Orban 674A User manual

674A Stereo Equalizer
OPERATING ANNUAL

Specifications:
All specifications apply when driving
600 ohms or higher impedances. Noise
measured on an average-reading meter
through a20-20,000Hz bandpass filter
with !8dB/octave Butterworth skirts.
ELECTRICAL
Input:
Impedance, Load (each leg): I00K in
parallel 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
transformer 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:
greater than +19dBm, 20-20.000H
z
Frequency Response:
±0.25dB; 20-20,000Hz: EQ controls set
at zero detents
Available Gain:
+!2dB; adjustable to -infinity by means
of front-panel GAIN control
Slew Rate:
Varies between 6and 13V/us depend-
ing upon setting of GAIN controls; slew-
ing is symmetrical. Internal bandlimiting
assures that slew rate limiting will not
occur even with the most severe
equalization and program material.
Square Wave Response:
Square wave exhibits no spurious ring-
ing at any output level. The only ringing
observable is that theoretically
associated with any given equalization
curve.
Total Harmonic Distortion:
Less than 0.08%, 20-20,000Hz
(+!8dBm)
SMPTE Intermodulatlon Distortion:
Less than 0.05% (+!8dBm: 60/700Hz,
4:1)
Noise at Output:
Less than -78d8m (EQ in, filters out,
controls centered)
Overload/Nolse Ratio:
Better than 113dB for any single band-
pass filter, for any settings of TUNING or
BANDWIDTH controls.
Equalization Ranges:
±!6dB peaking EQ, Reciprocal
Tuning Ranges:
20-60Hz; 40-150Hz; 1!0-310Hz;
230-750Hz; 480-l900Hz; l.l-4.5Hz;
2.8-9.0kHz; 5.9-21 kHz. Dials calibrated
at ISO preferred frequencies.
Crosstalk:
Typically better than -55dB@ 20kHz;
improves at 6dB/octave below that fre-
quency.
"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 But-
terworth)
High Pass Filter Section:
Tunable in 2ranges: 20-200Hz or
200-2000HZ, 12dB/octave, (2nd-order
Butterworth)
Overload Indicator:
Lamp lights for 200ms if the instan-
taneous peak output of any amplifier
rises to within IdB of its clipping point.
Circuit Design:
Active RC realized with FET-input
opamps. Line driver employs discrete
transistor current booster.
Operating Temperature:
0-50° C
Power Requirements:
115/230VAC ±10%; 50/60Hz; 12 watts
PHYSICAL
Operating Controls (each channel):
EQUALIZATION, TUNING, and BAND-
WIDTH for each of eight bands. TUN-
ING, RANGE (xI; X10), and FILTER
IN/OUT for each filter. EQUALIZATION
IN/OUT, POWER ON/OFF, and GAIN for
entire equalizer.
Panel:
19" XS'A" (48.3 X13.3cm): 3units
Chassis Depth Behind Panel:
5'A" (13.3cm)
Weight:
Net: Ilibs. (5 kg); Shipping: 13 'A lbs.
(6.1 kg)
AC Cord:
3-wire U-ground to USA Standard
Connectors:
140 type barrier strip (5# screw); holes
punched for XLR-type connectors
(Switchcraft D3F and D3M or equal)
Circuit Ground:
Available on barrier strip; normally
jumpered to chassis.
Options:
1) Plexiglass security cover for EQ and
filter sections
2) Balanced transformers in two or four
outputs
3) XLR-type connectors on input and
two or four outputs
4) Phone jacks on input and two or four
outputs

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
4Mechanical Installation
XLR Connector Installation
5Balanced Output Tranformer Installation
6Electrical Installation
Input
Output
Wiring With Two-Conductor Shielded Cable
Grounding Diagram
7Typical Input/Output Connection Rules
7Wiring With Single-Conductor Shielded Cable
8OPERATING INSTRUCTIONS
Using the EQ Section
Boost/Cut Curve Familes
Using the Highpass and Lowpass Filters
Using the 674A As An Electronic Crossover
Headroom
I I Part B: Specific Applications
| | Stereo
IISound Reinforcement
House Tuning
Notch Filtering
Full Electronic Crossover
Partial Electronic Crossover and Individual Driver
Equalizer
Stage Monitors
13Dance Bars
14 Recording Studios
15 Motion Picture Sound
15 Broadcasting
16 Electronic Music
17
Part C: Maintenance
17 Introduction
17 Performance Evaluation
General
Power Supply
Signal Processing Circuitry
23 Maintenance and Service
Preventive Maintenance
Access
Replacement of Components on Printed Circuit Boards
Troubleshooting 1C Opamps
Factory Service
Shipping Instructions
26 Circuit Description
General
Input Buffer
Equalizer Sections
EQ IN/OUT Switch
Lowpass Filter
Highpass Filter; Output Buffer
Overload Indicator
Power Supply
31Appendix: Interconnections And Grounding
31 Driving the Input From High-Impedance And/Or High-
Level Sources
31 Grounding
NOTE
Text references to (+) and (-) input and output terminals mean the
same as schematic and rear-panel designations "HI" and "LO",
respectively.

