Universal Audio 533 User manual

BEFORE PROCEEDING WITH COMPLETE UNPACKING AND SETUP
PLEASE READ THE SECTION ON UNPACKING AND INSPECTION
model 533
GRAPHIC EQUALIZER
model 535
DUAL GRAPHIC EQUALIZER
Mhd
UREI Incorporated
8400 BALBOA BOULEVARD
NORTHRIDGE, CA. 91325-4100 U.S.A.
PHONE: (818) 895-UREI (895-8734)
FAX: [phone]
© Copyright 1979, 1988 UREI Incorporated

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TABLE OF CONTENTS
PAGE
SECTION I. INTRODUCTION
1.1 DESCRIPTION 1
1.2 SPECIFICATIONS 2
1.3 CONTROLS 3
SECTION II. INSPECTION AND INSTALLATION
2.1 UNPACKING AND INSPECTION 4
2.2 ENVIRONMENTAL CONSIDERATIONS 4
2.3 POWERING 4
2.4 LINE VOLTAGE SWITCH 5
2.5 EXTERNAL CONNECTIONS 5
2.5.1 MODEL 301 XLR/QG ADAPTER 6A
2.6 IMPEDANCE AND TERMINATION 7
2.7 SIGNAL LEVEL AND OVERLOAD DETECTOR 8
2.8 ACCESSORIES 8
2.9 RACK MOUNTING A MODEL 533 8
2.10 RACK MOUNTING TWO UNITS (MODEL 533) 9
SECTION III. OPERATING INSTRUCTIONS
3.1 GENERAL 11
3.2 SYSTEM CHECK 11
3.3 EXPERIMENTATION 11
3.4 DOCUMENTATION 11
3.5 APPLICATION 11
3.5.1 OBJECTIVE MEASUREMENTS VS SUBJECTIVE HEARING 13
3.5.2 ADDITIONAL HINTS 13
3.5.3 SPECTRAL DISTRIBUTION IN THE AUDIO SIGNAL 14
3.5.4 EQ SYSTEMS AND LIMITERS 14
3.6 DISCUSSION OF EQUALIZATION 15
3.6.1 BEFORE EQUALIZATION 17
3.6.2 MEASUREMENT TECHNIQUES 18
3.6.3 ADJUSTMENT OF EQUALIZERS 19
3.6.4 SOME PROBLEMS WITH CORRECTIVE ROOM EQ 19
3.6.5 FEEDBACK SUPPRESSION 20
3.7 GAIN STRUCTURE AND SIGNAL-TO-NOISE 21
3.8 INPUT LEVEL 22
3.9 OVERLOAD INDICATOR 22
SECTION IV. THEORY OF OPERATION
4.1 GENERAL
4.2 INPUT AMPLIFIER
4.3 GAIN AMPLIFIER
4.4 GRAPHIC EQUALIZER
4.4.1 GENERAL
4.4.2 FILTERS
2323
23
23
23
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TABLE OF CONTENTS (Continued)
4.4.3 THE ACTIVE INDUCTOR
4.5 OUTPUT AMPLIFIER
4.6 OVERLOAD INDICATOR
4.7 POWER SUPPLY
PAGE
25
26
26
26
SECTION V. MAINTENANCE
5.1 GENERAL 27
5.2 REPAIRS AND WARRANTY 27
5.3 SERVICE ADJUSTMENTS 27
5.3.1 COMMON MODE BALANCE 27
5.3.2 OFFSET ADJUSTMENT 28
5.4 CLEANING 28
5.5 ON OP AMPS 29
SECTION VI. ILLUSTRATION
SCHEMATIC DIAGRAM OF UREI MODEL 533
SCHEMATIC DIAGRAM OF UREI MODEL 535

SECTION I
INTRODUCTION
FIGURE 1. MODEL 535 FRONT PANEL.
NOTE: This manual contains complete instructions for the operation
of the two-channel "Dual Graphic Equalizer" Model 535.
However, this manual is also intended for the single channel
"Graphic Equalizer" Model 533. If used with a Model 533, all
references to the two-channel unit should be disregarded.
1.1 DESCRIPTION
The Model 535 Dual Graphic Equalizer is a studio quality
product which has wide application in recording, sound reinforcement,
radio and TV production, and high fidelity music systems.
The system is designed to operate at any nominal level from as
low as -20 dB (Ref. 0.775 V) allowing its use with low level
mixers, high or low impedances, and balanced or single-ended
circuits. The Model 535 provides 12 dB of boost or cut at each of
its 10 frequencies which are centered at ISO 1 -octave increments
from 31.5 Hz to 16 kHz. The positions of the control arms of the
vertical, stepless controls provide a graphic representation of the
response curve selected.
The filters are active, minimum phase, 2 -pole synthesized LC
networks, whose skirts properly combine for minimum ripple and
phase shift when used in combination. A front panel control
adjusts the gain from -10 dB to +20 dB. Bypass switches permit
switching the equalization out for A-B tests. An optional (SC -1)
security cover may be installed to protect all operating controls
against inadvertent disturbance or tampering in fixed installations.
-1-

