Universal Audio 537 User manual

BEFORE PROCEEDING WITH COMPLETE UNPACKING AND SETUP,
READ THE SECTION ON UNPACKING AND INSPECTION
model 537
GRAPHIC EQUALIZER
fU
UREI Incorporated
8400 BALBOA BOULEVARD
NORTHRIDGE, CA. 91325-4100 U.S.A.
PHONE: (818) 895-UREI (895-8734)
FAX: [phone]
©Copyright 1978,1988 UREI Incorporated

TABLE OF CONTENTS
PAGE
SECTION I. INTRODUCTION
1.1 DESCRIPTION 1
1.2 SPECIFICATIONS 2
1.3 CONTROLS 3
1.4 PHYSICAL 3
1.5CONNECTIONS 3
SECTION II. INSPECTION AND INSTALLATION
2.1 UNPACKING AND INSPECTION 4
2.2 ENVIRONMENTAL CONSIDERATIONS 4
2.3POWERING 4
2.4 LINE VOLTAGE SWITCH 5
2.5 EXTERNAL CONNECTIONS 5
2.6 IMPEDANCE AND TERMINATION 7
2.7 SIGNAL LEVEL AND OVERLOAD DETECTOR 8
2.8 ACCESSORIES 8
SECTION III. OPERATING INSTRUCTIONS
3.1 GENERAL 9
3.2 SYSTEM CHECK 9
3.3EXPERIMENTATION 9
3.4DOCUMENTATION 9
3.5 APPLICATION 9
3.6 GAIN STRUCTURE AND SIGNAL-TO-NOISE 12
3.7 INPUT LEVEL 13
3.8 OVERLOAD INDICATOR 14
SECTION IV. THEORY OF OPERATION
4.1 INPUT AMPLIFIER 13
4.2 ONE-THIRD OCTAVE FILTERS 13
4.3 EQ IN-OUT SWITCH 16
4.4 OUTPUT AMPLIFIER 16
4.5 OVERLOAD DETECTOR 17
4.6 POWER SUPPLY 17
SECTION V. MAINTENANCE
5.1 GENERAL 18
5.2 REPAIRS AND WARRANTY 18
5.3 SERVICE ADJUSTMENTS 18
5.4 ON OP AMPS 20
SECTION VI. ILLUSTRATIONS
FIGURE 5. SCHEMATIC DIAGRAM OF UREI MODEL 537 21

SECTION I
INTRODUCTION
FIGURE 1. MODEL 537 FRONT PANEL.
1.1 DESCRIPTION
The Model 537 is an active filter set for general purpose
frequency response adjustment of audio signals. It offers alarge
control range with optimum signal-to-noise ratio at any filter
setting, and is therefore an ideal tool for creative equalization
in professional sound recording, sound reinforcement, broadcast,
motion picture and TV recoroing, and live performance.
The 537 provides 12 dB of boost or cut at each of its 27
frequencies, which are centered at ISO 1/3-octave increments
from 40 Hz to 16 kHz. The filter characteristic has been optimized
for excellent combining action with minimum phase shift. The
position of the 27 control arms provides agraphic representation
of the response curve selected. An adjustable front panel control
provides up to 20 dB gain. An overload detection circuit monitors
the signal throughout the unit. If the signal level approaches
overload, afront panel LED flashes to warn the user. Abypass
switch permits switching the equalization out for A-B tests. The
optional security cover (SC-1) may be installed to protect all
operating controls against inadvertent disturbance or tampering at
fixed installations.
Due to the front panel's uncluttered and logical arrangement,
operation is simple and self-explanatory. However, athorough
understanding of the Model 537 Graphic Equalizer and its application
can only benefit the user, so we recommend carefully reading this
manual
.
NOTE: Throughout this manual, where the expression "dB" is
used to denote asignal level, it is referenced to
0dB =0.775 volts rms.
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1.2 SPECIFICATIONS
ELECTRICAL:
INPUT; Balanced bridging, differential amplifier.
