páginas desderadio electronics april 1985 2

8/18/2019 Páginas DesdeRadio Electronics April 1985 2

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Analyze..

s

c l

..

Beal- Ii ,e Hi-fi

.

for yo

F

rE R YO U S ET UP ND

LISTE

N TO YOUR

stereo speakers in yo ur living roo m, yo u

realize that they do n t give the full , j ust

right so und that you we re hoping for. They

do n t so und ne arl y as goo d as they did in

the stereo showroo m. That situation has

becom e a much-rep eated one beca use

yo u r stereo syste m requires help in obtain

ing a co rrect respo nse depending on the

room in wh ich it s pl aced .

That s why the equalizer has be com e a

standard co m po ne n t of the audio sys tem .

But the equalize r isn tof any help unles s

it s adjusted properly to c or rect the re

sponse of the loudspeakers. Adju sting the

equalize r for maximum performance can

be a ted io us and fru strating jo b . But it

doe sn t have to be . Proper equ alizer ad

ju stm ent can be d

on e

easily in the stereo

listening roo m by usin g a real-time ana

lyzer.

We ll show you how to build an audio

spec trum analyzer that displays a pic ture

of any audio signal spec tru m in 10 oc

taves . It is an eco no m i ca l, lab- style mea

suring too l capa ble of cali brating a wide

variety of levels. For those of you with

rack-mou nte d systems, rest assured that

the PC board fits int o a 19-inch rack

mountable chassis.

Why use an analyzer

The additio n of the real-time audio ana

lyzer to yo ur stereo sys tem , PA, or record

ing co nso le allows yo u to see what you re

hearin g. You ca n use it as a tool when

taping : to match tap es with the or iginal

so urce . o r to di scover th e pl ayb ack

charac ter istics of a tap e machine . Be

cause it reveals the spectral content of the

music played . it can be used as an educa

tion al , entertaining, and colorful display.

Total sys tem/e nvironment co ntrol can

Once you can see the

response of your stereo

system you can control it

better with your equalizer

and flatten your speaker

response

ROGER

or

and LLOYD DDINGlON

be realized by using the analyzer with an

equalizer. An alyz i ng the frequency re

sp onse of your listening area and adjust

ing your equalizer is simplified because

the ten octave filte rs are tuned to the stan

da rd ISO ce nter frequencie s that are used

in most equali zers . Music can be ana

lyzed , tape copi e s ca n be comp ared to

originals, and equalization ofl ive vocal or

instrument al sound ca n be optimized. Be

ca use unwanted extraneo us noise wi ll be

displayed, it can be removed . Whe n using

a microphone with a known frequenc y

response, the bu ilt -in diagnostic signa l

ge nerator provid es a visual display of the

reproduct ion charac teristics in the listen

ing environment.

The analyzer, with its several input co n

nectors and selec tab le input mod e , allows

a variety of hookup option s . For exa mple,

you can use it with a receiver, preamp,

tap e r eco rder /player, equalizer, micro

phon e , co mpact disc player, hom e satel

lite receiver, mixing boa rd or rec ording

co nsole. The Sl-LED display form s a pic

ture of any audio s ignal over a 21-d B

range of ene rgy in 3-dB ste ps in eac h of

the ten standard ISO octaves .

The basics of

our

analyzer

Figure I is a block diagram of the ana

lyzer. As you ca n see, the re are two possi

ble input source s , line and microphone.

As we ll soo n see , there is re all y a third

possibl e input source that ca n be selected

by a front-panel rotary sw itch .

In the

LINE

mode , the anal yzer will

acce pt standard line-level I-volt nominal )

signals from devices such as preamp s ,

receivers, tape machines, con sole s, etc.

In the M IC mode, the analyzer accepts the

output of a dyn amic microphone, which

is fed into the built-in preamp. We ll see

how and when to use that input shortly.

The front-panel LEVEL control se ts the

level of the input signal so that the high

est-level signa l is st ill in the ran ge of the

LED display. The input signal is amplified

by the input driver and separated in to the

ten octaves by the ana log bandp ass-filter

networks. The signals are then rectified

and filtered so that the RMS amplitudes

ca n be determined. Next the signals are

multi plexed together by the diode analog

multiplexer, amplified , and fed to an ana

log-to-digital A/D) co nverter.