1.2 SPECIFICATIONS
ELECTRICAL (EACH CHANNEL):
INPUT: Balanced
INPUT IMPEDANCE:
MAXIMUM INPUT LEVEL:
EQUIVALENT INPUT NOISE:
GAIN:
FREQUENCY RESPONSE:
OUTPUT:
OUTPUT LOAD:
POWER OUTPUT:
DISTORTION:
FILTER TYPE:
FILTER BANDWIDTH:
CENTER FREQUENCIES:
FREQUENCY ACCURACY:
RANGE OF BOOST AND CUT:
CALIBRATION OF BOOST
AND CUT:
bridging, differential amplifier.
40 kohms, used as balanced input.
20 kohms, used as unbalanced
(single-ended) input.
+20 dB (7.75 volts).
Less than -90 dBm. 15.7 kHz noise bandwidth
with all controls set flat, output termina-
ted with 600 ohm load (Model 535, -88 dBm).
Variable, -10 dB to +20 dB, with front
panel control.
+1 äuß 20-20,00n Hz (with no EQ).
Floating, transformer isolated.
150 ohms or greater.
+24 dBm into 600 ohm load (12.28 volts),
+20 dB into 150 ohm load (7.75 volts).
Less than 0.25% THD, 30 Hz to 15 kHz at
maximum rated output.
2 -pole synthesized L -C.
1 -octave at -3 dB points with 8 dB
boost or cut.
Standard ISO (Hz) 31.5 63 125 250 500
lk 2k 4k 8k 16k.
±3% of center frequency.
12 dB, single filter section.
Within 1 dB of indicated setting, 0 to
±6 dB, within 1.5 dB of indicated
setting, ±6 to ±12 dB, (single
filter section).
CHANNEL SEPARATION: 60 dB worst case at high frequencies,
typically better than 80 dB.
1MlISCELLANEOUS:
POWER REQUIREMENTS: 100-125 VAC or 200-250 VAC, 50/60 Hz,
switch selectable, less than 10 W.
-2-

ENVI RONMENT :
CONNECTIONS:
Operating 0°C to +50°C (+32°F to +122°F);
storage -20°C to +60°C (-4°F to +140°F).
Input and output, through rear chassis
barrier strip, or XLR/QG connectors
through Model 301 accessory adapter;
Power through 3 -wire power cable;
Model 533 utilizes IEC style connector
and power cord. (See Installation
Instructions, Section 2.5.)
PHYSICAL CHARACTERISTICS:
DIMENSIONS: Model 535, 483 x 89 mm rack panel; depth
behind panel 203 mm. (19" x 3-1/2" x 8");
Model 533, 216 x 89 mm rack panel; depth
behind panel 190 mm (8-1/2" x 3-1/2" x
7-1/2").
FINISH: Panel is 3.18 mm (1/8") brushed clear
anodized aluminum, in two shades.
Chassis is cadmium plated steel.
WEIGHT: Model 535, 4.31 kg (9.5 pounds);
Model 533, 2.95 kg (6.5 pounds).
SHIPPING WEIGHT: Model 535, 6.35 kg (14 pounds);
Model 533, 4.08 kg (9 pounds).
ACCESSORIES: Model SC-1 Security Cover, smoke gray
transparent plastic; covers all operating
controls (Model 535 only).
1.3 CONTROLS:
EQUALIZATION:
Model 301 XLR/QG adapter; two needed for
Model 535. One adapter provides connec-
tions for signal input and output.
Vertical slide controls, continuously
variable 0 to ±12 dB.
GAIA: Screwdriver adjustable from -10 dB 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 3 circuit points;
MAINS VOLTAGE: Rear panel slide switch, 115/230 volts.
-3-