INPUT IMPEDANCE: 40 kohms ,used as balanced input.
20 kohms, used as unbalanced
(single-ended) input.
MAXIMUM INPUT LEVEL; +20 dB (7.75 volts).
EQUIVALENT INPUT NOISE: Less than -90 dB. Typically -94 dB;
15.7 kHz bandwidth with all controls set
flat, output terminated with 600 ohm
load
.
GAIN: from unity to +20 dB, ±1 dB.
FREQUENCY RESPONSE: ±1 dB, 20-20,000 Hz (with no EQ)
.
OUTPUT: Floating, transformer isolated.
OUTPUT LOAD: 150 ohms or greater.
POWER OUTPUT: +24 dBm into 600 ohm load (12.28 volts),
+26 dBm into 150 ohm load (7.75 volts).
DISTORTION: Less than 0.5% THD, 30 Hz to 15 kHz at
maximum rated output.
POWER REQUIREMENTS: 100-125 VAC or 200-250 VAC, 50/60 Hz,
switch selectable, less than 10 W.
ENVIRONMENT: Operating 0°C to +50°C (+32°F to +122°F);
storage -20°C to +60°C (-4°F to +140°F)
.
FILTER TYPE: 27 individual L-C type active filters.
FILTER BANDWIDTH: 1/3-octave at -3dB points with 8dB
boost or cut.
CENTER FREQUENCIES: Standard ISO (Hz) 40 50 63 80 125 160 200
250 315 400 500 630 800 IK 1.25K 1.6K 2K
2.5K 3.15K 4K 5K 6.3K 8K lOK 12. 5K 16K.
FREQUENCY ACCURACY: ±3% of center frequency.
RANGE OF BOOST AND CUT: 0to ±12 dB, single filter section.
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CALIBRATION OF
BOOST AND CUT: Within 1dB of indicated setting,
0to ±5 dB,
Within 1.5 dB of indicated setting,
±5 to ±12 dB, single filter section.
1.3 CONTROLS
:
EQUALIZATION: 27 vertical slide controls, continuously
variable 0to ±12 dB.
GAIN: Screwdriver adjustable from unity to
+20 dB.
EQ IN/OUT: Front panel toggle switch.
POWER: Front panel toggle switch.
INDICATORS: LED, Power ON;
LED, Overload Condition, levels monitored
at 4circuit points;
MAINS VOLTAGE: Rear panel slide switch, 115/230 volts.
CONNECTIONS
:
Input and output, through rear chassis
barrier strip;
Power through 3-wire power cable.
1.4 PHYSICAL:
DIMENSIONS: 4tt3 X89 mm rack panel; Depth behind panel
203 mm. (19" x3-1/2" x8")
FINISH: Panel is 3.18 mm (1/8") brushed clear
anodized aluminum in two shades.
Chassis is cadmium plated steel.
WEIGHT: 5.00 kg (11 pounds).
SHIPPING WEIGHT: 7.23 kg (16 pounds).
ACCESSORIES: Model SC-1 Security Cover, smoke gray
transparent plastic; covers all operating
controls.
1.5 CONNECTIONS:
All connections for
barrier strip at the rear
structions. Section 2.5.,
input and output are made through a
of the chassis. (See Installation In-
Figures 2and 3.)
-3-

SECTION II
INSPECTION AND INSTALLATION
2.1
UNPACKING AND INSPECTION
Your Model 537 was carefully packed at the factory, and the
container was designed to protect the unit from rough handling.
Nevertheless, we recommend careful examination of the shipping
carton and its contents for any sign of physical damage which
could have occurred in transit.
If damage is evident, do not destroy any of the packing mater-
ial or the carton, and immediately notify the carrier of apossible
claim for damage. Shipping claims must be made by the consignee.
The shipment should include:
Model 537 Graphic Equalizer
UREI Instruction Manual (this book)
Two-part Warranty Card bearing the
same serial number as the Model 537.
Rack mounting hardware.
