Universal Audio 562 User manual

UREI ELECTRONIC PRODUCTS
r/^) FEEDBACK
J\j£ SUPPRESSOR
FEATURES:
5independently adjustable notch filters.
Narrow notches minimize sound coloration.
Built in peak clipper for driver protection during setup
tuning.
High cut and low cut end filters.
Headroom indicator monitors signal levels and approach
of overload.
Bypass switch for easy setup and A-B comparison.
The Model 562 Feedback Suppressor has been design-
ed to reduce feedback in sound reinforcement systems
such as fixed and portable installations, in auditoriums,
concert halls, meeting rooms, schools and churches.
Experience with narrow band notch filters has demon-
strated that in atypical sound reinforcement system, gain
before feedback can be significantly improved by center-
ing anarrow band-reject filter on four or five of the most
prominent system resonant frequencies. It has also been
found that rolling off the low and high frequency ends of
the spectrum to pass only arequired program’s band-
width, increases gain before feedback at the band
edges. In addition, many sound systems are improved
through the application of broad or narrow band
equalization.
UREI
J-ELECTRONIC
PRODUCTS

m
The 562 contains 5active notch filters, each continu-
ously adjustable in notch depth from 0to 20 dB, and in
frequency from 60 Hz to 6000 Hz. Low cut and high cut
filters with 12 dB/octave slopes are tunable from
20 to 200 Hz and 2.5 to 20 kHz respectively. The Model
562 has front panel adjustable gain, ranging from —10 to
+20 dB and headroom indicators to optimize the signal
level. Abypass switch allows A-B comparison, and asetup
switch provides system protection during tuning. XLR/QG
connectors and terminal strips are provided for both
input and output.
In using the Feedback Suppressor, the system's gain
is slowly increased until the first sustained feedback
occurs. One notch filter is then "tuned’’ until the feedback
ceases. The gain is increased again, and the next reso-
nance attenuated with the second filter. This procedure
is repeated with filters 3through 5. If feedback occurs
at the extreme high or low end it can be reduced by
using the end cut filters. Atypical result is an increase of
several dB in actual system gain before feedback. In addition
intelligibility is improved without undesirable changes
in fidelity.
SPECIFICATIONS
ELECTRICAL:
INPUT:
Input Impedance: 40 kohms balanced, 20 kohms unbalanced.
Maximum input Level: +20 dBu
Gain: Variable —10 dB to +20 dB with front panel level
control.
Headroom Indicator: 4LED indicators showing maximum signals:
0, —10, —20, —30 dB relative to overload.
NOTCH FILTERS
Frequency Range
Notch Depth
Filter Bandwidth
60 Hz to 6kHz, continuously variable in two ranges.
0to —20 dB, continuously variable.
Approx. 1/6 octave at 5dB notch depth (3 dB points).
END CUT FILTERS
Low Cut Filter
High Cut Filter
12 dB per octave filter slope, continuously adjustable
from 20 Hz to 200 Hz.
12 dB per octave filter slope, continously adjustable
from 2.5 kHz to 20 kHz.
OUTPUT
Output Circuit
Output Load
Power Output
Distortion
Frequency Response
Output Noise
Power Requirements
Environment
Floating, transformer isolated.
600 ohms or greater.
+20 dBm
Less than 0.5% THD, 30 Hz to 15 kHz at maximum
rated output.
±0.5 dB, 20 Hz to 20 kHz (EQ out).
+0.5 dB, -1.5 dB. 20 Hz to 20 kHz (EQ in).
Less than —90 dBm (15.7 kHz noise bandwidth,
input and output terminated with 600 ohms: controls
set for unity gain).
Less than 10 W, 100-125 VAC or 200-250 VAC.
50/60 Hz. switch selectable.
Operating 0°C to +50°C (+32°Fto +I22°F).
Storage -20°C to 60°C (~4°F to +140°F).
PHYSICAL:
Connections Input and output on rear panel barrier strip and 3-pin
XLR/QG connectors.
Power through 3-wire IEC style connector.
Dimensions 483 x44 mm rack panel ,depth behind panel 248 mm
,
(19 in x13/4inx9 3/4 in ).
Finish Panel is 3.18 mm (l/8in )brushed clear anodized aluminum
in two shades. Chassis is cadmium plated steel.
Weight 4.31 kg (9.5 lb).
Shipping Weight. 6.35 kg (14 lb).
Accessory Model SC-3 Security Cover: smoke gray transparent
plastic: covers all operating controls.
Note: 0dBm =ImW
OdBu =0.775 volts
IBL/UREI continually engages in research related to product improvement. New materials, pro-
duction methods and design refinements are introduced into existing products without notice as
aroutine expression of that philosophy. For this reason, any current JBL/UREi product may differ
in some respect from its published description but will always equal or exceed the original
design specifications unless otherwise stated.
URE!
ELECTRONIC
PRODUCTS
JBL Professional. 8500 Balhna Rnulpvarr) PO Roy 9900 MnrthriHno Qaiifnmia a-noa 11ca