SECTION II
INSPECTION AND INSTALLATION
2.1 UNPACKING AND INSPECTION
Your Model 535 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 a possible
claim for damage. Shipping claims must be made by the consignee.
The shipment should include:
Model 535 Dual Graphic Equalizer
UREI Instruction Manual (this book)
Two-part Warranty Card bearing the
same serial number as the Model 535.
Mounting screws and finish washers
(for rack mounting of 535 only).
2.2 ENVIRONMENTAL CONSIDERATIONS
The system will operate satisfactorily over a range 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 together with
high heat producing equipment (such as power amplifiers), adequate
ventilation should be provided to prolong the life of components.
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 535 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 a rear panel
switch. BE SURE TO VERIFY BOTH THE ACTUAL LINE VOLTAGE, AND THE
SETTING OF THE VOLTAGE SELECTOR SWITCH BEFORE CONNECTING THE
535 TO THE MAINS.
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To comply with most Electrical Codes, the 535 is supplied
with a three-wire AC cord, the grounding pin of which is connected
to the chassis. In some installations this may create ground -loop
problems. Ground loops can become very evident (as hum and buzz)
if a significant potential difference exists between the AC conduit
ground and the grounded metal enclosure in which the chassis is
installed. If hum is experienced, check for the possibility of
ground loops by using a 3 -prong to 2 -prong AC adapter, ungrounding
the AC plug temporarily. This ungrounds the Model 535, and will
probably cure the hum or buzz, but is not a substitute for proper
system grounding. Be aware that unless the Model 535 Dual Graphic
Equalizer is AC grounded, a safety 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 a tag on the line cord specifies otherwise, the
Model 535 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 a window next to the
switch slot. Be sure to change the fuse to the correct value:
1/8 -amp slo-blo when changing to 230 V operation or 1/4 -amp
slo-blo for 115 V operation. A small screwdriver should be
used to move the recessed switch.
2.5 EXTERNAL 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 XLR/QG
connectors are to be used, install the accessory Model 301
according to the instructions suppplied with the adapter.
(See also "Mounting Instructions," page 6A.)
If the Model 535 output is connected to a high impedance
circuit, we recommend shunting the "±" and "COM" output terminals
with a 620 ohm, 1/2 watt resistor. This assures optimum loading
for the 535. (See section 2.6 regarding input termination).
(See Figures 2 and 3 on the following page)
-5-

OUTPUT GRD INPUT
± COM COM ±
10401
OUTPUT / INPUT
/ OPTIONAL
FIGURE 2. CONNECTING THE MODEL 535 WITH BALANCED
INPUT AND BALANCED OUTPUT CIRCUITS.*
OUTPUT GRD INPUT
± COM COM ±
OUTPUT INPUT
FIGURE 3. CONNECTING THE MODEL 535 WITH UNBALANCED
INPUT AND UNBALANCED OUTPUT CIRCUITS.*
*With a balanced 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.
-6-

2.5.1 MODEL 301 XLR/QG ADAPTER
Pgg

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 a few 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 a rated
minimum, with no change in frequency response.
Modern audio systems, therefore, utilize amplifiers and other
active devices which have very low output impedances and high
(10k to 50k) input impedances. These products may thus be cascaded
(operated in series), or many inputs may be connected to a single
output of a preceding 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.
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 535 has input impedances of 40 kohms (40,000 ohms)
when used in a balanced, differential input configuration, and 20
kohms (20,000 ohms) when used unbalanced (one side grounded). This
makes the 535 suitable for use with any nominal source impedance,
low or high. Only when it is used from a source which requires a
low impedance termination (such as a 600 -ohm transmission line or
older vacuum-tube equipment) will a source termination resistor be
required at the 535 input.
-7-

