Orban 672A User manual

672A Equalizer
OPERATING MANUAL

EQUALIZER
Specifications: 672A Equalizer
All specifications apply when driving 600
ohms or higher impedances. Noise mea-
sured on an average -reading meter through
a20-20,000Hz bandpass filter with 18dB/
octave Butterworth skirts.
Electrical
Input:
Impedance, Load (each leg): 100K in paral-
lel with lOOOpF, electronically balanced
Impedance, Driving: Ideally 600 ohms or
less, balanced or unbalanced
Nominal Input Level: Between -10 and
+4dBm
Absolute Overload Point: +26dBm
Output:
Impedance, Source: 47 ohms in parallel
with lOOOpF, unbalanced (Optional trans-
former balanced 600 ohm outputs)
Impedance, Load: Should be 600 ohms or
greater-will not ring into any capacitive
load
Nominal Output Level: +4dBm
Max. Output Level Before Clipping:
+19 dBm, 20 -20,000Hz
Frequency Response:
10.25dB, 20 -20,000Hz: EQ controls set at
“O” detents
Available Gain:
+12dB; adjustable to —infinity by means
of front -panel GAIN control
Slew Rate:
Varies between 6and 13V/uS depending
upon setting of GAIN control; slewing is
symmetrical. Internal bandlimiting assures
that slew rate limiting will not occur even
with the most severe equalization and pro-
gram material.
Square Wave Response:
Square wave exhibits no spurious ringing at
any output level. The only ringing observ-
able is that theoretically associated with any
given equalization curve.
Total Harmonic Distortion:
Less than 0.05%, 20 -20,000Hz (+18dBm)
SMPTE Intermodulation Distortion:
Less than 0.05% (+18dBm: 60/7000Hz,4:l)
Noise at Output:
Less than -75dBm (EQ in, Filters out,
Controls centered)
Overload /Noise Ratio:
Better than 113dB for any single bandpass
filter, for any settings of TUNING or BAND-
WIDTH controls.
Equalization Ranges:
+16dB peaking EQ, Reciprocal
Tuning Ranges:
20 -60Hz;40 -150Hz; 110-310Hz; 230-
750Hz; 480 -1900Hz; 1.1- 4.5Hz; 2.8-9.0kHz;
5.9 -21kHz. Dials calibrated at ISO preferred
frequencies.
“Q” Range:
Greater than 0.5 to 10 for any setting of the
TUNING control
Low Pass Filter Section:
Tunable in 2ranges: 200 -2000Hz or 2.0-
20kHz, 12dB/octave, (2nd-order
Butterworth)
High Pass Filter Section:
Tunable in 2ranges: 20 -200Hz or 200 -
2000Hz, 12dB/octave, (2nd-order
Butterworth)
Overload Circuit:
Lamp lights for 200mS if the instantaneous
peak output of any amplifier rises to within
ldB of its clipping point.
Circuit Design:
Active RC realized with FET-input opamps.
Line driver employs discrete transistor cur-
rent booster.
Operating Temperature:
0-50°C
Power Requirements:
115/230VAC 110%; 50/60Hz; 6watts
Physical
Operating Controls:
EQUALIZATION, TUNING, and BAND-
WIDTH for each of eight bands. TUNING,
RANGE (xl; xlO), and FILTER IN/OUT
for each filter. EQUALIZATION IN/OUT,
POWER ON/OFF, and GAIN for entire
equalizer.
Panel:
19” x5V4” (48.3 x13.3cm): 3units
Chassis Depth Behind Panel:
51/4”(13.3cm)
Weight:
Net: 8lbs. (3.6kg); Shipping: 12 lbs. (5.4kg)
AC Cord:
3-wire U-ground to USA Standard
Connectors:
140 type barrier strip (#5screw) plus paral-
lel-wired V4” 3ckt. phone jacks (Switchcraft
12B or equal). Holes punched for XLR-type
connectors (Switchcraft D3F and D3M or
equal)
Circuit Ground:
Available on barrier strip; normally jumpered
to chassis.
Specifications subject to change without notice.

Table of Contents
Specifications
<Registration Card
Warranty
*IPart A: Installation and Operation
IIntroduction
1Performance Highlights
2Front Panel Description
3Rear Panel Description
3Block Diagram
3AC Power
3Mechanical Installation
4Installation of Options
XLR Connector Installation
Balanced Output Tranformer Installation
5Electrical Installation
Input
Output
Wiring With Two-Conductor Shielded Cable
Grounding Diagram
Typical Input/Output Connection Rules
Wiring Wi‘h Single-Conductor Shielded Cable
8OPERATING INSTRUCTIONS
Using the EQ Section
Boost/Cut Curve Familes
Using the Highpass and Lowpass Filters
Using the 672A As An Electronic Crossover
Headroom
I|Part B: Specific Applications
IIStereo
||Sound Reinforcement
House Tuning
«Notch Filtering
Full Electronic Crossover
Partial Electronic Crossover and
iIndividual Driver Equalizer
Stage Monitors
13 Dance Bars
14 Recording Studios
14 Motion Picture Sound
15 Broadcasting
15 Electronic Music
17Part C: Maintenance
17 Introduction
17 Performance Evaluation
Genera I
Power Supply
Signal Processing Circuitry
20 Maintenance and Service
Preventive Maintenance
Access
Replacement of Components on Printed Circuit Boards
Troubleshooting 1C Opamps
Factory Service
Shipping Instructions
23 Circuit Description
Genera I
Input Buffer
Equalizer Sections
EQ IN/OUT Switch
Lowpass Filter
Highpass Filter; Output Buffer
Overload Indicator
Power Sipply
27 Appendix: Interconnections And Grounding
27 Driving the Input From High-Impedance
And/Or High-Level Sources
27 Grounding
29 PARTS LIST
35 SCHEMATIC
36 PARTS LOCATOR