2.2
ENVIRONMENTAL CONSIDERATIONS
The system will operate satisfactorily over arange of ambient
temperatures from 0°C to +50°C (+32°F to 122°F), and up to 80%
relative humidity.
If the system is installed in an equipment rack with high heat
producing equipment (such as power amplifiers), adequate ventilation
should be provided in order to assure longest component life.
Also, while circuitry susceptible to hum pick-up is sufficiently
shielded from moderate electromagnetic fields, installation should
be planned to avoid mounting the system immediately adjacent to
large power transformers, motors, etc.
2.3
POWERING
The 537 may be operated from either 100-125 VAC or 200-250
VAC mains (50 or 60 Hz, single phase.) As indicated in section
2.4, the nominal line voltage may be selected with arear panel
switch. BE SURE TO VERIFY BOTH THE ACTUAL LINE VOLTAGE, AND THE
SETTING OF THE VOLTAGE SELECTOR SWITCH BEFORE CONNECTING THE
537 TO THE MAINS.
To comply with most Electrical Codes, the 537 is supplied
with athree-wire AC cord, the grounding pin of which is connected
to the chassis. In some installations this may create ground-loop
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problems. If ground problems are experienced, check for the
possibility of ground loops by using a3-prong to 2-prong AC
adapter. This ungrounds the Model 537, and will sometimes cure the
problem, but is not asubstitute for proper grounding. Be aware
that unless the Model 537 mainframe is properly grounded, asafety
hazard can exist. UREI accepts no responsibility for legal actions
or for direct, incidental or consequential damages that may result
from violation of any electrical codes.
2.4 LINE VOLTAGE SWITCH
Unless atag on the line cord specifies otherwise, the
Model 537 was shipped ready for operation with nominal 115 VAC
power mains. In order to change this for nominal 230 V(50 or
60 Hz), slide the VOLTAGE SELECTOR switch on the rear panel to
the 230 position. The voltage is visible in awindow next to the
switch slot. Be sure to change the fuse to the correct value:
1/8-amp slo-blo when changing to 230 Voperation or 1/4-amp slo-
blo for 115 Voperation. Asmall screwdriver may be used to move
the recessed switch.
2.5EXTERNAL CONNECTIONS
Permanent input and output signal wires should be shielded
cable, and connected in accordance with standard wiring practice,
as indicated on the rear panel barrier strip.
If the Model 537 output is connected to ahigh impedance
circuit, we recommend shunting the ''±" and "COM" output terminals
with a620 ohm, 1/2 watt resistor. This assures optimum loading
of the output stage. (See section 2.6 regarding input termination).
(See Figures 2&3, next page)

OUTPUT GRD INPUT
FIGURE 2. CONNECTING THE MODEL 537 WITH BALANCED
INPUT AND BALANCED OUTPUT CIRCUITS.*
FIGURE 3. CONNECTING THE MODEL 537 WITH UNBALANCED
INPUT AND UNBALANCED OUTPUT CIRCUITS.*
*With abalanced input and unbalanced output, or vice-versa,
use the appropriate connections suggested by each of the above
diagrams. No special switching or transformers are needed.
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2.6 IMPEDANCE AND TERMINATION
Audio engineering had its roots in the telephone industry, and
"600 ohm circuits" (together with their predecessors, "500 ohm
circuits") are carry-overs from telephone transmission practices.
Long audio transmission lines, like their video counterparts, must
be properly sourced from and terminated in equipment which matches
their characteristic impedance, if optimum frequency response and
noise rejection are to be achieved.
However, transmission line theory and techniques are not
only unnecessary but impractical within modern recording studios,
broadcast studios and other local audio systems where transmission
circuits are seldom more than several hundred feet in length. The
advent of negative feedback circuitry and solid-state electronics
has spawned modern audio amplifiers and other signal processing
devices having source impedances of only afew ohms. They are
essentially indifferent to load impedances and by varying their
output current inversely to changes in load impedance, maintain
the same output voltage into any load impedance above arated
minimum, with no change in frequency response. Most new designs
(all at UREI) have high input impedance to allow use with various
source impedances.