REGISTRATION CARD The original purchaser should have received apostpaid Registration Card packed
with this manual.
Registration is of benefit to you because it enables us to tell you of new
applications, possible performance improvements, service aids, etc., which may be
developed over the life of the product, it also provides us with the date of sale so
that we may more promptly respond to possible claims under Warranty in the
future (without having to request acopy of your Bill of Sale or other proof of
purchase).
Please fill in the Registration Card and return it to us.
If the Registration Card has become lost or you have purchased the unit used,
please photocopy the image of the card reproduced below and send it to us in an
envelope. Use the address shown on the title page.
Model #—Serial #
Name or Title ———
Organization
Street ——
City/State/Country ————
Zip or Mail Code ——
Purchased from City Date of Purchase
Nature of your application ——
How did you hear about it?
Comments: ———
Fig. I: REGISTRATION CARD
WARRANTY The Warranty, which cpplies only to the first end-user of record, is stated on the
Warranty Certificate on aseparate sheet packed with this manual. Save it for
future reference.

INTRODUCTION
PERFORMANCE
HIGHLIGHTS
Part A: Installation and Operation
The Orban 674A is adual-channel eight-band quasi-parametric equalizer of high
professional quality, and is the stereo version of Orban's popular 672A equalizer.
Because optimizing the controls for convenient stereo operation required placing
corresponding "A" and "B" controls physically close to each other, there is slight
crosstalk between the channels at very high frequencies (typically better than -55dB
at 20kHz, falling at 6dB/octave at lower frequencies). Asmall amount of leakage
from one channel to the other may be heard if you attempt to use each 674A
channel to equalize entirely independent program material, and we recommend that
the 674A be used only with stereophonic program material. If you require absolute
isolation between the two channels, we recommend use of apair of 672A
equalizers instead.
The 674A 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 each EQ section for added
versatility. When these filters are overlapped (200-2000Hz), the 674A can be used
as a combined graphic/parametric equalizer and tunable electronic crossover.
Separate lowpass and highpass outputs are provided for this purpose.
Each channel of the 674A 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. AH 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 it becomes audible.
The flexibility offered by the 674A makes it aparticularly powerful tool in nearly
all areas of audio: sound reinforcement, public address, recording studio,
broadcasting, motien picture sound, dance bars (discos), theater
The 674A 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 674A are fully described in this manual. It will
familiarize you with the unit's potential and enable you to imaginatively use the
674A 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: 1frequency 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
l2dB 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 (retrofit kit available)
FRONT PANEL
DESCRIPTION
The GAIN control adjusts the 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
+!6dB. 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 EQ
section. The "Q" becomes broader when turned clockwise; sharper when turned
counterclockwise.
2

REAR PANEL
DESCRIPTION
AC POWER
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 switch 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 the front panel.
The FUSE used in the 674A is a3AG slo-blo type: 1/4 amp for 115V operation
and 1/8 amp for 230V operation. Replace with the same type only.
The INPUT and OUTPUT connectors provided allow connection via barrier strip (#5
screw). In addition, acover plate masks holes for user-installed XLR-type
connectors and/or 1/4" phone plugs.
The BLOCK DIAGRAM outlines the signal flow through the various sections of
each channel of the 674A. It is reproduced here:
Please refer to the Electrical Installation section for connection instructions.
The power transformer can be strapped 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.
The two primary windings of the power transformer are connected in parallel for
115 volt operation, and in series for 230 volt operation. (See the Schematic
Diagram at the back of this manual.)
To strap the power transformer for adifferent voltage, remove the top cover of
the 674A. Strapping instructions are found on the insulating fishpaper around the
power tranformer. 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 green/yellow) wire (which is connected to the long prong) is connected directly
to the 674A 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.
3

