Dorrough Electronics 40-A User manual

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Dorrough
Loudness Monitor
Instruction Manual
Model 40-A
DORROUGH ELOCTKONICS
5221 C.O llier Place
Woodland Hills, r.A 91364
lelephone: (213) 999-11.32

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Dorrough Loudness Monitor
Model40-A
Instruction Manual
Theory of Operation
The input ,s fed into the left and right differential
amplifiers. The differential inputs are configured
so that
the
input t mpedance Is 20K for balanced i nputs and 10K
for unbalancedinputs(negative terminal Is grounded for
unbalancedoperation).
Theleft
input levelIscontrolledbyR- 13 andthe right input
byR- 14 .Theoutput of thesetwo controlsarefed 10 a summing
amphfier. The feedback resistor
R-18 controls the input
sensitivity
of the instrument. It Is shipped with R- 18 as 20K.
The
input sensitivity with this value feedback resistor is
-18d8m Changing the value of
R-18 to 100Kincreases the
sensitivity to
-30d8m .
The output
of the summing amplifier feeds a precision
rectifier.Theoutputoftheprecisionrectifierfeedsa split
to the
PEAK and PERSISTENCE circuits.
Sincethedesireddisplay
is to belogarithmic,thesignals
are fed
10 corresponding"linear/logarithmic"converters. The
outputs
of these converters feed the display drivers. The
driverfor the PEAK display is configured as a dot, while
the driver for the PERSISTENCE
disp l ay is configured as
a bargraph.
Each of the
40 LEDs derives an i nput from both of the
drivers,with the exception
of the red LEDs designated as
-
1, 0, +1 These particular LEDs have no input from the do t
driver, and therefore
do not display PEAK information. This
provision was made
to eliminate possible confusion relative
to PEAK
and PERSISTENT amplitudesat this reference, With
the exception
of these LEDs, a steady state sinewave w il l
illuminate a bargraph at a given amplitude, with the last
displayed LED brighterthan the remainde
r. This gives
indication that the amplitude of the PERSISTENCE and the
amplitude
of the PEAK areequal. showing proper alignment
of the instrument.
Operation
Stereophonicmaterial,in theform ofleftandri ght inputs,
is fed to themeterattheterminalstriplocatedin therear of the
meter. Individual gain
con trols f or the leftand right i nput s are
al
so loca ted on the rear panel.
These sources feed a 20K balanced
bridg i ng amplifi er .
Unbalanced
10K bridging inputIsaccomplished by convert-
Ing the minus(-) inputterminal to chassisgroundontherear
terminal s
tr ip .
The meteris shipped with the input controls set for
+8d8m as reference"0" on the meter face. Levels as low as
-30d8m can be referenced to "0" by changing the value of
R-
18 fr om 20K to 100K.
If yourprogramreference Is dif ferentthanthatof +BdBrn,
simpleadjustments
or some otherstandard can be made by
the following procedure.
Initial Set-up
Feed a 1000Hzsinewave,observing polarity, intothe left
i
nput at a st andard operating level (i.e ., +4, +8, e tc .).
Adjust the i
nput level control on the rear panel for an
indication
of ''0"on the PERSISTENCEscale, The peak
excursion and the persistence amplitude are equal under
steady state slnewave conditions.
Next, remove the signal from the left channel and feed
the signal in the same polarity to the right input,
mak i ng the
same adjustment.
Followi
ng the adjustments for both the left and right
inputs, f
eed the signal to both Inputs simultaneously. The
summing amplifier will algebraically
add the two inputs for
an
indica tion of +6 as read on the PEAK scale. Di ff erent
amplitudesand phaseanglesbetweentheleftand ri
ght input
signals will produce different algebraicsums. This display
is essential in indicating centerchannel buildup in some
program material.Thus, thisindication wi
ll aid the operator
in making the
proper adjust ment to programoperating level.
field Alignment
All adjustmentsforthealignment ofthe LED disp l ay are
loca t ed internal to the enclosure.
To gain access to these controls. the instrument is
placed bottom-side-up and the six screws are removed.
Once the screws are removed, place the unit right-side- up
and the cabinet can be hfted from the remainderof the
chassis,
expos ing the adjustments (see illustration
"CHASSIS LAYOUT').
Procedure
1. Connect the left input to a test oscillator.
2. Feed a sinewave tone of 1000Hz at a reference level
ol
"0"dBm into the instrument,
3 Ad1ust the left in
pu t ga in control R-13 for DS-1 ro be off
and DS-2,3.4 to be on.
4. Adjust the input signal to -25dBm
5 Adjust the low PERSISTENCE level control R-44 so that
only the first LED
(- 25d8) at the bottom of the PERSIS•
TENCE scale is illuminated.
6 Adjust the t
ow PEAK level control A-46 so that only the
first LED (-
25d8) at the bottom of the PERSISTENCE
scale brightens. This
1nd1cates that both the PEAK
and the PERSISTENCE
dr ivers are providing current
to this LED
7 Change the input level to +SdBm.
8.
AdJUSl the high PERSISTENCE level control R- 48 so that
the bargraph ,s illuminated
up to +SdB .
9 Adjustthe
high PEAKlevelcontrolR- 49 sothatthe +SdB
LED brightens. This indicates that both the PEAK and
the PERSISTENCE drivers are
bo th providing current
to this LED,
10 Increaseinputlevel to +14d8m .The +14d8 LEDshould
indicate
dr i ve from both PEAK and PERSISTENCE
drivers.
11. Reduce inputlevel to Reference level ( i. e ., +4, +8 ,etc.)
and set the meterinputs for''0" indication on PERSIS-
TENCE
sca le as descr i bed in the"INITIAL SET -UP ':
12 . Alignment Is now complete.

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MODEL40-A

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R-13 R-14
Chassis Layout

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DORROUGH t:LOCTRONICS
5221 Co llier Pl ace
Woodland Hill s, CA 91364
leleph one: (213) 999-ill2