INTRODUCTION
PERFORMANCE
HIGHLIGHTS
Part A: Installation and Operation
The Orban 672A is asingle-channel eight-band quasi-parametric equalizer of high
professional quality. It is equipped with graphic-type EQ controls providing up to
l6dB of reciprocal boost or cut. The center frequency and bandwidth of each band
are continuously variable to enable precise control of the audio spectrum. Wide-
range high- and low-pass filters (12dB/octave) follow the EQ section for added
versatility. When these filters are overlapped (200-2000Hz), the 672A can be used
as acombined graphic/parametric equalizer and tunable electronic crossover.
Separate lowpass and highpass outputs are provided for this purpose.
The 672A has an electronically-balanced bridging input, and unbalanced outputs
(which can be balanced by the addition of optional output transformers). Input,
outputs, and power line connections all contain effective RF filtering. l2dB of gain
is available. All potential overload points in the equalizer are monitored by an
extremely fast "peak-stretching" overload detector, so that peak clipping can be
detected and corrected before if becomes audible.
The flexibility offered by the 672A makes it aparticularly powerful tool in nearly
all areas of audio: sound reinforcement, public address, recording studio,
broadcasting, motion picture sound, disco, theater.
The 672A easily meets the quality, performance, and reliability requirements of
the demanding professional, and is also well-suited for use in semi-pro or audiophile
applications.
The controls and features of the 672A are fully described in this manual. It will
familiarize you with the unit's potential and enable you to imaginatively use the
672A for your specific installation and application.
EQ Section
Eight bands, each with TUNING and BANDWIDTH control
+l6dB equalization range
Reciprocal curves
Each band tunes over 3:1 frequency range
"Q" is variable between 0.5 and 10
—"Tic" marks on TUNING and BANDWIDTH controls guide you to octave-band
graphic equalization settings
Bands are totally non-interacting
LP/HP Filter Sections
Each section is continuously tunable over 100:1 range in 2decades
!

Each section is independently switchable
l2dB/octave slopes
—Separate highpass and lowpass outputs permit use as full electronic crossover
in the range of 200-2000Hz
General
I2dB available gain
Very low noise and distortion
High slew rate for minimum TIM (SID)
—"Peak-stretching" overload lamp warns of clipping anywhere in equalizer before
distortion is audible
—EQ controls are long-throw dust-shielded sliders for good resolution
—Industrial-grade parts and construction, including socketed IC's
RFI suppression on input, output, and power leads
—Balanced output optional (order retrofit kit as required)
—Holes for XLR-type connectors are provided
FRONT PANEL
DESCRIPTION
The GAIN control adjusts tne drive level to the filters and equalizers.
The OVERLOAD indicator monitors all critical points to warn of excessive signal
amplitude due to excessive input amplitude or to large amounts of peak boost
equalization. Overloads are eliminated by turning down the GAIN control.
The EQ IN/OUT switch defeats the graphic EQ section.
The EQ controls adjust the maximum peak or dip in each band over arange of
+l6dB. Center detent corresponds to flat output from the band.
The TUNING controls adjust the center frequency of each of the eight bands in
the EQ section.
The BANDWIDTH controls adjust the "Q" (sharpness) of each band in the EG
section. The "Q" becomes broader when turned clockwise, sharper when turned
counterclockwise.
The HIGHPASS FILTER IN/OUT switch defeats the highpass filter action.
The HIGHPASS FILTER TUNING control adjusts the corner frequency of the
highpass filter over two ranges. The RANGE switcn determines whether the range
is 20-200Hz or 200-2000Hz.
The LOWPASS FILTER IN/OUT switch adjusts the corner frequency of the lowpass
filter in two ranges. The RANGE switch determines whether the range is 2-20kHz
or 200-2000Hz.
The POWER switch and green LED pilot lamp complete tne front panel.
2

REAR PANEL
DESCRIPTION
AC POWER
MECHANICAL
INSTALLATION
The FUSE used in the 672A is a3AG 1/8 amp slo-blo t/pe, used for both I15V
and 230V operation. Replace with the same t/pe only.
The INPUT and OUTPUT connectors provided allow connection via barrier strip (//5
screw) or standard 1/4" phone plugs. Each input or output of both connector types
are connected in parallel. In addition, acover plate masks holes for user-installed
XLR-type connectors.
Please refer to the Electrical Installation section for connection instructions.
The power transformer can be strupped for 115 volt or 230 volt 50 or 60Hz AC
operation. If the unit was ordered for 230 volts, atag on the power cord warns of
the modification.
To strap the power transformer for adifferent voltage, remove the bottom cover
of the 672A. Strapping instructions are found on the insulating fishpaper around the
power transformer. It is not necessary to rearrange the heavy insulated wiring; all
strapping can be performed with bare jumper wire. Take care not to burn the
insulation.
The power cord is terminated in a"U-Ground" plug to USA standards. The green
(or yreen/yellow) wire (which is connected to the long prong) is connected directly
to the 472A chassis. If it becomes necessary to lift this ground to suppress ground
loops, this should be done with athree-prong to two-prong adapter plug, rather
than by damaging the power plug. It is not recommended that this ground be
defeated unless absolutely necessary because it eliminates the intrinsic safety
feature of the three-wire system.
WARNING!
IF THE GROUND IS DEFEATED, CERTAIN FAULT CONDITIONS IN
fill: UNIT OR THE SYSTEM TO WHICH IT IS CONNECTED CAN
RESULT IN APPEARANCE OF FULL LINE VOLTAGE DETWEEN
CHASSIS AND EARTH GROUND. SUCH VOLTAGE IS CAPABLE OF
CAUSING SEVERE INJURY OR DEATH!
Vertical space of three standard rack units (5 l/4"/l3.3cm) is required.
Mounting the unit directly over large heat-producing devices like avacuum-tube
power amplifier may shorten component life and is not recommended. Ambient
temperature should not exceed 113 degrees F(45 degrees C) when equipment is
powered.
3