Most modern audio systems, therefore, utilize amplifiers and
other active devices which have very low output impedances and high
(lOK to 50K) input impedances. These products may thus be cascaded
(operated in tandem), or many inputs may be connected to asingle
output of apreceeding device, without regard to impedance "matching"
Switching, patching, etc. is simplified because "double loads" and
"unterminated" bugaboos are essentially eliminated. "Floating"
(ungrounded) transformer outputs minimize ground loop problems, and
differential transformerless input circuitry (or input transformers)
minimize common mode noise or interference which may be induced
into the interconnecting wires or cables, or produced by different
ground potentials.
Where audio must be transmitted through cables or wire pairs
of more than several hundred feet in length, however, transmission
line termination practices should still be observed.
The Model 537 has an input impedances of 40,000 ohms when used
in abalanced, differential input configuration, and 20,000 ohms
when used unbalanced (one side grounded). This makes the equalizer
suitable for use with any normal source impedance, low or high.
Only when it is used from asource which requires alow impedance
termination (such as a600-ohm transmission line or older vacuum-
tube equipment) is asource termination resistor required at the
537 input.
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2.7 SIGNAL LEVEL AND OVERLOAD DETECTOR
The Model 537 's differential input amplifiers are capable of
being driven by signals up to alevel of +20 dB (Ref. 0.775 Vrms),
above which clipping and distortion occurs. The overload LED turns
on when peak signals exceed the clipping level, and it will remain
on long enough to be seen even if the excess signal is only abrief
transient. To avoid audible distortion, when the LED indicator
flashes more than occasionally, either lower the output level of
the source feeding the equalizer, or use an external attenuation
pad
.
Because of the possible combination of control settings, the
overload is monitored at several points in the circuit;
1. Input amplifier (maximum signal level +20 dB,
ref .0.775V rms )
.
2. Filter amplifiers, which could be driven into clipping as
aresult of boost equalization.
3. Output amplifier, which is capable of delivering +24 dBm
into a600-ohm load (12.3 Vrms), or +26 dBm into a150-ohm
load (7.75 Vrms )
.
2.8 ACCESSORIES
Asecurity cover may be installed over the 537 front panel.
This protects all controls from unwanted adjustments. Only the power
switch remains accessible. The smoke gray plexiglas cover is trans-
parent enough to allow viewing of the control settings.
-8-

SECTION III
OPERATING INSTRUCTIONS
3.1
GENERAL
After the Model 537 Graphic Equalizer has been installed
and is connected to both the signal source and the succeeding
equipment according to Section II, power may be switched ON. Set
all controls to aminimum, i.e.:
Gain Control —Counterclockwise, which is unity gain.
All Frequency Controls —To the zero line of the front panel,
producing no effect on the frequency
response
.
3.2
SYSTEM CHECK
Apply program material and monitor the output signal while the
EQ switch is in the OUT position. The signal should be passed
through the Model 537 without loss or gain in amplitude, or
frequency alteration. No change should be noticeable when the EQ
switch is set to the IN position.
3.3
EXPERIMENTATION
To become familiar with the range and effect of the individual
filters and their almost unlimited number of combinations, it is
helpful to "play" with the controls and listen to the change in
some familiar program material. The EQ switch is very useful to
make an instant A-B comparison while altering adjustments.
3.4
DOCUMENTATION
The very nature of a27 1/3-octave graphic equalizer will
result in many different control settings. For later duplication
of acertain sound, or of acorrective filter shape, it is necessary
to document the position of the controls (frequency, boost and/or
cut, and gain). This is made convenient by the graphic layout of
the Model 537' sfront panel and its calibrated markings. In
addition, afrequency curve may be drawn with asuitable sweep
generator and X-Y recorder system, such as the UREI Model 200/2000.