WARNING!
IF THE GROUND IS DEFEATED, CERTAIN FAULT CONDITIONS IN
THE UNIT OR THE SYSTEM TO WHICH IT IS CONNECTED CAN
RESULT IN APPEARANCE OF FULL LINE VOLTAGE BETWEEN
CHASSIS AND EARTH GROUND. SUCH VOLTAGE IS CAPABLE OF
CAUSING SEVERE INJURY OR DEATH!
MECHANICAL
INSTALLATION
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.
INSTALLATION
OF OPTIONS
Remove the cover plate from the rear chassis apron and install each connector
with apair of #4-40-1/4" flat-head screws, nuts, and lockwashers. On each channel
in turn, connect jumper wires from the barrier strip to the XLR's as shown in Fig.
I.
XLR Connector Installations To install the optional XLR connectors, obtain (2 ea.)
Switchcraft D3M and (2 ea.) Switchcraft D3F (or equivalent) connectors from a
local supplier. These connectors are also available directly from Orban as retrofit
kit RET-28.
Phone Jack Installation: To install the optional phone jacks, obtain (4 ea.)
Switchcraft #I2B (tip-ring-sleeve) phone jacks from alocal supplier. These jacks are
also available directly from Orban as RET-29.
Remove the hole plugs from the rear panel and install the jacks using the
hardware supplied with the jacks. Then connect jumper wires from the barrier strip
to the jacks as shown in Fig. I.
4

CUA55X5
MVE.VM FROfA \NS\OE OF (oTA-A CHAB5\B
\M\RE U&T
XLR CONNECTOR INPUT XLR CONNECTOR HP OUT XLRCONNECTOR fAA\N/LP OOT
FROtA TO COLOR FROtA TO COLOR FRO/A TO COLOR
3S-\ (A,) 35- &(&N0) BLR Jl-' (Ay) 3t-e> (<*«?) &LK 3V\ (Ay) JB-G(GNP-) BLK
35-2. (U» T&\-Z\n(LC>’) BRNT 3\-Z (L0*> TBl-fc oin(i) gioA 3%-Z CLO') TBI-N OUT (4-) &RVT
JS-3 (NO TOW IU (W) REOJ3\-S (WV) tb\-s<*jt(rO 6R*J' 3VB TB\-5Cm(R^ ^J
TABLE SHON*JKi FOR LEFT CHANURL,REPL\CATE FOR R\&*T CHANNEL
Inpot
pin Ichassis ground
pin 2LO
pin 3HI
CXitput
pin Ichassis ground .
pin 2circuit ground (unbalanced) or LO (transformer-balanced)
pin 3HI (balanced or unbalanced)
Balanced Output Transformer Installation: If transformers were not installed at
the factory, refer to the installation instructions furnished with the transformer
retrofit kit, available directly from Orban. The transformers supplied with this kit
have been designed to have anegligible effect on published specifications. Should
you wish to use some other transformers, it would be wise to make careful
performance measurements with special attention to LF distortion and HF response
at high output levels, thus determining the output level achievable with
performance acceptable for your application. Atransformer meeting Orban
standards should produce approximately +20dBm (limited by clipping in the output
amplifier) without significantly compromising performance.
The Electrical Installation section describes grounding procedures in the event
transformers are used.
5

ELECTRICAL
INSTALLATION
Input
Output
Connecting the 674A Equalizer to other equipment is quite straightforward.
Relatively uncomplicated systems (such as home playback systems, "semi-pro"
recording studios, electronic music studios, dance bars, etc.) tend to come together
without serious grounding problems even if fhe 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 inferconnections and grounding can be found in the Appendix.
The electronical ly-balanced input of each channel of the 674A equalizer is
compatible with most professional and semi-professional sound equipment, balanced
or unbalanced, whose source impedance is 600 ohms or less. If if 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 +4dBm. The absolute overload point is
+26dBm.
The two outputs of each channel of the 674A 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/LOWPASS outputs for normal operation when the crossover feature
is not used.
If the 674A is being used as afull electronic crossover, use the MAIN/LOWPASS
outputs to drive the low-frequency amplifiers, and the HIGHPASS outputs to drive
the high-frequency amplifiers.
The LOWPASS and HIGHPASS outputs of agiven channel are in-phase .Since
correct frequency response from al2dB/ocatve crossover requires that the drivers
be out of phase,phase reversal must be effected elsewhere in the system, such as
at the outputs of the power amplifiers.
Wiring the 674A 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
6