BEFORE PROCEEDING WITH COMPLETE UNPACKING AND SETUP
PLEASE READ THE SECTION ON UNPACKING AND INSPECTION
model 562
FEEDBACK SUPPRESSOR
UREI Incorporated
8400 BALBOA BOULEVARD
NORTHRIDGE, CA. 91325-4100 U.S.A.
PHONE: (818) 895-UREI (895-8734)
FAX: [phone]
©Copyright 1980,1988 UREI Incorporated

TABLE OF CONTENTS
PAGE
SECTION I. INTRODUCTION
1.1 DESCRIPTION 1
1.2 SPECIFICATIONS 2
1.3 CONTROLS AND INDICATORS 3
1.4 CONNECTIONS 4
SECTION II. INSPECTION AND INSTALLATION
2.1 UNPACKING AND INSPECTION 5
2.2 ENVIRONMENTAL CONSIDERATIONS 5
2.3 POWERING 5
2.4 LINE VOLTAGE SV5ITCK 6
2.5 EXTERNAL CONNECTIONS 6
2.6 IMPEDANCE AND TERMINATION 8
2.7 SIGNAL LEVEL AND HEADROOM INDICATOR 9
2.8 ACCESSORIES 9
SECTION III. OPERATING INSTRUCTIONS
3.1 SETUP PROCEDURE 10
3.2 DISCUSSION OF FEEDBACK 11
3.2.1 GENERAL 11
3.2.2 SUSCEPTIBILITY TO FEEDBACK 12
3.2.3 HOV; MUCH ADJUSTMENT 12
3.2.4 INTELLIGIBILITY AND FREQUENCY MASKING 13
3.2.5 USE OF ENDCUT FILTERS 13
3.3 CAIN STRUCTURE AND SIGNAL-TO-NOISE 14
SECTION IV. THEORY OF OPERATION
4.1 GENERAL 15
4.2 INPUT AMPLIFIER 15
4.3 GAIN AMPLIFIER 15
4.4 FEEDBACK SUPPRESSOR 15
4.5 ENDCUT FILTERS 15
4.6 INVERTER 16
4.7 OUTPUT AMPLIFIER 16
4.8 HEADROOM INDICATOR 16
4.9 POWER SUPPLY 16
SECTION V. MAINTENANCE
5.1 GENERAL 17
5.2 REPAIRS AND WARRANTY 17
5.3 SERVICE ADJUSTMENTS 17
5.3.1 COMMON MODE BALANCE 17
5.3.2 OFFSET ADJUSTMENT 18
5.4 CLEANING 18
5.5 ON OP AMPS 19
SECTION VI. SCHEMATIC AND TYPICAL ATTENUATION CURVES