3.5
APPLICATION
It will be helpful to study the editorial in the following
paragraphs. Although the section is not intended to be acomplete
textbook, it shares some of the experiences we have gained during
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the years our equalizers have been in use. Also, it should serve
as astimulant to experiment with the instrument. We believe that
it is of primary importance for the user to know and understand the
equipment and its capabilities. Some sections may not be of
interest at this time, however it is beneficial to read through the
entire text.
As in any other field, "practice is the key to success." Just
as it is not reasonable to expect to get beautiful music out of a
musical instrument the first time you play on it, it is also not
reasonable to expect to immediately master the techniques necessary
to adjust the Model 537 for the desired sound the first time you
work with it.
3.5.1. OBJECTIVE MEASUREMENTS vs .SUBJECTIVE HEARING
Since the hearing process is subjective, each individual
will have adifferent opinion of what constitutes "good sound" or
"bad sound". Until science has found all the answers about the
phenomenon of hearing, we will have to use the available tools to
modify sound with acertain imprecision. This lack of ability to
quantify the relationship which exists between the subjective
response of the listener, and the objective measurements of
ampl itude-vs-frequency response, phase, distortion, etc., need not
be abarrier to our efforts to "improve" the sounds that we hear.
We have amost sophisticated quality assessment mechanism
which is specifically suited to do this job... the human ear.
Therefore, the educated listener with good hearing will probably
always be the standard of reference to decide if asound has been
accurately reproduced. However, accurate faithful reproduction
is not always the goal, and in many circumstances it may not even
be possible or desirable.
3.5.2. ADDITIONAL HINTS
The Model 537 Graphic Equalizer is apowerful tool for the
audio engineer to modify, adjust, alter, enhance, destroy or
reclaim the audio signal. The extreme result would be asound
that never existed before. In such instances, there are very
few rules for what to do and what not to do with the equalizer
controls ... in creative equalization whatever works is right.
It is good to begin the work in acontrolled environment
where there is enough time to experiment, and the freedom to
do it "wrong" without offending someone else. Doing it the
wrong way means to know what does not work! Start out with
simple program material of various types and try to make it
sound "better." Different program material will require dif-
ferent control settings.
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Extreme filter settings are generally to be avoided. Of course
special effects and corrective equalization of program material
which must be saved may require such settings. The important thing
is to avoid creating anew problem while correcting another.
Whenever abig notch in the frequency response of the filter is
selected, it can cause important parts of the program material to be
removed. It is possible to make adjacent notes on the musical scale
differ considerably in level. If that's what is desired, fine.
However, if you are trying to correct for problems which would better
be dealt with by choice of microphone, microphone positioning,
acoustic control, etc., you may run into trouble. The 537 (or any
other such device) is not alifesaver in an ocean of problems.
3.5.3. SPECTRAL DISTRIBUTION IN THE AUDIO SIGNAL
It may seem obvious, but we'll point it out anyway; amajor
advantage of agraphic equalizer is to be able to see at aglance
what the instrument is doing. The shape of the filter response is
apparent immediately from the positions of the slide controls. This
information, together with the knowledge of the spectral distribution
of the program material, is very advantageous in arriving at a
desired frequency response.
NOTE: The UREI Model 970 Vidigraf™ is an especially useful
tool to view the program spectrum in arecording studio
situation. It allows the operator to monitor the signal from
aprogram source and to determine visually the relative levels
of frequency bands.
We can arbitrarily divide the audible frequency spectrum into
four sections: bass, lower midrange, upper midrange, and high
frequencies. When we are concerned with wide bandwidth program
material, we should remember that each of these frequency bands is
important for its contribution to afull and balanced sound. Lack
of bass frequencies will make the sound thin, while too much will
make it boom. The lower midrange, which provides much of the rich
and mellow texture to music, may make the program material sound
muffled if too abundant. The upper midrange is necessary for
clarity and articulation, but gives the program a"canned" quality
if overused. The high frequencies in excess create aharsh sound,
but if balanced, give the audio program aspaciousness which it
would not otherwise have.