INSTALLATION
OF OPTIONS XLR Connector Installation: To install the optional XLR connectors, obtain (2 ea.)
Wchcraft D3M and (2 ea.) Switchcraft D3F (or equivalent) connectors from a
connectors are o,so avciiCie directly front Orbon os retro,,,
i-:+RPT.Oft
with apair ithe XLR's as shown mFig.
Phone Jack Installation: To install the optional
Switchcraft #I2B (tip-ring-sleeve) phone jacks from a
also available directly from Orban as RET-29.
phone jacks, obtain (4 ea.)
local supplier. These jacks are
Remove the hole plugs from
hardware supplied with the jacks,
to the jacks as shown in Fig. I.
the rear panel and install the jacks using the
Then connect jumper wires from the barrier strip
VIP roMtURCTOR \upvn 1XLR CONNECTOR UP OUT YsLR COKitSECTOR WWIo/rP oo
10 COLOR FROM 10 COLOR PRO,* 10 COLO
3'-' (At) 3\-G (&kW) BLR 3Z-\ (Ai) J7.-G (GNV) BLR 3V\ (At) 33 -G (GMVt BLR
3Z~Z (Lot TBI-G (cF) BLVOj>!\AiVbl 3VB (UO) TBI-IO (A) BLK^lr
GRKiJ
rs\-z aot TB\-Z \«(L0) }T\W\ST
RfcP J3VS (RO TB\-9 (Rl)
3T--Z (R0 T&V-5 (Rif
(rO TB\-' IN (Wt
Fig. I: WIRING OF OPTIONAL CONNECTORS
4

BLOCK
DIAGRAM
ELECTRICAL
INSTALLATION
Balanced Output Transformer Installation: If transformers were not installed at
the factory, refer to the installation instructions furnished with the transformer kit,
known as "RET 6". The transformer supplied with this kit has been designed to
have <3 negligible effect on published specifications. Should you wish to use some
other transformer, it would be wise to make careful performance measurements
with special attention to Lt distortion and Mb response at high output levels, thus
determining the output level achievable with performance acceptable for your
application. Atransformer meeting Orban standards should produce approximately
+20dDm (limited by clipping in the output amplifier) without significantly
compromising performance.
The Electrical Installation section describes grounding procedures in the event
transformers are used.
The BLOCK DIAGRAM outlines the signal flow through the various sections of the
672A. It is reproduced here:
Connecting the 672A (equalizer to other equipment is quite straightforward.
Relatively uncomplicated systems (such as home playback systems, "semi-pro"
recording studios, electronic music studios, discos, etc.) tend to come together
without serious grounding problems even if the wiring practices are somewhat
casual, provided that high RF fields are not present. Unusual situations can be
analyzed if you are familiar with the standard rules governing grounding and
interfacing between balanced and unbalanced systems.
The instructions below will apply to the majority of cases. Acomprehensive
discussion of interconnections and grounding can be found in the Appendix.
Input
The electronically-balanced input of the 672A equalizer is compatible with most
professional and semi-professional sound equipment, balanced or unbalanced, whose
source impedance is 600 ohms or less. If it is greater (as in some vacuum-tube
audiophile preamps), aminor modification may be made to the input to accomodate
the situation. Please refer to the Appendix for further details.
Nominal input level is between -10 and tAdOm. The absolute overload point is
r26dBm.
5

Output
The two outputs of the 672A are unbalanced (unless fitted with the optional
transformers), and the source impedance is 47 ohms in parallel with lOOOpF to the
chassis (for RFI suppression).
Use the MAIN/HIGHPASS output for normal operation when the crossover feature
is not used.
If the 672A is being used as afull electronic crossover, use the LOWPASS output
to drive the low-frequency amplifier, and the MAIN/HIGHPASS output to drive the
high-frequency amplifier. These outputs are out-of-phuse, as is correct with
I2dB/octave crossovers.
Wiring the 672A With Two-Conductor Shielded Cable
We recommend wiring with two-conductor shielded cable (such as Belden 8451 or
equivalent) because signal current flows through the two conductors only. The shield
does not carry signal, is used only for shielding, and is ordinarily connected to
ground at one end only. The following table and diagram are applicable to agreat
majority of installations.
Fig. 3: GROUNDING
INPUT
TABLE I:
TYPICAL INPUT/OUTPUT CONNECTION RULES
1) Always use HI and LO as the two input terminals to the 672A.
2) When the 672A is driven from an unbalanced source, connect shield both to
circuit ground of source, and to chassis ground of 672A.
3) When the 672A is driven from abalanced source, connect shield at source end
to chassis ground. Do not connect shield at 672A end.
4) If the source is balanced by means of atransformer, a620 ohm l/2w 5%
load resistor is probably required. Consult the manufacturer's manual.
6