SECTION I
INTRODUCTION
FIGURE 1-1. MODEL 562 FRONT PANEL.
1.1 DESCRIPTION
The Model 562 Feedback Suppressor has been designed to reduce
feedback in sound reinforcement systems without affecting room
equalization. Experience with narrow band notch filters has
demonstrated that, in atypical sound reinforcement system, gain
before feedback can be significantly improved by centering anarrow
band reject filter on four or five of the most prominent system
resonant frequencies. It has also been found that rolling off the
low and high frequency ends of the spectrum, to pass only aprogram's
required bandwidth, increases gain before feedback at the band
edges
.
The Model 562 contains 5active notch filters, each contin-
uously adjustable in notch depth from 0to 20 dB, and from 60 Hz
to 6,000 Hz. In addition, low cut and high cut filters with 12 dB/
octave slopes are tunable from 20 Hz to 200 Hz and 2.5 kto 20 kHz
respectively. The Model 562 has front panel adjustable gain,
ranging from -10 to +20 dB, and headroom indicators to optimize the
signal level. This allows the unit to be sourced from low level
"semi-pro" equipment and high level professional equipment. A
bypass switch allows A-B evaluation of feedback suppression, and a
setup switch provides system protection during tuning by limiting
maximum output level. XLR/QG connectors and terminal strips are
provided for both input and output signals.
In using the Feedback Suppressor, the system's gain is slowly
increased until the first sustained feedback occurs. One notch
filter is then "tuned" until the feedback ceases. The gain is
then increased again, and the next resonance attenuated with the
second filter. This procedure is repeated with filters 3through
5. If feedback occurs at the extreme high or low end, it can be
reduced by using the end cut filters, thus making all 5notch
filters available for use in the passband. Atypical result
is an increase of 12 dB or more in actual system gain before
feedback. In addition, intelligibility is improved without
undesirable changes in fidelity.
-1-

1.2 SPECIFICATIONS
ELECTRICAL:
INPUT CIRCUIT:
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).
GAIN: Variable, -10 dB to +20 dB, with front
panel control.
HEADROOM INDICATOR: 4LED indicators showing maximum signals;
0, -10, -20, -30 dB relative to overload.
Notch Filters
FREQUENCY RANGE: 60 Hz to 6kHz, continuously variable
in two ranges.
NOTCH DEPTH: 0to -20 dB, continuously variable.
FILTER BANDWIDTH: Approximately 1/6-octave at 5dB
notch depth (-3 dB points).
End Cut Filters
LOW CUT FILTER: 12 dB/octave filter slope, continuously
adjustable from 20 Hz to 200 Hz.
HIGH CUT FILTER: 12 dB/octave filter slope, continuously
adjustable from 2.5 kHz to 20 kHz.
Output
OUTPUT CIRCUIT: Floating, transformer isolated.
OUTPUT LOAD: 600 ohms or greater.
POWER OUTPUT: +20 dBm into 600 ohm load (7.75 volts).
DISTORTION: Less than 0.5% THD, 30 Hz -15 kHz at
maximum rated output.
FREQUENCY RESPONSE: ±0.5 dB, 20 Hz -20 kHz (EQ out),
+0.5 dB, -1.5 dB, 20 Hz -20 kHz (EQ in).
-2-

OUTPUT NOISE: Less than -90 dBm (15.7 kHz noise bandwidth,
input &output terminated w/ 600 ohms,
controls set for unity gain).
POWER REQUIREMENTS: 100 -125 VAC, or 200 -250 VAC, 50/60 Hz,
switch selectable, less than 10 watts.
ENVIRONMENT: Operating, 0°C to +50°C (+32°F to +122°F);
Storage, -20°C to +60°C (-4°F to +140°F).
PHYSICAL:
DIMENSIONS: 483 x44 mm rack panel, depth behind panel
248 mm, (19" x1-3/4" x9-3/4").
FINISH: Panel is 3.18 mm (1/8") brushed clear
anodized aluminum in two shades.
Chassis is cadmium plated steel.
NET WEIGHT: 4.31 kg (9.5 lbs).
SHIPPING WEIGHT: 6.35 kg (14 lbs).
ACCESSORY: Model SC-3 Security Cover; smoke gray
transparent plastic; covers all operating
controls.
1.3 CONTROLS AND INDICATORS
GAIN: Continuously variable control of input
stage gain, from -10 dB to +20 dB.
HEADROOM INDICATOR: 4LED indicators showing maximum signal
levels: 0, -10, -20, -30 dB relative to
overload
.
NOTCH DEPTH: Continuously variable attenuation from
0to -20 dB.
FREQUENCY: Varies the notch frequency from 60 Hz
to 600 Hz or 600 Hz to 6kHz.
FREQUENCY RANGE: Toggle switch selects range for XI or X10.
SUPPRESSOR IN/OUT: Toggle switch allows Feedback Suppressor
to be bypassed.
SETUP/OPERATE: Toggle switch activates clipper to protect
loudspeakers during adjustment of Feedback
Suppressor.
-3-