2.7 SIGNAL LEVEL AND OVERLOAD DETECTOR
The Model 535's differential input amplifiers are capable of
being driven by signals up to a level of +20 dB (Ref. 0.775 V rms),
above which clipping and distortion occurs. The overload LED turns
on when peak signals approach the clipping level, and it will remain
on long enough to be seen even if the excess signal is only a brief
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. 0.775 V rms).
2. Filter amplifiers. which could be driven into clipping as
a result of boost equalization.
3. Output amplifier, which is capable of delivering +24 dBm
into a 600 -ohm load (12.3 V rms), or +26 dBm* into a
150-ohm load (7.75 V rms).
*NOTE: +26 dBm into 150 ohms is equivalent to +20 dB
referenced to 0.775 volts, i.e., 7.75 V rms.
2.8 ACCESSORIES
A security cover (SC -1) may be installed over the front panel
of the Model 535 only. This protects all controls from unwanted
adjustments. Only the power switch remains accessible. The smoke
gray plexiglas cover is transparent enough to allow viewing of the
control settings. The Model 301 adapter is available for the use
of XLR/QG connectors; two are needed for the Model 535.
2.9 RACK MOUNTING A MODEL 533
The SR-21 kit (optional) allows mounting of one 533 in a
standard 19" USASI rack. The unit occupies two standard spaces
(3-1/2" high). The SR-21 kit should contain the following items:
QUANTITY DESCRIPTION UREI PART NUMBER
2 Rack Adapter Angles 25-12108
2 Rack Extensions 25-12109
4 #6-32 Oval Head Screws
4 #6 Lock Washers
4 #6-32 Nuts
4 #8-32 Pan Head Screws
4 #8 Lock Washers
-8-

The SR-21 Installation Procedure is as follows:
(Refer to Figure 4)
1. Using the #6-32 screws, lockwashers, and nuts, mount the
angles to the back side of the rack extensions. Do not
tighten the screws at this time.
2. Using the #8-32 screws and lock washers, mount the angle
and rack extension assemblies to the tapped holes in the
sides of the 533. Do not tighten the screws at this time.
3. The 533 front panel is adjustable to allow optimum
alignment of the front panel with the rack extensions. For
precise alignment, loosen the front panel mounting screws, as
well as the top and bottom cover screws (1/2 turn is adequate).
Adjust the relative positions of the brackets and panels, and
then tighten all hardware. SEE SECTION 2.5 BEFORE INSTALLING
THE ASSEMBLY IN A RACK.
2.10 RACK MOUNTING TWO UNITS (MODEL 533)
The DR -21 kit (optional) allows mounting of two 533's
side by side in a standard 19" USASI rack. The assembly occupies
two standard spaces (3-1/2" high). The DR -21 kit should contain
the following items:
QUANTITY
2
2
2
4
4
DESCRIPTION
Joining Plates
Rack Mounting Angles
Spacer Plates
#8-32 Pan Head Screws
#8 Lock Washers
UREI PART NUMBER
25-11168
25-12106
25-12107
The DR-21 Installation Procedure is as follows:
(Refer to Figure 5)
1. Place the two 533's side by side, with front panels
touching. Remove four screws from the top of each unit, as
illustrated, to permit positioning of the joining plate.
Replace the screws through the plate and covers, but do not
tighten completely at this time.
2. Turn the assembly upside down, and install the bottom
joining plate in the same fashion as the top joining plate
(step 1).
-9

3. Using the #8-32 screws and lock washers, mount one angle
and one spacer plate to each side of the 533 assembly. The
spacer plates should be located between the angles and the
533 chassis. The mounting holes in the 533 are tapped,
so nuts are not required. Do not tighten the screws at this
time.
4. The 533 front panels are adjustable to allow optimum
alignment of the front panels with the rack mounting angles.
For precise alignment, loosen the front panel mounting
screws, as well as the top and bottom cover screws (1/2 turn
is adequate). Adjust the relative positions of the angles
and panels, and then tighten all hardware. SEE SECTION 2.5
BEFORE INSTALLING THE ASSEMBLY IN A RACK.
18-32 Pan Head Screw
18 Lock Washer
16-32 Nut
#6 Lock Washer
16-32 Oval Head Screw
cover screws
Angle
Rack Extension
front panel mounting screws
FIGURE 4. MOUNTING DETAILS FOR SR-21 RACK MOUNT KIT.
cover screws
18-32 Screw
18 Lock Washer
front panel mounting screws
Joining Plate
Spacer Plate
Angle
FIGURE 5. MOUNTING DETAILS FOR DR-21 RACK MOUNT KIT.
-10-