OUTPUT
1) On the 672A output, connect shield at 672A end to chassis ground (whether
driving balanced or unbalanced). Oo not connect shield at other end.
2) When driving abalanced load, jumper circuit ground to chassis ground on 672A
(on rear panel). When driving an unbalanced load, do not attach jumper.
3) 672A chassis should always be earth-grounded (i.e. through third wire in power
cord or through rack.) I- or maximum protection from shock, float this ground
only as last resort.
4) If optional output transformer(s) are installed on 672A, jumper the circuit
ground to chassis ground on the 672A.
Because it is not always possible to determine if the pieces of equipment driving
or being driven by the 672A have their circuit grounds internally connected to their
chassis grounds (which are always connected to the ground prong of the AC line
cord), and because the use of the AC power line ground often introduces problems
because it can be noisy or otherwise imperfect, the wiring techniques in the
diagram are not universally applicable.
If you follow the diagram and hum or noise appears, don't be afraid to
experiment. If the noise sounds like alow-level crackling buzz, then prob<±>ly there
isn't enough grounding. Try connecting the LO input of the 672A to achassis
ground terminal on the 672A's barrier strip and see if the buzz goes away. You can
also try strapping the 672A's chassis and circuit grounds together, and see if this
helps.
Aground loop usually sounds like asmooth, steady hum rather than acrackly
buzz. If you have aground loop, you can often break it by disconnecting the
jumper between circuit and chassis grounds on the 672A's rear-panel barrier strip.
In either case, think carefully about what is going on, and keep in mind the
general principle: one and only one circuit ground path should exist between each
piece of equipment!
Wiring The 672A With Single-Conductor Shielded Cable: Sometimes, particularly if
you are using the 672A with musical instruments or home-type equipment, you will
find yourself with no time to correctly connect the 672A, and will find instead
that you must use single-conductor shielded ccbles (usually terminated by 1/4"
phone plugs which can be plugged into the 672A's auxiliary phone-plug input and
outputs). If this happens, connect the inner conductors of the shielded ccbles to the
HI sides of the 672A input and outputs). Connect the shields to the LO sides. (The
HI side appears on the ti£ of the phone jacks; the LO side on the ring. The sleeve
is chassis ground).
The shield will ordinarily receive chassis ground from the external equipment
which it is connecting to the 672A. The chassis ground/circuit ground jumper on
the rear barrier strip of the 672A should be left in whichever configuration gives
minimum hum or buzz. To minimize hum or buzz, it may be necessary to jumper
one or more shields to chassis ground. IF ATWO-CONDUC TOR PHONb PLUG IS
IJSFO AS THE EXTERNAL CONNECTOR, THIS WILL HAPPEN AUTOMATICALLY,
AND MAY INTRODUCE AGROUND LOOP. Because use of single-conductor cables
virtually eliminates any possibility of carefully controlling the system grounding
scheme, it is NOT RECOMMENDED!
7

OPERATING
INSTRUCTIONS
Using ,h. EQ Section: To ose .ho K« section clone, switch the HP cod LP filter
switches OUT. Switch the EQ section switch IN.
L°i„rLtort',,”L
er «rA
zxjlzzz r*.wrwc
S.' shd SOOORrt. Once yog hove gotten the "(eel" of the 672A in this
.mode, W
experimenting with the TUNING and BANDWIDTH controls to see how they
the sound.
..t:
Q
discover the subtle shelving effects available from
broadband" pe^g (BANDWIDTH control close to full ™
the center detent). Contrast this with the "rmgy", colored quality of setting
BANDWIDTH control toward narrow (ccw).
When you cut the EQ observe the effects you now achieve. Narrowed dips are
essentially inaudible, but permit suppressing sounds of fixed ^quency
,Jeurn
to 16dB in each band. (If the sound to be suppressed ,s rich mharmonics,
band per dominant harmonic.)
The FQ curves are reciprocal: boost and cut are mirror images of each other A
tof CQ(in recordinq atrack, for example) can be precisely cance ec
“L, £^„tuning »d»,MD(h
controls set os they were doting the original recording, bot with oil LQ contro sse
bo“ ogpositl to their original settings. Boos, becomes cot, and mce-verso.
._ori lnchieved by tuning adjacent bands to the same
r^quencT^dB of EQ is then available. But beware of overload and noise buildup
when boosting!
While in the narrowband boost mode, aband's TUNING control can be con.inooosly
swept to give asound similar to "phasing".
The "Q" increases (i.e., the bandwidth in terms of fractions of an octave
decreases)as the TUNING control is moved higher in frequency. But the a_so_u_e
bandwidth (in Hertz) does not vary.
Specified "Q" range for an* setting of the TUNING control exceeds 05to 10 This
means that with TUNING centered,the available "Q" range is typically 0.3 to 20.
When varying the BANDWIDTH control, the peak gain remains constant while the
skirts of the curve vary.
To provide best value, the 672A uses controls of high reliability but modest
calibration accuracy. Calibrations are approximate, and are intended primari yas
reference guides. However, placing an EQ contro, on its center detent ac^^
defeats th! equalization in that band. When operating with narrow bandwid s
accurate stereophonic matching of several channels of 672A equoRation onithe
basis on panel calibrations alone is impractical. However, if tne BANDWID I
H
control are operated on, or more clockwise than, their HTc mar s, en no
problems will occur. (See the subsection on Stereo ,n Part Bfor 9
instructions.)
8