3.5.4. EQ SYSTEMS AND LIMITERS
The use of acompressor or limiter in the audio chain is
frequently dictated by the system's headroom restraint, the dynamic
range of program material, or aesthetic reasons. The question then
arises, "do Iput the limiter before or after the equalizer?"
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The answer is that generally the equalizer should be ahead of the
limiter in the chain.
The equalizer, by its very nature, changes the frequency
ance and harmonic relationship of the input signal. Frequencies
that were greatest in amplitude at the input may not be so at the
output. Conversely, some frequencies may be at ahigh level at
the output due to the equalizer's action. The limiter, which
is actuated by the level of the signal, should see the equalized
or balanced signal.
The use of the equalizer prior to the limiter offers afurther
advantage, i.e., the program material may be equalized to optimize
the limiter output by adjusting the input signal to the limiter.
As an example, if arecording has alarge dynamic range and an
excess of low frequencies, the equalizer would be able to reduce
the level of the low frequencies fed to the limiter, thereby
reducing the amount of limiting needed. It also enables the
user to select afaster attack and release time on the limiter
if needed for peak limiting. The excessive low frequencies would
have restricted the speed at which the limiter could operate and
still achieve the desired low distortion.
There are afew instances where the equalizer might be placed
after the equalizer, and that would be in asystem where extreme
uncontrollable dynamics are expected. These might otherwise
overload the Model 537. However, these cases are very rare, and
the exceptionally wide dynamic range of the 537 makes this
situation even more unlikely.
3.5.5. ROOM EQUALIZATION
Just ashort note on the use of the Model 537 in corrective
room equalization. If applied judiciously and with caution, the
equalizer may prove to be successful in these applications. For
example, athorough analysis of the room's frequency response is
necessary, boost of frequencies and extreme filter settings must
be avoided, etc. Instead of the Model 537, we recommend adedicated
instrument such as aUREI Model 539 Room Equalizer when working
with acoustic problems.
3.6 GAIN STRUCTURE AND SIGNAL-TO-NOISE
Only the correct gain structure throughout the entire audio
system will enable the user to take advantage of the optimum
signal-to-noise ratio built into the individual components. In
this discussion, the matching between the equalizer and afollowing
power amplifier is considered.
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The Model 537 was designed for an average signal level at
its input of approximately 0dB (Ref. 0.775 Vrms ).This guarantees
20 dB of headroom; that is, peaks may be 20 dB above the average
level before any clipping could occur. Residual output noise from
the Model 537 is 90 dB below the average signal level, or 110 dB
below maximum output level, and therefore inaudible.
The specification of the power amplifier may state an input
sensitivity of 0.775 volts (0 dB) for maximum output power. This
means that any signal level which is greater than 0dB causes
overload of the amplifier. As stated before, the maximum output
level from the equalizer may have peak values of approximately
+20 dB. Obviously, the equipment is not properly matched!
Two solutions shall be discussed:
(1) We reduce the input level to the equalizer to -20 dB.
Since it is aunity gain device, its output is also
-20 dB average signal level. This value is safe to
provide us with 20 dB of headroom in the power amplifier.
At the same time, however, we reduce our signal-to-noise
ratio. As was mentioned before, the residual noise from
the equalizer is 90 dB below a0dB reference level. If
our average signal level is -20 dB, only 70 dB of
signal-to-noise ratio remains, which may be adequate,
but is alimitation.
(2) Abetter solution would be to leave the average signal
level through the equalizer at the optimum level of 0dB,
but add a20 dB attenuator between its output and the
power amplifier's input. This reduces the average signal
level and the residual noise from the equalizer (and
other possible sources). The original 90 dB below
average signals or 110 dB below peak signals is main-
tained. In many cases the gain control of the power
amplifier may be used as the suggested attenuator (so
long as an input stage in the power amplifier will not
be overdriven)
.
3.7 INPUT LEVEL
If the equalization controls are in the flat position or
attenuated, no special care is necessary with regard to input
signal level other than keeping it below +20 dB (Ref. 0.775 Vrms).