SECTION III
OPERATING INSTRUCTIONS
3.1 GENERAL
After the Model 535 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
controls:
Gain Control -- To 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 535 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 a graphic equalizer will result in many
different control settings. For later duplication of a certain
sound, or of a corrective 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
535's front panel and its calibrated markings. In addition, a
frequency curve may be drawn with a suitable 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 a complete
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 a stimulant 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 535 for the desired sound the first time you
work with it.
Some of its many uses are listed below:
(1) Recording studio frequency shaping of recorded material. The
very low output noise and high signal handling capability make
the Model 535 suitable for use in any studio situation.
(2) Reduction of proximity effect due to close-talking use of
cardioid microphones. A decrease of only 2 dB or so in the
200 to 400 Hz region may significantly improve the intelligi-
bility of what otherwise would be a very "boomy" sound.
(3) Elimination of excess high frequencies and sibilance, especially
at high sound pressure levels. A slight reduction in level at
8 kHz may eliminate much of the harshness which is experienced
when loudspeakers and amplifiers are operated close to their
power limits.
(4) Sound effect processing such as "Telephone Sound."
(5) Room equalization.
(6) Sound effects tapes for theatrical productions may be made
through the Model 535 to ensure consistency of quality of the
recorded material, and to pre-equalize for correct sound
from the particular loudspeakers to be used. If the tapes
are correctly equalized, adjustments during the show are
reduced to volume adjustments and speaker selection. If
the show goes on the road to some other auditorium, the Model
535 may be inserted in the line to account for different
acoustics while retaining the original total balance of sounds
from effect to effect.
(7) Many sound systems are required to serve rooms in which dif-
ferent acoustic environments prevail in different areas: for
example, the under balcony area of a theatre versus the
bal cony area. For such installations, use of one half of a
Model 535 for one area and the other half of a Model 535 for
another area may allow for more consistent response throughout
the room.
-12-

3.5.1. OBJECTIVE MEASUREMENTS vs. SUBJECTIVE HEARING
Since the hearing process is subjective, each individual
will have a different 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 a certain imprecision. This lack of ability to
quantify the relationship which exists between the subjective
response of the listener, and the objective measurements of
amplitude -vs-frequency response, phase, distortion, etc., need not
be a barrier to our efforts to "improve" the sounds that we hear.
We have a most 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 a sound 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 535 Graphic Equalizer is a powerful tool for the audio
engineer to modify, adjust, alter, enhance, destroy or reclaim the
audio signal. The extreme result would be a sound 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 a controlled 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 different control settings.
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 a new problem while correcting another.
Whenever a big 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 535 (or any
other such device) is not a lifesaver in an ocean of problems.
-13-

3.5.3. SPECTRAL DISTRIBUTION IN THE AUDIO SIGNAL
It may seem obvious, but we'll point it out anyway; a major
advantage of a graphic equalizer is to be able to see at a glance
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.
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 a full 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 a harsh sound,
but if balanced, give the audio program a spaciousness which it
would not otherwise have.
3.5.4. EQ SYSTEMS AND LIMITERS
The use of a compressor 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 I put the limiter before or after the equalizer?"
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
balance and harmonic relationships 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 a high 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 a further
advantage, i.e., the program material may be equalized to optimize
the limiter output by adjusting the input signal to the limiter.
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As an example, if a recording has a large 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 a faster 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 a few instances where the equalizer might be placed
after the limiter, and that would be in a system where extreme
uncontrollable dynamics are expected. These might otherwise
overload the Model 535. However, these cases are very rare,
and the exceptionally wide dynamic range of the 535 makes this
situation even more unlikely.
3.6 DISCUSSION OF EQUALIZATION
The main reason for frequency response tailoring of audio
systems is to compensate for response anomalies occurring in the
system and its environment, and to adjust the system response curve
to some desired shape. In some systems this means adjusting for
best overall flatness of response through the entire audio range,
and in other instances some different response shape is desirable.
Three important characteristics of audio systems may be
improved by judicious application of corrective equalization
techniques. They are:
(1)
(2)
DISTORTION OF FREQUENCY RESPONSE BALANCE.
Sources of sound which may be amplified through a sound
system start out with some specific frequency balance.
If, in being processed through an audio system this
balance is disturbed by variation in frequency response,
then truly the sound has been distorted. For example,
an instrument has a specific characteristic with regard
to the levels of the various harmonics compared to the
fundamental tone. If these signals are processed through
an audio system with non-linear frequency response, then
the resultant sound is simply not the distinctive sound
which that instrument orignally produced.
SUSCEPTIBILITY TO FEEDBACK.
In public address and sound reinforcement systems, the
maximum acoustic gain that may be obtained for a micro-
phone in the vicinity of a loudspeaker which is part of
that system, will be determined by the positive feedback
loop created when sound from the loudspeaker enters the
microphone, reinforcing the signal level until the
system goes into oscillation. This positive feedback
problem is aggravated if some frequencies in the
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