Fig. 4: BOOST/CUT CURVh FAMILIES
Using the Highpass And Lowpass Filters
Be sure that the HP and LP filter switches are IN as appropriate. This creates a
passband: the low frequency cutoff is determined by the HP TUNING knob, and the
high frequency cutoff by the LP TUNING knob, both in conjunction with their
associated RANGE switches.
Because these filters are in series atter the EQ section, EQ is possible within the
passband. The EQ controls can also modify the l2dB/octave cutoff characteristics
of the HP and LP filters when tuned near or at the corner frequency of the
filter(s).
The ability of these filters to roll off the frequency extremes obviates the need
to use the first and last EQ bands for this function. (The filters are more
effective, as well.)
Deliberately narrowing Ihe passband radically (by tuning the filters close to each
other) is useful for special effects like telephone, transistor radio, and "old-time"
recordings. The authenticity of such effects can often be augmented by deliberately
driving the equalizer into clipping distortion (by advancing the GAIN control beyond
the point where the OVERLOAD lamp lights).
Using The 672A As An Electronic Crossover
If you do not otherwise need the lowpass and highpass filters, they may be used
as afull 12dB/octave Butterworth electronic crossover in bi-amped systems.
Connect the MAIN/HIGHPASS output to the high frequency amplifier, and the
LOWPASS output to the low frequency amplifier. Turn the LOWPASS FILTER
IN/OUT switch to OUT, and the HIGHPASS FILTER IN/OUT switch to IN. Select
the xO.I range on the LOWPASS FILTER RANGE SWITCH, and the xIO range on
the HIGHPASS FILTER RANGE SWITCH. Now adjust both HIGHPASS TUNING and
LOWPASS TUNING to the desired crossover frequency. Any frequency between
200Hz and 2kHz can be accomodated.
Refer to the speaker manufacturer's literature to determine the recommended
crossover frequency. Typical crossover frequencies used with two-way horn-loaded
systems range from 500 to 700Hz.
Ideally, the crossover frequencies can be slightly trimmed for maximally flat
response around the crossover frequency by observing areal-time analyzer as the
TUNING controls are adjusted. If areal-time analyzer is unavailable and you wish
to adjust the crossover frequency to accuracies superior to those offered by the
panel calibrations, use an oscillator and AC voltmeter (with dB scale) as follows:
9

First, disconnect 672A outputs from the amplifiers. Then, switch both filters OUT
temporarily.
CAUTION!
DO NOT SWITCH THE HIGHPASS FILTER SWITCH OUT
WHILE DRIVING ATWEETER. YOU ARE LIKELY TO
DESTROY IT!
Connect the oscillator to the 672A input, and the AC VTVM to the 672A's
MAIN/HIGHPASS output. Adjust the oscillator's output frequency to IkHz, and its
output level for aOdB reading on the AC VTVM's dB scale. Adjust the oscillator
frequency to the desired crossover frequency. Now switch the HIGHPASS filter IN,
and adjust its TUNING control until the AC VTVM reads -3dB. Then switch the
HIGHPASS filter OUT and switch the LOWPASS filter IN. Adjust the LOWPASS
FILTER TUNING until the AC VTVM reads -3dB. Both filters are now matched, and
the crossover frequency is correct within the frequency calibration of the oscillator
and the accuracy of the AC VTVM.
When the trimming procedure is complete, switch the LOWPASS IILTLK IN.
Reconnect the amplifiers to the 672A outputs. The two outputs are already out-
of-phase as required by the l2dB/octave configuration. DO NOT REVERSE THE
PHASE OF THE DRIVERS.
Notes On Headroom
The overload-to-noise ratio available from the 672A varies from about 100 to
105dB depending on the settings of the controls. To minimize audible noise while
driving power amplifiers, for example, the overload point of the 672A should be
matched to the overload point of the power amplifier. This is done by adjusting the
INPUT ATTENUATOR settings on the power amp(s) to make the amp(s) barely clip
when asinewave input to the 672A is adjusted in level such that it just bare ly
lights the 672A OVERLOAD lamp. Clipping in the amplifier may be detected by
means of an oscilloscope, power output meters, or other overload indicator,
depending on the amplifier used. This procedure assures minimum noise and
distortion from the 672A/power amp combination.
WARNING!
PRIOR TO TESTING, BE SURE THAT THE POWER AMP LOAD (SUCH
AS ALOUDSPEAKER) IS NOT DAMAGED BY FULL-POWER SINEWAVE.
IF YOU THINK THAT IT MIGHT BE, USE ADUMMY LOAD OR ATEST
SIGNAL WITH AHIGHER PEAK-TO-AVERAGE RATIO (LIKE PINK
NOISE LOWPASS FILTERED AT IKHZ) SO AS NOT TO BURN OUT
TWEETERS.
10

Part B: Specific Applications
This part of the manual provides very specific instructions and suggestions on how
to use the 672A in the fields of Sound Reinforcement, Recording Studios, Motion
Picture Sound, Broadcasting, Dance Bars, and provides comments on Electronic
Music. We recommend that anyone involved in pro audio (and certainly those
involved in its more eclectic aspects, such as Theater) read all the sections. The
applications information in each will undoubtedly provide much food for thought.
1:STEREO For broadband equalization (BANDWIDTH controls at or clockwise of the "tic"
marks), no difficulty should result when the equalizers are matched on the basis of
panel calibrations alone. However, if any bandwidth control is adjusted "narrowband"
(i.e., 0through 5), test instruments should be used to assure matching between
672A's when two or more are employed in multi-channel sound systems.
The following procedure assumes that one equalizer has been adjusted by ear or
instrument to achieve the desired effect. The other equalizer(s) must then be
instrument-matched to this reference. This is most readily done by the "differential
method": the reference unit and the equalizer under adjustment are swept out-of-
phase, and their outputs are summed. The equalizer under adjustment is tweaked to
minimize the amplitude of the sum. This method is very sensitive because it
automatically indicates both phase and amplitude mismatches. It is performed as
follows:
1) Connect asinewave sweep generator with logarithmic sweep to the inputs of
the reference equalizer such that the hot output is connected to the reference
equalizer's HI input, and its LO input is grounded.
2) Connect another output from the sweep generator to the equalizer under
adjustment. Connect the hot output to this equalizer's LO input; ground its HI
input.
3) Sum the outputs of the two equalizers by connecting each hot output to one
side of a22K 5% I/A watt carbon resistor (the value is not particularly
critical). Connect the other side of both resistors to the vertical input of an
oscil loscope.
4) Sweep the equalizers from 20-20, 000Hz, and adjust the scope sensitivity to
easily see the swept component.
5) Switch alj IN/OUT switches on both equalizers OUT. Adjust the GAIN control
on the equalizer under adjustment to null the output observed on the scope.
6) Switch all IN/OUT switches IN (unless some of these switches are normally
out on the reference equalizer). Adjust the controls of the equalizer under
adjustment until they are visually as close as possible to the control settings
on the reference equalizer.
7) Slightly adjust the various controls on the equalizer under adjustment to
minimize the amplitude of the differential component as observed on the scope.
2: SOUND The 672A is an extremely versatile sound reinforcement equalizer. It can function
REINFORCEMENT as anotch filter, broadband equalizer, filter, and electronic crossover. Many of
these functions are simultaneously available.
House Tuning: In an economy installation, the 672A can do asurprisingly effective
job of tuning areinforcement system to aroom without the use of athird-octave