However, if any of the controls are adjusted for boost, then
it must be remembered that at the frequencies affected by that
control the gain of the amplifier is greater than unity, and the
maximum input signal level which is allowable at those frequencies
is reduced by the amount of boost at those frequencies.
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Por examplef if the 400 Hz control is set for 5dB of boostf an
input signal level of +15 dB at 400 Hz would result in an output
signal level of +20 dB. Because +20 dB is the maximum capability
of the filter amplifier, the maximum input level permissible
at 400 Hz without overload has become +15 dB. At moderate amounts
of boost, this should not present aproblem, but if, for example,
all controls are in full boost, some frequencies will be boosted
as much as 15 to 16 dB due to the combining effect of adjacent
filters. This would bring the maximum input level down to +4 or +5
dB, and under such circumstances, the input level should be reduced
to asafe level to avoid overload.
3.8 OVERLOAD INDICATOR
Overload conditions are monitored throughout the equalizer.
The LED flashes ON when peak signals exceed the clipping level in
any part of the circuit. It will remain ON long enough to be seen
even if the excess signal is only abrief transient. To avoid
audible distortion, when the LED indicator flashes more than
occasionally, the signal level must be lowered.
Alistening test may show that the LED flashes before
noticeable distortion is audible. This may be due to
high amplitude, short duration transients, and you may never hear
the resulting overload, depending on program material, listening
conditions, and yes, even your own ears. Therefore the overload
LED, like all such indicators, should not be ignored.
-14 -

SECTION IV
THEORY OF OPERATION
4.1 INPUT AMPLIFIER
The signal is applied to adifferential input amplifier (IC-1,
sections Aand B) .The input accepts either balanced or unbalanced
sources (see also Installation 2.5 and 2.6). Common mode rejection
is typically better than 60 dB.
The factory set offset adjustment of R-3 assures that no DC
voltage is at the output of the following amplifier stage, thus
preventing audible clicks when operating the EQ IN-OUT switch. The
gain of the input amplifier is -6 dB.
4.2 ONE-THIRD OCTAVE FILTERS
There are 27 individual L-C networks in the equalizer. They
are divided into four groups. Two groups each are designed to be
the feedback/feedforward circuit around amplifiers IC-2, sections
Cand D. Figure 4(next page) shows asimplified schematic of the
circuit.
Since resistors Rd and Re are of equal value, the op amp
functions as aunity gain inverting amplifier. The boost-cut
potentionmeter is shown as two separate resistors, Ra and Rb, with
point "M” denoting the electrical midpoint of the pot.
The potentiometer is fed by two signals which are equal in
amplitude but 180° out of phase. Therefore the voltage at point
"M” is zero. The wiper of the potentiometer is connected to the
series resonant circuit consisting of Rc, C, and L. When the
wiper is at point "M” ,no signal flows through the series circuit.
This is the 0dB position of the 1/3-octave control on the front
panel
.
Referring to Figure 4(next page) ,as the wiper is moved to
the right, increasing negative feedback is applied to the op amp
for those frequencies which are passing through the tuned circuit,
selectively reducing the gain of the amplifier. The result is the
desired attenuation in the equalizer's frequency response.
Conversely, if the wiper is moved to the left, more input signal
is selectively fed to the op amp, increasing the gain at the
resonant frequency of the tuned circuit. Adjacent filter sections
combine smoothly without unwanted characteristics (excessive
ripple, phase shift, etc.) The potentiometers are fed through
isolating buffer amplifiers (IC-3 and IC-4). In addition, phase
correcting networks (R-42,43, C-25, etc.) ensure excellent loop
stability and good transient response.
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FIGURE 4. SIMPLIFIED 1/3-OCTAVE FILTER SECTION.