LOW CUT FILTER: Continuously adjustable from 20 Hz to
200 Hz.
HIGH CUT FILTER: Continuously adjustable from 2.5 kHz to
20 kHz.
POWER: Toggle switch with LED to indicate when
the Model 562 is powered.
1.4 CONNECTIONS
All connections for input and output are made on the rear
panel through barrier strip and 3-pin XLR/QG connectors. Power is
applied through a3-wire IEC-style connector. (See Installation
Instructions, Section 2.5, Figures 2-1 and 2-2.)
-4-

SECTION II
INSPECTION AND INSTALLATION
2.1
UNPACKING AND INSPECTION
Your Model 562 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 562 Feedback Suppressor
UREI Instruction Manual (this book)
Two-part Warranty Card bearing the
same serial number as the Model 562.
Screws and finish washers for mounting
the unit into astandard 19" rack.
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 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 562 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 562 TO THE MAINS.
-5-

AC mains power is supplied to the Model 562 through an IEC-
style 3-conductor receptacle mounted on the rear of the chassis,
and amatching 3-wire power cable. The ground pin of the connector
is tied to the chassis of the Model 562 in order to comply with
electrical codes. In some installations this may create ground-loop
problems. Ground loops can become very evident (as hum and buzz)
if asignificant 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 a3-prong to 2-prong AC adapter, ungrounding
the AC plug temporarily. This ungrounds the Model 562, and m^y
cure the hum or buzz, but is not asubstitute for proper system
grounding. Be aware that unless the Model 562 Feedback Suppressor
is AC grounded# asafety hazard can exist .UREI accepts no respon-
sibility for legal actions or for direct, incidental or consequen-
tial 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 562 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 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.
Connections may be made to either the barrier strip or the XLR/QG
connectors on the rear panel
•
NOTE: The pins of the XLR*s are wired as follows:
#3 to "±" of input and output,
#2 to "Common" of input and output,
#1 to Ground (chassis).
To accommodate other wiring practices, the connection to
the barrier strips may be changed inside the Model 562.
If the Model 562 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
for the 562. (See Section 2.6 regarding input termination).
(See Figures 2-1 and 2-2 on the following page)
-6-

FIGURE 2-1. CONNECTING THE MODEL 562 WITH BALANCED
INPUT AND BALANCED OUTPUT CIRCUITS.*
FIGURE 2-2. CONNECTING THE MODEL 562 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.

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.
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 asingle
output of apreceding 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 562 has input impedances of 40 kohms (40,000 ohms)
when used in abalanced, differential input configuration, and 20
kohms (20,000 ohms) when used unbalanced (one side grounded). This
makes the 562 suitable for use with any nominal source impedance,
low or high. Only when it is used from asource which requires a
low impedance termination (such as a600-ohm transmission line or
older vacuum-tube equipment) will asource termination resistor be
required at the 562 input.
-8-

2.7 SIGNAL LEVEL AND HEADROOM INDICATOR
The Model 562' sdifferential input amplifier is capable of
accepting signals up to alevel of +20 dBm, above which level clip-
ping and distortion occurs. The Headroom LEDs indicate the level
of the signal relative to overload or clipping. The red "0" LED
indicates overload, while the green "10," "20," and "30" LEDs
indicate dB level below clipping.
2.8 ACCESSORIES
An optional Security Cover, Model SC-3, is available to cover
all operating controls of the Model 562, and to protect against
any inadvertent misad justment of critical settings.
-9-