TABLE I:
TYPICAL INPUT/OUTPUT CONNECTION RULES
INPUT
1) Always use "HI" and "LO" as the two input terminals to the 674A.
2) When the 674A is driven from an unbalanced source, connect shield both to
circuit ground of source, and to chassis ground of 674A.
OUTPUT
1) When the 674A is driven from abalanced source, connect shield at source end
to chassis ground. Do not connect shield at 674A end.
2) On the 674A output, connect shield at 674A end to chassis ground (whether
driving balanced or unbalanced). Do not connect shield at other end.
3) When driving abalanced load, jumper circuit ground to chassis ground on 674A
(on rear panel). When driving an unbalanced load, do not attach jumper.
4) 674A chassis should always be earth-grounded (i.e. through third wire in power
cord or through rack.) For maximum protection from shock, float this ground
only as last resort.
5) If optional output transformer(s) are installed on 674A, jumper the circuit
ground to chassis ground on the 674A.
iBecause it is not always possible to determine if the pieces of equipment driving
Ior being driven by the 674A 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 probably there
jisn't enough grounding. Try connecting the "-" input of the 674A to achassis
|ground terminal on the 674A's barrier strip and see if the buzz goes away. You can
also try strapping the 674A's chassis and circuit grounds together, and see if this
jhelps.
|
|Aground loop usually sounds like asmooth, steady hum rather than acrackly
ibuzz. If you have aground loop, you can often break it by disconnecting the
jumper between circuit and chassis grounds on the 674A's rear-panel barrier strip,
tIn either case, think carefully about what is going on, and keep in mind the
jgeneral principle: one and only one circuit ground path should exist between each
piece of equipment! (Bear in mind that the circuit grounds of the two channels of
the 674A are connected together internally, and could conceivably introduce a
ground loop if you do not take this connection into account in planning your
wiring.)
;Wiring The 674A With Single-Conductor Shielded Cable: Sometimes, particularly if
you are using the 674A with musical instruments or home-type equipment, you will
find yourself with no time to correctly connect the 674A, and will find instead
that you must use single-conductor shielded cables (usually terminated by 1/4"
phone plugs which can be plugged into the 674A's optional phone-plug inputs and
7

outputs). If this happens, connect the inner conductors of the shielded ccbles to the
HI sides of the 674A inputs and outputs. Connect the shields to the LO sides. (The
HI side appears on the tjg 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 674A. The chassis ground/circuit ground jumper on
the rear barrier strip of the 674A 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-CONDUCTOR PHONE PLUG IS
USED 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!
OPERATING The operating controls of the 674A have been configured to permit easy
U^yi^jj£-y|Q|^j£ adjustment of corresponding controls in channels "A" and "B" in stereo applications.
In the EQ section, "A" and "B" EQ sliders are located next to each other. TUNING
and BANDWIDTH controls are (non-clutched) concentric, so that they can be readily
manipulated either simultaneously or independently.
For the same reason, the "A" and "B" channels of the LOWPASS and HIGHPASS
filters are grouped together by function in adjacent areas of the front panel.
To provide best value, the 674A uses controls of high reliability but modest
calibration accuracy. Calibrations are approximate, and are intended primarily as
reference guides. However, placing an EQ control on its center detent accurately
defeats the equalization in that band. When operating with narrow bandwidths,
accurate stereophonic matching on the basis on panel calibrations alone is
impractical. However, if the BANDWIDTH controls are operated on, or more
clockwise than, their "tic" marks, then no problems will occur. (See the subsection
on Stereo Matching in Part Bfor matching instructions.)
The following instructions apply to each channel of the 674A.
Using the EQ Section
To use the EQ section alone, switch the HP and LP filter switches OUT. Switch ,
the EQ section switch IN.
For those who have never used aParametric Equalizer before, the easiest way to
become familiar with the 674A is to set the TUNING and BANDWIDTH controls at
the "tic" marks. The 674A will now behave like an eight-band octave graphic
equalizer (on ISO standard octave frequencies of 63, 125, 250, 500, 1000, 2000,
4000, and 8000Hz). Once you have gotten the "feel" of the 674A in this mode, try
experimenting with the TUNING and BANDWIDTH controls to see how they affect
the sound.
When you boost the EQ, discover the subtle shelving effects available from
broadband peaking (BANDWIDTH control close to full clockwise; EQ control above
the center detent). Contrast this with the "ringy", colored quality of setting the
BANDWIDTH control toward narrow (ccw).
When you cut the EQ observe the effects you now achieve. Narrowband dips are
essentially inaudible, but permit suppressing sounds of fixed frequency (like hum), up
to l6dB in each band. (If the sound to be suppressed is rich in harmonics, use one
band per dominant harmonic.)
8