filter set. Use three or four of the bonds in their narrowband notching mode
(BANDWIDTH full counterclockwise) to notch out the major ring modes. This is
most effectively and safely done if there is alimiter or compressor in the circuit
before the power amplifier to prevent speaker damage. Advance system gain until
it feeds back at asingle frequency. Make sure that the limiter is in gain reduction
to protect the speakers. Estimate the frequency of the feedback, and choose a
band on the 672A which covers it. Turn the BANDWIDTH control to broad,and pull
the EQ control down to -l6dB. Adjust the TUNING control until the feedback
changes frequency. Continue to adjust the TUNING control until the feedback
returns at its original frequency. Set the TUNING control half-way between the
two settings where the feedback was observed to change frequency. Now reduce
the BANDWIDTH to full CCW (narrowband), and readjust the TUNING as necessary
to keep the notch centered on the ring frequency. Finally, move the EQ control up
toward center until the ring frequency reappears. Back off about AdB. This will
leave headroom so that the ring frequency will not reappear when the system gain
is increased when more ring modes are tuned out later. This procedure may be
repeated with the new ring frequencies as they appear. Usually after the first four
ring frequencies have been eliminated, the "point of diminishing returns" is reached.
NOTE:
If you are doing apermanent installation, be aware of the fact that air
temperature changes may significantly shift the frequency of the room
ring modes. To obtain added stability over time, you may wish to use
slightly broader bandwidths than the very narrow (typical "Q"=20)
bandwidths created by the full CCW rotation of the BANDWIDTH
controls.
The remaining bands on the 672A are now available to perform broadband
equalization of the system. Use apink-noise generator and third-octave real-time
analyzer for most precise results. However, if your budget doesn't permit athird-
octave analyzer, use your ears. Fortunately, you only have four or five bands to
deal with —not awhole third-octave set!
The filters on the 672A may be used to roll-off the very low and high
frequencies. Low frequencies tend to rob amplifier power and can damage speakers
(particularly bass horns) if substantial power is applied to the loudspeaker below its
cutoff frequency. Removing unnecessary high frequencies can protect tweeters and
can help control feedback problems in certain specialized applications like stage
monitors.
Notch Filtering: If athird-octave filter set is available for broadband equalization,
the 672A may be used solely as anarrowband notch filter set to eliminate up to
eight ring modes. The notches obtainable are considerably sharper than those
created by athird-octave equalizer. The 672A's highpass and lowpass filters may
also be employed to roll-off the system in acontrolled way at low and high
frequencies.
Full Electronic Crossover: If the highpass and lowpass filters are not otherwise
required, the house tuning, notch filtering, and electronic crossover functions can
all be performed simultaneously by the 672A. Set up the crossover function as
described in the Operating Instructions (in PART A), and follow the guidelines
above for House Tuning and Notch Filtering.
Partial Electronic Crossover and Individual Driver Equalizer: Substantial
advantages are sometimes obtained if eac hind ividua ldriver in asound
reinforcement system is assigned its own equalizer. Some practitioners feel that
12

this is the most powerful way of equalizing asystem. The 672A is ideal for this
application, because its lowpass and highpass filters can also be used as a
l2dB/octave electronic crossover, and because it is cost-effective.
For this application, use the MAIN/HIGHPASS output on each 672A.
Ordinarily, using I2d0/octave crossovers requires adjacent frequency range drivers
to be connected out-of-phase. This phase reversal is readily obtained either at the
power amplifier output terminals or at the input terminals of the 672A, exploiting
the balanced input feature.
To use a672A as ahigh frequency equalizer/crossover, adjust the HIGHPASS
FILTER RANGE switch to xIO, and switch the HIGHPASS FILTER IN/OUT switch
IN. Adjust the HIGHPASS FILTER TUNING control to the desired crossover
frequency (200-2000Hz). The bands in the EQ section which are tunable above the
cutoff frequency can be used for EQ. The lowpass filter is still available to shape
the upper cutoff frequency in the system.
To use a672A as alow frequency equalizer/crossover, adjust the LOWPASS
FILTER RANGE switch to xO.I, and switch the LOWPASS FILTER IN/OUT switch
IN. Adjust the LOWPASS FILTER TUNING control to the desired crossover
frequency (200-2000Hz). The bands in the EQ section which are tunable below the
cutoff frequency can be used for EQ. The highpass filter is still available to shape
the lower cutoff frequency of the system.
To use a672A as amidrange equalizer and crossover, use both the highpass and
lowpass filters to create the desired bandpass for the midrange driver. The bands in
the EQ section which can be tuned within the passband can be used for EQ.
Stage Monitors: The 672A is the ideal stage monitor equalizer because of its low
cost and notching capabilities. Follow the instructions in House Tuning above. Use
the highpass and lowpass filters to limit the frequency response of the stage
monitors in the extreme high and low frequencies. This will result in maximum
intelligibility, and will minimize feedback problems.
3: DANCE BARS The sound generally desired in adance bar is one that will make the customers
get out and boogie (or sit down and drink....). This usually implies alarge bass
boost in the region of 40-80Hz, and asmaller amount of treble boost. Bands Iand
8of the 672A can be employed for this purpose, leaving the remaining bands
available for "house tuning". In this case, we mean smoothing out midbass and
midrange acoustic response of the loudspeakers in the room.
We do not recommend that the D.J. be permitted to operate the 672A in the
course of his normal activities. The 672A should be adjusted once by the installing
contractor to the manager's specifications, and then locked up. If the D.J. is to be
provided with equalization, sufficient power to correct the sound of inadequate
records can be supplied by asimple 5-band graphic equalizer with more limited
range.
If the sound contractor has access to athird-octave real-time analyzer and pink-
noise source, these can be of substantial aid in adjusting bands 2-7 for flat acoustic
response in the midbass and midrange. Coloration here is undesirable, and can cause
customer edginess. It can also interfere with conversation. For the same reason,
excessive treble boost should be avoided because of its potential for subliminal
irritation.
13