SECTION III
OPERATING INSTRUCTIONS
3.1 SETUP PROCEDURE
This portion of the manual gives astep-by-step procedure for
setting of all controls on the Model 562.
1. Set the Feedback Suppressor's Bypass to "In" position.
2. Set the Setup/Operate switch to "Setup" position.
3. Set the Gain control to mid position.
4. Set the Depth control fully counterclockwise.
5. Set the Frequency control fully counterclockwise.
6. Set the Frequency Range switches to "XI" position.
7. Slowly increase the microphone level (using the mixer's
level control) until the first feedback frequency
becomes detectable and has stabilized.
8. Turn the first Depth control fully clockwise (maximum
depth) and tune the associated Frequency knob until the
feedback disappears. If the feedback cannot be tuned
out, set the Range switch to "X10" and turn the
Frequency knob until the feedback disappears.
With maximum Depth the feedback frequency may be at the
skirt of the filter; for amore accurate tuning, reduce
the Depth setting and retune the Frequency, if necessary,
to eliminate the feedback.
9. Increase the microphone level so the next feedback
frequency will appear, and repeat the procedure of
step 8. Similarly, continue to increase the
microphone level and eliminate the third, fourth
and fifth feedback frequencies.
10. The Suppressor In/Out switch can be set to "Out" to
assess the effect of the Feedback Suppression settings.
CAUTION: After setup, be sure to de-activate the
clipper by returning the Setup/Operate switch to the
Operate position. Otherwise severe signal distortion
will result.
11. Use the Endcut filters to restrict the bandwidth of the
processed program material as desired.
-10-

12. INPUT AMPLIFIER: With program applied, set the Gain
control so that the green "10" LED lights as often as
possible without the red "0" LED turning on at all. If
the red LED does turn on, the Gain setting should be
reduced
.
CAUTION REGARDING FEEDBACK
Feedback is not only annoying to the ear, but is also dangerous
to unprotected amplifiers and loudspeakers. The Feedback
Suppressor section of the Model 562 incorporates aprotection
circuit which is activated in the Setup position of the front
panel toggle switch. In Setup mode, the output voltage to the
power amplifier is limited to asafer value by means of
clipping excessive levels. The protective circuitry described
is not guaranteed to prevent damage to amplifiers or speakers.
Power amplifiers could be set too high and signal levels at
the protective circuit may be too low for it to be activated
before dangerous levels are achieved. BE CAREFUL OF YOUR
AMPLIFIERS AND SPEAKERS.
3.2 DISCUSSION OF FEEDBACK
3.2.1 GENERAL
The quality of amplified sound in aroom depends not only on
the microphone, amplifier, loudspeakers, etc., but also to alarge
degree on the acoustical characteristics of the room itself.
Sounds reflected by walls and ceiling add to and subtract from
the direct sound from aloudspeaker as it arrives at alistener's
ear. Various frequencies reflect in different amounts, and resonant
peaks and dips will occur. Peaks in the frequency response cause
coloration of the original sound. They will mask adjacent frequen-
cies of the program material. Aside from adversely influencing the
quality of music and the intelligibility of speech reproduction,
these peaks in the frequency response also reduce the maximum
obtainable average sound level throughout the audio spectrum. The
average sound level is limited by those frequencies whose amplitude
approach or exceed asystem gain of unity.
This condition is marked by ringing and feedback. Ringing is
aprolongation of the reverberation time for those frequencies
which are approaching unity gain. Feedback is aspontaneous
oscillation at the frequency where loop gain (including the acoustic
environment) exceeds unity.
Reduction of acoustic resonances will produce the following
desirable results:
a) More sound level through higher achievable gain.
b) More freedom for performers.
-11-

