies the

signal

so

that the

meter

can indicate

a somewhat aver-

age power

rather than peak

power,

which

the 80

-Hz

(low -pass) filter pro-

vides. The filter was

selected to give

good

LED response

and

still

provide

a

reasonably

slow

LED

switching time

for

the eye to follow.

The

eleven differential comparators

in ICi to

/C3

deliver

logic

outputs

based

on

the

states

of their inputs.

If

a

noninverting ( +) input is higher than

the associated inverting ( ) input, the

comparator output is high, and

vice

versa.

The inverting

inputs

of all

corn

-

parators are connected

together and

driven by the input

signal.

The nonin-

verting

inputs

are

threshold

voltages

determined

by the

voltage

divider

made up of R1

through R16.

The

di-

vider

values

are based

on the desired

power

-level increments. Since the

in-

puts to

the

comparators should

not

be

higher

or

lower than the supply

volt-

age, the

input

circuit forms a

5:1 ratio

and the corresponding

thresholds are

similarly

reduced. As the output of

each comparator

switches

from high

to low, its

associated

transistor is

turned on, causing the

LED to

glow.

Each LED requires

about

20 to

40

mA,

while the comparators can sink

only

2

mA.

Thus

the transistors

must

be used,

with

series

resistors

(R17

to

R27) to limit the LED current.

If

more

brightness

is required, the series

re-

sistors

can

be

reduced

to 22

ohms.

The

power supply

is a conventional

transformer

-diode -filter combination

with a

voltage regulator (IC4)

to

pro-

vide 5

volts. Capacitor

C2

is used

for

transient suppression.

Construction.

The meter

circuit can

be assembled on

perforated

board or

a pc board (Fig.

2)

can

be used.

Note

that the

11

LED's

are

not mounted

on

the

board.

They

are connected to

the

appropriate

pads on the board by

lengths

of

insulated

wire.

Before

installing the

comparators

(ICi to 1C3),

build and test the power

supply.

(The

transformer is not

on

the

board.) Install

IC3, and the

1/16

-watt

LED (LED11). Connect the

positive

lead

of one of

the meter

inputs

(BP1 or

BP3) to the 5 -volt dc line

and

tie the

other input leads to

ground.

When

power

is

supplied,

LED11

should

light.

If it

does

not,

it may be

connected

backwards.

Remove

power

from the

circuit

and

install

the other two IC's

and

the remainder

of

the LED's. The

AC

INPUT

L

LEFT-

RIGHT- --

LEFT+ -

RIGHT+-

C3

J

-CI-

D2-

-4.04

-D3-

-DI -44

-RI6

-

-R15-

- R14

-

-813-

-RI2--

-R11-

-RIO-

-R9 -

-RES-

- R7-

.-- R6 -

-R5 -

-R4 -

-R3 -

-R2-

-RI-

-P3-

-R2-

-RI-

-P33-

-R31-

-P29-

-R28--

D5-

-832-

-R30-

PIN

2

PIN

3

IC4

PIN I 1

IC2

r1

ICI

rTh

J

C2

Qn

C

)

010

)

C

LED II

-R27-

1------

LEDIO

09 LED9

)

-R25-

LED 8

-R24-

LED

7

-R23-

t

LED

6

)-R22- r------

LED5

)-R21-

t\

04

LED4

LED 3

-R19J--,

LED 2

-RI8- «

LEDI

-RI7-

08

07

)

06

C

C

C

03

Q5

C

)

02

01

)

*LED'S

ARE NOT MOUNTED

ON

THE

BOARD

Fig.

2. Foil

lulitern (top)

(111(1

rnliilmIieilt

la !pod

for

the

nnrlio

power meter.

leads

for

the

LED's

should

be

long

enough for them to be

mounted

on

the

front

of

the chassis

selected.

With

one of

the positive

inputs to the

meter connected

to the 5 -volt

supply,

the lowest four LED's should

glow.

With

both

positive inputs connected

to 5 volts, the lowest

six

LED's

should

light. This test indicates whether

or

not the

circuit

is

operating

properly. If

desired, the threshold

voltages

can be

checked with Table II, which

gives

the

ideal values- though

a

5%

tolerance

is

permitted.

The

basic accuracy of the system is

directly related to the input divider

MARCH

1976

37

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