4: RECORDING
STUDIOS
5: MOTION PICTURE
SOUND
The amount of bass boost is primarily limited not by subjective considerations but
by available amplifier power and loudspeaker bass power-handling capacity. For
maximum capability, bi-amping is recommended because the bass amplifier can be
clipped occasionally without causing obvious harshness. In this case, the 672A can
also be used as an electronic crossover.
In all cases, the setting of the 672A low frequency TUNING and BANDWIDTH
controls is quite critical in obtaining bass that is punchy, "tight", and sensual,
without being "boomy" (like "jukebox bass"). Correct settings will vary considerably
with loudspeaker type and room acoustics. In particular, satisfactory results cannot
be obtained with ahorn-loaded bass system with acutoff frequency above 40Hz.
Trying to boost bass below the cutoff frequency of the horn will cause severe
distortion and also has the potential of damaging the drivers.
The 672A can be used in the recording studio just about anywhere that a
conventional equalizer is used. Most studios find at least two channels of patchable
Parametric Equalization to be invaluable in cleaning up "difficult" tracks that the
internal console equalizers can't handle.
If atrack is plagued by hum or other interference of fixed pitch, the 672A can
be used in its narrowband notching mode (BANDWIDTH full CCW; tQ at -I6d8) to
reduce the level of the sound and its most important harmonics by l6dB, which is
often sufficient to render it inaudible. The notching of each harmonic requires use
of aseparate band.
The 672A is capable of producing awide variety of special effects. Sweeping
wideband notches gives atrue "phasing" sound. Sweeping narrowband peaks gives a
different sound with asimilar flavor.
Telephone, transistor radio, and "old-time" recording effects are most easily
generated by adjusting bands 5and 6close together in frequency (around 1.3kHz,
for example). The BANDWIDTH controls are adjusted to give moderately narrow
bandwidths and both EQ controls are set for l6dB boost. This produces the sound of
aringy bandpass filter with approximately l2dR/octave slopes and gives the
distinctive sound desired. If you want further selectivity, use the highpass and
lowpass filters. Authentic-sounding distortion can be generated by overdriving the
input of the 672A beyond the point where the OVERLOAD lamp flashes.
The 672A can also be effectively employed as amonitor system
equalizer/electronic crossover, particularly when athird-octave equalizer is too
costly. See the Sound Reinforcement section above.
The 672A can be used to particular advantage as adialog equalizer because of its
many versatile features, such as:
1) Fine-tuning capability, allowing the mixer to obtain the best possible sound for
difficult or poorly-recorded location recordings;
2) Instant notch filtering which can reduce hum and camera whine;
3) Instant lowpass and highpass filters which effectively remove both low and
high frequency acoustical noise from the track without affecting dialog;
4) Ability to use the equalizer as a standard octave-band graphic by setting
TUNING and BANDWIDTH controls to their standard settings.
The scoring mixer can similarly benefit from the fine adjustability of the 672A.
The 672A can be used to equalize the motion picture monitoring environment by
adjusting the "B" chain in the re-recording theater to the studio's accepted acoustic
response standard. The music scoring stage monitoring system can be similarly
14

6: BROADCASTING
7: ELECTRONIC
MUSIC
adjusted. In many cases, the lowpass filter can be used to simulate the Academy
Rolloff used in mixing tracks intended for optical release prints. Acurve
corresponding to typical studio practice is generated by setting the lowpass filter to
4.5kHz.
Those interested in motion picture applications should also review the section on
Recording Studios immediately above.
In AM, FM, and TV broadcasting applications, the integral RF suppression will
greatly facilitate installation of the 672A. The optional output transformer is
particularly recommended for stations with studio and transmitter at the same site.
Normal precautions regarding grounding and shielding should be followed when the
672A is installed.
The 672A is invaluable in the production studio. It can be used to "sweeten"
records (if two channels are available for stereo), and to equalize the announce
microphone for maximum punch.
If si bi lance is aproblem, the best solution is the use of an Orban Dynamic
Sibilance Controller. If one is unavailable, tuning a672A band to 6kHz, adjusting
the BANDWIDTH close to the "tic" mark, and lowering the EQ control as necessary
will help control sibilance -- although at the expense of vocal presence.
Use of narrowband peaking, and sweeping can create some wild gimmicks.
However, be aware that excessive high frequency boost can saturate the 7.5ips tape
cartridges usually used. This can be avoided by following the 672A with the Orban
Stereo Compressor/Limiter, which assures clean carts under all EQ conditions.
The 672A can also be used to enhance the sound of telephone calls, remotes,
satellite feeds, shortwave broadcasts, and network feeds. Use the LP and HP filters
to remove out-of-band noise. Presence and intelligibility can often be enhanced by
boosts in the 4-5kHz region.
The 672A is amost effective phone line equalizer. Unlike simpler phone company
equalizers, the 672A's flexibility can effectively deal with minor response "glitches".
In the main studio, the 672A can be used to notch out cart or tape machine
motor hum and ventilation system noise with minimal effect on voice quality. Use
one band for each major 60Hz harmonic, with BANDWIDTH control adjusted close
to "narrowband".
The 672A may also be used in the AM program line to equalize the air sound,
thus partially compensating for the inadequacies of typical consumer AM radios. A
certain amount of high frequency boost is essential to counteract the extremely
rolled-off performance of such radios. However, extreme boosts can cause problems
with conventional compressors and limiters (such as severe pumping, "gulping" on
material with large amounts of high frequency energy, and audibly obvious de-
essing). Therefore, for ultimate performance, we recommend using the Orban
OP TIMOD-AM, which is acomplete signal processor in cluding adjustable
equalization.
The 672A is ahighly valuable adjunct to any electronic music synthesizer. It is
particularly useful as a formant filter, and can be used as a resonator to simulate
certain instrument body sounds. The synfhesist will find the more extreme
equalization settings to be particularly useful in approaching live instrument sound
and getting away from the raw sound of typical synthesizer systems.
As this whole area is highly specialized, we will not explore it further. We suggest
that the beginner familiarize himself with the literature on musical instrument
physics.
15