c) Improvement of sound quality (i.e., greater clarity of
music and words).
The 562 Feedback Suppressor is not acure-all for serious
sound system deficiencies. It will not make inferior components
perform like superior ones, nor will it completely correct poor
acoustic conditions or poorly engineered installations.
3.2.2 SUSCEPTIBILITY TO FEEDBACK.
In public address and sound reinforcement systems, the maximum
acoustic gain that may be obtained for amicrophone in the vicinity
of aloudspeaker 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 audio range are reproduced at
ahigher level than the rest of the frequency band. If the level
of these frequencies which are being reproduced at an exaggerated
level can be reduced, then the sound system gain may be increased
to some degree without feedback.
An improvement of asystem's tendency to feedback should be
attempted after the general equalization of the frequency response
is performed. The result will be ahigher amplifier gain setting
than was possible before feedback suppression. Remember: an
improvement of 3dB is equal to twice the previously available
power. To prevent ringing, it is best to adjust the gain at
least 3dB below the threshold of feedback.
3.2.3 HOW MUCH ADJUSTMENT?
Theory says that for agiven change in amplitude response,
using aminimum-phase filter network, there will be acorresponding
phase change. While you may not hear the amplitude response
change, you may hear the effects of the phase angle rotation.
Since the minimum amplitude response change generally causes the
minimum change in phase response, the shallower the notches for
feedback suppression, the less likely that they will be audible.
When adjusting the Model 562 in the field, it is often possible
to ignore the above considerations. Program material and deficien-
cies in the speaker system and acoustic environment are usually
several orders of magnitude larger than the effects introduced by
the notch filters, even at extreme settings. This is especially
true in vocal reinforcement; music reinforcement systems are
typically less tolerant. When using the Model 562 in asystem
intended to reproduce music, care should be taken not to notch out
too much program material. Once the feedback modes have been found
and the filters are adjusted according to the procedure in Section
3.1, the depth controls may be backed off to determine if anotch
less deep will still suffice.
-12-

3.2.4 INTELLIGIBILITY AND FREQUENCY MASKING
Most of the discussion on the use of the Model 562 has focused
on the reduction of feedback. However, in some instances this may
not be its most important function. Sometimes asystem may already
provide adequate gain before feedback, and other times asystem may
have so many modes that notching out four or five of them with a
Model 562 will not provide asignficant increase in useable gain.
In either of these cases, however, there may be masking of important
mid and high frequency sounds by peaks in the bass range. Bass
peaks tend to occur due to typically longer reverberation times
at low frequencies. Here, the Model 562 may be used to reduce the
bass peaks and hence improve speech intelligibility and reduce
frequency masking of music.
3.2.5 USE OF ENDCUT FILTERS
The endcut filters of the Model 562 are used to restrict the
bandwidth of the program material for one of several reasons:
1) It may be necessary to remove feedback modes at very
high and very low frequencies that are beyond the range of
the notch filters. This might occur in asituation where
the microphone and loudspeaker are fairly close to each
other. For example, consider astage monitor system
where aspeaker is located close to the microphone and
aimed directly at it. In this instance, feedback at
frequencies (above 6kHz) may occur due to an unusually
high level of high frequency gain available for feedback.
(Very high frequencies do not generally reach the micro-
phone due to their greater absorption in air, and due to
greater speaker directivity at high frequencies.)
2) In small rooms, feedback at very low frequencies may be
aproblem. Bear in mind that the acoustical treatment of
many smaller rooms that have not been designed for music
utilizes acoustic materials and methods that are only
effective at middle and upper frequencies. For instance,
it would not be unusual to find aconference room designed
for NC-30* which actually has 90% absorption of signals in
the mid to high frequencies, but less than 10% absorption
for low frequencies. Additionally, low frequency reflec-
tions can be greater because of the proximity of the walls.
*NC-30 is astandard Noise Criteria curve of frequency vs. level
showing an average of 30 dB SPL permissible background noise.
3) It is usually desirable to restrict the upper and lower
bandwidths to only that necessary for the program material.
In the case of voice this may be aconsiderable reduction.
This avoids the amplification of such extraneous sounds as
rustling paper, power line hum, air conditioning noise,
mic stand rumble, and leakage from on-stage bass instrument
amplifiers
.
-13-