4.3 EQ IN-OUT SWITCH
Aloss pad of 4.26 dB at the output of the filter section
matches the signal level to the output of the isolation amplifier
(IC-l-C). The result is that no level change occurs when switching
the equalizer section IN or OUT if none of the frequency controls
are adjusted for boost or cut. In the EQ IN position, the signal
is coupled through C-14 and R-24 into the output amplifier. In the
EQ OUT position, the output of the isolation amplifier (IC-l-C) is
also connected to the output amplifier. However, since the output
impedance of IC-l-C is very low, the signal from the filter section
is effectively short circuited.
4.4 OUTPUT AMPLIFIER
The output stage is ahigh performance op amp (IC-5) which
drives acomplementary pair of output transistors. The gain is
adjustable between +1 dB and +21 dB, depending on the ratio of
R-26, 27 &28. The output transformer has again of 3dB when
terminated with a600 ohm load, thus restoring unity gain through
the entire Model 537 Equal izer# when the gain is set to minimum.
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4.5 OVERLOAD DETECTOR
Under normal signal level conditions, the output of IC-2-B
is positive due to the negative bias at its inverting input. Thus,
the LED is turned OFF. If the signal levels from the input amplifier
through diode CR-10, from IC-2-C through CR-13 and R-36, from IC-2-D
through CR-14 and R-36, or from the output amplifier through CR-11
and R-36 exceed the threshold set by the ratio of R-35 and R-37, the
overload detector amplifier changes its output state for approxi-
mately 0.1 seconds, and the LED indicates the overload condition.
The pulse-stretching network included in the positive feedback
loop of the amplifier ensures that the LED remains ON long enough to
be observed, even though the overload may have been caused by avery
short duration signal peak.
4.6 POWER SUPPLY
The power supply is bipolar, employing two integrated circuit
voltage regulators VRl and VR2 to provide low-ripple, ±18 volt DC.
Additional filter capacitors assure power supply stability and
low noise.
The pilot LED is connected to the positive side of the
power supply to indicate power ON condition.
-17-

SECTION V
MAINTENANCE
5.1
GENERAL
The Model 537 is an all solid-state unit, ruggedly constructed
of only the highest quality components, and as such, it should
provide years of trouble free use with normal care. All parts used
are conservatively rated for their application, and workmanship
meets the rigid standards you have learned to expect in UREI
products
.
NO SPECIAL PREVENTIVE MAINTENANCE IS REQUIRED.
5.2
REPAIRS &V7ARRANTY
This product is factory warranted to the original purchaser
against defects in material and workmanship for one year after
initial purchase. Save your sales slip for proof of warranty if
it should be necessary. Should amalfunction ever occur, the
dealer from whom the unit was purchased will be glad to handle
return for factory repair. Please call or write to the factory for
aReturn Authorization Number which must accompany all repairs .
For prompt service ship the unit prepaid directly to the factory with
the RA Number visible on the shipping label .Be sure it is well
packed in asturdy carton, with shock-absorbing material such as
foam rubber, styrofoam pellets or "bubble-pack" completely filling
the remaining space. Particular attention should be paid to
protecting the controls, switches, etc. Tape anote to top of the
unit describing the malfunction, and instructions for return. We
will pay one-way return shipping costs on any in-warranty repair.
Because of specially selected components in this product,
field repairs are not authorized during the warranty period, and
attempts to perform repairs may invalidate the warranty.
5.3
SERVICE ADJUSTMENTS
These controls have been carefully set at the factory and
should not require adjustments except after service work.
5.3.1 OFFSET ADJUSTMENT
The internal trimpot R— 3affects this adjustment. It should
only be necessary to change the factory setting if IC-1 is replaced.
The adjustment is correct when the DC output of IC-l-C is 0VDC.^^
Since there is no offset voltage at the input of IC-5, no "click
will be generated when the EQ switch is operated.
NOTE: If the offset control is severely misadjusted, large
voltage pulses are generated in the equalizer when the EQ
switch is operated. This could overload or damage unprotected
equipment which may be connected to the output of the Model
537. UREI cannot be responsible for consequential damages
due to misadjustment of this control.
1r\_ o—