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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
8/18/2019 Páginas DesdeRadio Electronics April 1985 2
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I
I
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___
J
DISPLAY
80LED
MI
XED
DISPLAY DRIVER
MIC
PRE-
AMP
MIC
IN
L_
I
I
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
8/18/2019 Páginas DesdeRadio Electronics April 1985 2
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8/18/2019 Páginas DesdeRadio Electronics April 1985 2
4/5
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
o
~
•
o
o
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
.047 IlF
.022 Il F
.01 Il F
.0056 IlF
.0022 IlF
.0015 IlF
680 pF
330 pF
180
pF
82 pF
R2
680K
R3
680K
=
FREOUENCY
31.5Hz
62
Hz
125 Hz
250 Hz
500
Hz
1000
Hz
2000 Hz
4000 Hz
8000
Hz
6
Hz
f
1
o - 2rrC V RTff2
FROM
INPUT
DRIVER C
~
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
8/18/2019 Páginas DesdeRadio Electronics April 1985 2
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PARTS LIST
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I 0 -0
- =
+
w => E = ± ~
0
ONa ~
O lll
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