NOTE: Care should be taken in adjusting the endcut filters
so that the program material is not adversely affected. Use
your ears to evaluate the adjustments. If it sounds good,
it is good.
3.3 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.
The Model 562 was designed for amaximum 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 562 is 90 dB below the average signal level,
or greater than 110 dB below maximum output level, and therefore
inaudible
.
For signal levels from equipment with less output capability,
the Input Level control of the Feedback Suppressor provides a
convenient preamplifier for additional gain, up to 20 dB above
unity, without reducing the excellent signal-to-noise ratio.
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SECTION IV
THEORY OF OPERATION
4.
1
GENERAL
This section provides ageneralized description of the 562
circuitry, and should be used in conjunction with the schematic at
the end of the manual
.
4.2
INPUT AMPLIFIER
The signal is applied to adifferential input amplifier (IC 1,
sections Cand D). The input accepts either balanced or unbalanced
sources (see also Installation, Section 2.5). Common mode rejec-
tion is factory adjusted with R1 and is typically better than 40 dB.
4.3
GAIN AMPLIFIER
IC 1A is avariable gain amplifier providing -12 to +18 dB
of gain. Thus the stage has again range of 30 dB, allowing
adjustment through the Model 562 from from -10 to +20 dB, referenced
to unity gain.
4.4 FEEDBACK SUPPRESSOR
There are five identical notch filter sections in the Model 562.
The filter type is "State Variable." Each filter utilizes four
operational amplifiers which are on asingle IC; filter #1 will be
described
.
Frequency Selection is variable with the dual front sections
of R36 over the ranges from 60 to 600 Hz and 600 to 6000 Hz,
depending on the setting of the multiplier switch.
The signal output of amplifier section D(at pin 14) consti-
tutes abandpass function with the center frequency selected as
described previously. The signal is 180° out of phase with the
input signal. To achieve anotch, some bandpass signal is mixed
with the input signal ahead of the summing amplifier (see typical
attenuation curves in Section VI). Notch Depth is variable from
0dB to 20 dB with the third section (center shaft) of the poten-
tiometer R36 .Aloss of approximately 0.8 dB is taken through each
filter section for atotal of 4dB through the 5sections.
4.5 ENDCUT FILTERS
IC 8A and IC 8B and associated components are avoltage
controlled voltage source high pass (low frequency cutoff) and low
pass (high frequency cutoff) filters respectively. Both filters
are 12 dB/octave Butterworth types. The high pass section has 4dB
of voltage gain; the low pass section is unity gain.
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4.6
INVERTER
IC 8C is configured as an inverting, unity gain buffer ampli-
fier. Its purpose is to maintain correct input/output polarity
with the suppressor section switched OUT. Components R85, R86 and
R87 form an offset adjustment circuit which allows any DC offset at
the input of the output amplifier to be matched to an equal offset
at the output of IC 8C for minimal "click" when the suppressor is
switched IN or OUT.
4.7
OUTPUT AMPLIFIER
IC 8D, Ql, Q2 and additional components comprise the output
amplifier. It has avoltage gain of 1.4 dB and drives transformer
Tl. With switch SI in the IN position, input signal is connected
from the low pass filter through C84 and R84. C84 provides AC
coupling for the signal. R84 is essentially ignored by the high
impedance input of the Op amp. With switch SI in the OUT position,
the signal path is through IC 8C. The relatively low output
impedance of IC 8C effectively short circuits the signal coming
through R84. This provides proper isolation between the IN/OUT
conditions of the bypass switch.
Switch S2 in the setup position places diodes CRl and CR2
across the signal line, limiting the maximum peak-to-peak signal
to approximately 1.2 volts for setup purposes.
4.8
HEADROOM INDICATOR
IC IB is ahalf wave rectifier with its input driven from the
variable gain amplifier. The output is filtered by R8 and C4, and
drives the negative inputs of IC 2, sections A, B, Cand D, which
are used as window comparators. The thresholds for the individual
sections are derived from the voltage divider consisting of R9
through R13.
4.9
POWER SUPPLY
The power supply is bipolar, employing two integrated circuit
voltage regulators, VR1 and VR2, to provide low ripple, ±18 volt DC.
Additional filter capacitors ensure power supply stability and low
noise
.
The pilot LED is connected across the positive and negative
sides of the power supply to indicate the power ON condition.
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