Lo gi c circuitry is used to co ntrol bot h

the diode mult iplexer and the mult iplexed

disp lay driver. The co ntrol lo gi c co nsists

of a timer, a d ivider, and a l-of-Iadecoder.

The I co nverter take s the ana log

voltage and drives the SO-LED disp lay.

Eac h LED -step vertica lly repres ent s a

ga in in amplitude of 3 dB . The hor izon tal

axis of the LED matrix represent s fre

quen cy. When a signal in any frequenc y

range drives the device higher than the 2 1

d B range , the OVERSC LE LED light s.

When that happ en s, simply use the LEV EL

co ntro l to bring the sig na l back into the

range of the analyzer

43


2/5

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DISPLAY

80LED

MI

XED

DISPLAY DRIVER

MIC

PRE-

AMP

MIC

IN

L_

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IDIAGN iiSTICOiTTPUT

r . . . .

I DSC

UT

FIG l UDIO ANALYZER BLOCK DIAGRAM The diagnost ic output can also be used asan inputto

the analyzer for calibration purposes

A di gnostic sweep sign l which is

used to calibrate the equalizer, is con

trolled by the clock andthe divider. A 555

timer is used as an oscillator, which is

filtered to obtain frequencies for testing at

all 10 octaves . The generated signals are

mixed together, filtered , and then sent to

the oscillator output. That output can be

fed to speake rs viathe stereo system) and

picked up by a microphone in order to

calibrate the equalizer . That diagnostic

signal can also be chosen as an input by

the front-panel mode switch. That letsyou

view the response o

ft he analyzer as all the

frequencies are swept.

ow the ir u it works

Figure 2 is the sche ma tic of the ana

lyzer . As you can see there, a three-posi

ti on ro tary

swi

tch,

Sl

,

select

s the

appropriate input. The LINE input is con

figured to allow either separate right- and

left-channel input s or a balanced input. In

other words , the inpu t can be the right

channel and ground, the left channel and

ground, or the right plus le ft for balanced

line in . In either case , the input signal

goes int o a line buffer or mixing amplifier

made up of Rll- RI5, C6, C7, and ICl-b .

A microphone input is also included for

low- im pe da nce dynamic microphones.

Since the ou tp ut of a dynamic mic ro

phone is at a very low leve l , the signal

must be preamplified. The microphone

preamp section is made up of R2 , R3 , R8,

R9, C2, C56 , ICl-c , and ICI-d.

you

want to use a co ndenser microphone, then

you ll have to add

Rl ,

R4, R5 and C l , as

shown in the das hed box in Fig. 2.

The lOOK front-panel L EV EL control,

RIl3, se ts the input level for the input

driver stage which consists

ofRl6,

R18,

R19, C13, and IC4-a.) That stage supplies

a l ow- imp ed an ce si gna l s ou rc e for the

analog filters. Each basic filter has the

same configuration, but the frequenc y is

selected by the value of the capacitors .

Figure 3 shows the basic filter, while Table

1gives the values of C and corresponding

filter frequencies .

The rectifier filters and the diode multi

plex network are identical for all frequen

cies. The output of the analog filter op

amp is rectified by a small-signal diode in

series with a 10K resistor and a l I L ca

pacitor connected to the negative suppl y.

For the

30-Hz frequenc y, for example , the

rectifier filter is

05

, R52, and C14. The

diode multiplexer bu ff er amp IC4 -b is

driven by the diode networks and consists

ofR17

, R20, R21, and IC4-b .

The control logic determines which fre

quency s signal is presented to the multi

plexer buffer amp. The 555 timer, IC8, is

controlled by R74 , R75, and C44 to ope r

ate as a 16-kHz oscillator, triggered and

reset on the trailing edge of each pulse.

The output of the 555 feeds IC5, a 4040

12-stage ripple-carry binary counter. As

the counter counts up, resets and repeats ,

the ou tpu t pin 1 is fed back through R80

and C3 to the frequenc y-modulating pin 5

of IC8. That causes the oscillator s output

to warble up and down about 1/2 oct ave.

The output pin s of IC5 pins 2, 3, 4, and

13) are fed to IC6 , a 74C42 BCO-to-deci

mal decoder. The 74C42 converts the sig

nal at its A, B , C, and 0 input s to a logic

zero on the appropriate output from

0-

9 .