Part C: Maintenance
Introduction: This part of the manual provides instructions on how to maintain the
672A, how to make sure that it is working according to specifications, and how to
repair it if something goes wrong.
Factor/ service is available throughout the life of the 672A. Please refer to
Factory Service subsection of MAINTENANCE AND SERVICE below for further
information.
1:PERFORMANCE
EVALUATION General:
This section provides aseries of thorough, definitive bench tests which will verify
whether or not the 672A is operating normally. The 672A has no trimmers, and
requires no alignment during its life.
Power Supply:
Equipment Required:
1) VTVM or DVM
2) Oscilloscope
The following tests will verify correct operation of the Power Supply:
1) Using the DC voltmeter, measure the voltage from circuit ground to both
positive and negative unregulated supplies. This can be readily measured across
the two large filter capacitors. This voltage may be expected to vary widely
depending on line voltage; it should measure between +18 and j+26 volts DC.
2) Measure the voltage between circuit ground and the outputs of the positive
and negative voltage regulators, VRI and VR2. The supplies should put out
between j- 14.25 and U5.75 VDC. If either supply exceeds 15.75 VDC, it
implies that its associated 1C regulator is defective. If either supply is lower
than 14.25 VDC, refer to the Power Supply portion of Part 3in this section
for troubleshooting hints.
3) Using the oscilloscope, measure the ripple and noise on the regulated positive
and negative power busses. Ripple and noise should be less than 2mV peak on
each bus.
Signal Processing Circuitry:
Equipment Required:
1) Oscilloscope with DC-coupled display
2) Audio sweep generator with sinewave output and logarithmic sweep (A
Tektronix 5L4N Low-Frequency Spectrum Analyzer in aTektronix 5111
Bistable Storage mainframe may be substituted for items (I) and (2).)
3) 20-20,000Hz bandpass filter, l8dB/octave slopes
4) VTVM or DVM
17

5) Harmonic distortion analyzer with built-in 400Hz and 80kHz filters and
residual THD below 0.0015%
6) Low-distortion oscillator with residua' FHD below 0.0015%
(A Sound Technology 1700A or H-P 339 will satisfy (5) and (6))
For the following tests, a600 ohm load must be provided across each 672A
output. 620 ohm 1/2 watt 5% carbon resistors are suitable.
1) Signal Passage Test
a) Connect the HI side of the 672A main output to the input of the harmonic
distortion analyzer, and also to the vertical input of the oscilloscope. Connect
the ground side of the 672A to instrument ground.
b) Ground the LO input of the 672A and connect the output of the low-
distortion oscillator to the HI input of the 672A.
c) Move all IN/OUT switches to OUT, and center all LG controls.
d) Adjust the GAIN control clockwise, and verify that an undistorted sinewave
appears at the output.
e) Continue to advance the GAIN control until clipping is barely visible. Verify
that this clipping level exceeds H9dBm for 20Hz, IkHz, and 20kHz input
signals.
f) Verify that the OVERLOAD lamp flashes at, or slightly below, the dipping
level.
2) DC offset
a) Disconnect the oscillator from the 672A input.
b) Move all IN/OUT switches to IN. Measure the DC voltage appearing at each
output of the 672A. Either voltage should not exceed 30mV DC .
3) Distortion and inverting Operation
a) Ground the HI input of the 672A and connect the low^listortion oscillator to
the LO input of the 672A. Adjust the oscillator to IkHz.
b) Move the LOWPASS and HIGHPASS FILTER IN/OUT switches to OUT, but
leave the EQ IN/OUT switch IN.
c) Using the same procedure as in the Signal Passage Test above, verify that
the gain is the same as previously measured +0.3dB.
d) Apply a20kHz signal and adjust the output level of the oscillator and the
672A's GAIN control until +I8d8m is observed at the 672A main output.
e) Measure the Total Harmonic Distortion, using the 400Hz and 80kHz filter on
the distortion meter. The THD should not exceed 0.08%.
f) Repeat at 20Hz; +l8dBm output; 400Hz filter OUT. The THD should not
exceed 0.05%.
g) Repeat step (f) using the low pass output of the 672A.
4) Noise
a) Adjust the 672A GAIN control to produce unity input/output gain.
b) Switch OUT the 400Hz and 80kHz filters in the analyzer. Connect the
20-20,000Hz bandpass filter between the 672A main output and the analyzer
input.
c) Disconnect the oscillator from the 672A, and connect both HI and LO 672A
inputs to circuit ground.
18