Those outputs are connected both to the

diode multiplexer and the display multi

plexing network . As the 74C4 2 counts

from 0-9 itenables each of the frequencies

in turn to feed through the multiplexe r

buffer amp, thereby presenting each oc

tave s analog voltage to IC7 , an

ID

con

verter. Resistors R77 and R78 form a

voltage divider to provide IC7 with a refer

ence vo ltage; the IC senses the analog

voltage input and fires the output LED

corresponding to that voltage. Each out

put cor responds to a 3 dB step in a 21dB

range.

At the same lime , the 74C42 enables

the par ticular octave to be sensed by the

ID it also enables the display driver for

that frequency , The multiplexed display

driver con

sists of

PNP tr ans i st ors

Q2-Qll,

which are biased by R81

-R90

.

Overscale is indicated by Ql , R76 , and

036 . The base of Ql is enabl ed by the

4040 lC5), and the collector is connected

to the o ut pu t of the- IDcon verter corre

spo nd ing to the highe st ana lo g level.

When that output is acti vated the tran

sistor is forward biased and turns on 03 6 ,

the

OVERSCA

LE LED .

The diagnostic sweep sig na l is gener

ated and controlled by the logic circuitry

as well. Ten oscillator filters are formed

by R91-RIIO and C26 , C29 , C32, C35,

C38, C41, C45, C48, C51, C54 . Those

filters convert the squarewave outputs of

IC5 to ramp wave s. Table 1 shows the

relations hip bet we en filter capacitance

and frequency.

The 555 timer fires and pulses a signal

to the first filter then , as the c ou nter

counts, the 16. kHz is divided down to

produce a center oscillating frequency for

all the octaves. The resulting signals are

presented to the mixin g amp formed by


3/5

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FIG. THE BASIC ANALOG FILTER configur -

tion

is the

s me

fo r

all

frequencies

However the

values of

the c p citors

ch nge as shown in the

table.

o

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Note that you' ll be installing some off

board jacks such as the line and micro

phone inputs and the oscillator output. Be

sure that the wires you moun t in the cir

cuit-board holes are long enough

I·

3 INCH S

FIG.

THE SOLDER SIDE of the analyzer cir-

cuit

board is

shown

here half sized.

•

6. Note that the LED's are mounted on the

solder side of the circuit board so that

they' ll be seen from the front of the cab

ine t. As with any diodes , be sure to install

them in the proper direction .

t

•• •

r 3 INCHES I

FIG. THE COMPONENT SIDE of the n lyzer

c

ircuit

bo rd is shown here half sized.

TO DIODE

RECTIFIER

FILTER

CAPACITANCE

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125 Hz

250 Hz

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2000 Hz

4000 Hz

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FROM

INPUT

DRIVER C

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RIO, RII 2, C12, and l -a, and then to

the output terminals and the input-select

switch. Since the top-octave frequency is

being warbled, all the octave frequencies

warble, giving a diagnostic signal with

energy across the audio band from 25 Hz

to 19 kHz .

The 1081 uses a standard

±

15 volt

supply. A 22-volt , 50-rnA center-tapped

transformer is used to step down the line

voltage to a useab le value that is rectified

by

01

0 4 , and filtered by C8

C

lI along

with R6 and R7.

Building the audio analyzer

The foil pattern for the com ponent

side of the circuit board is shown in Fig.

4.

The so lder side is shown in Fig. 5. It is

not entirely correct to call one side the

solder side and the other the Circuit side

because each side has components sol

dered

to it.

The

d isp lay LED 's are

mounted on the solder side.

Since the parts count is high and parts

are very close together be

very ref ul

not

to cause solder bridges. It also helps to

install the lower-profile parts first to be

sure the larger parts are not damaged by

trying to solder around them. However, at

this time , do not install R91, R93 , R95 ,

R97 , R99 , RIO , R103, R105 and R109 .

You can put them in position, but do not

solder them . We will return to those de

vices when we calibrate the unit.

We should remind you that the 74C42

and 4040 ( lC6 and IC5) are CMOS de

vice s. Because

CMOS

devices can be

easily damaged by static disc harges, they

must be handled with proper care.

A parts-placement diagram for the au

dio-analyzer circuit board is shown in Fig.

f

6


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PARTS LIST

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páginas desderadio electronics april 1985 2

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