The EQ curves are reciprocal: boost and cut are mirror images of each other. A
given amount of EQ (in recording atrack, for example) can be precisely cancelled
later by passing the track through the equalizer with all TUNING and BANDWIDTH
controls set as they were during the original recording, but with all EQ controls set
equal but opposite to their original settings. Boost becomes cut, and vice-versa.
More boost or cut can be achieved by tuning adjacent bands to the same
frequency. +32dB 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 continuously
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 absolute
bandwidth (in Hertz) does not vary.
Specified "Q" range for any setting of the TUNING control exceeds 0.5 to 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.
672/4A EQUALIZER CURVES AT 0-8.5,1.4,4,410
Fig. 4: BOOST/CUT CURVE 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 after 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 the 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).
9

Using The 674A As An Electronic Crossover
If you do not otherwise need the lowpass and highpass filters, they may be used
as afull !2dB/octave Butterworth electronic crossover in bi-amped systems.
Connect the MAIN/LOWPASS output to the low frequency amplifier, and the
HIGHPASS output to the high frequency amplifier. Turn the HIGHPASS FILTER
IN/OUT switch to OUT, and the LOWPASS 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:
First, disconnect 674A outputs from the amplifiers. Then, switch both filters OUT
temporarily.
Connect the oscillator to the 674A input, and the AC VTVM to the 674A's
MAIN/LOWPASS 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 LOWPASS filter IN,
and adjust its TUNING control until the AC VTVM reads -3dB. Then switch the
LOWPASS filter OUT and switch the HIGHPASS filter IN. Adjust the HIGHPASS
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 FILTER IN/OUT
switch IN. Reconnect the amplifiers to the 674A outputs. The two outputs are
PHASE. The !2dB/octave configuration requires an out-of-phase connection to the
drivers for proper blending at the crossover point. Phase reversal can be effected
by reversing the connections at the output of one power amplifier (either low or
high frequency) only .
Notes On Headroom
The overload-to-noise ratio available from the 674A varies from about 100 to
lOSdB depending on the settings of the controls. To minimize audible noise while
driving power amplifiers, for example, the overload point of the 674A 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 674A is adjusted in level such that it just barely
lights the 674A 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 674A/power amp combination.
WARNING!
PRIOR TO TESTING, BE SURE THAT THE POWER AMP LOAD (SUCH
AS ALOUDSPEAKER) WILL NOT BE DAMAGED BY FULL-POWER
SINEWAVE. IF YOU THINK THAT IT MIGHT BE, USE ADUMMY LOAD
OR ATEST SIGNAL WITH AHIGHER PEAK-TO-AVER AGE RATIO (LIKE
PINK NOISE LOWPASS FILTERED AT IKHZ) TO AVOID DAMAGE TO
TWEETERS. 10

Part B: Specific Applications
This part of the manual provides very specific instructions and suggestions on how
to use the 674A 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 MATCHING For broadband equalization (BANDWIDTH controls at or clockwise of the "tic"
marks), no difficulty should result when the channels are matched on the basis of
panel calibrations alone. In applications other than monitor tuning or sound
reinforcement, however, if any BANDWIDTH control is adjusted "narrowband" (i.e.,
0through 5), then test instruments should be used to assure matching between
stereo channels, assuring precise stereo imaging and accurate mono summing of the
channels. (See the section on Dance Bars below for adiscussion of matching
requirements in monitor tuning and reinforcement applications.)
The following procedure assumes that both channels have been adjusted by ear to
achieve the desired equalization, and that both are adjusted identically with
reference to panel calibrations. One channel must be arbitrarily chosen as the
"reference", and the other channel's controls must then be slightly tweaked to
achieve aprecise instrument-match to this reference. This is most readily done by
the "differential method": the reference channel and the channel under adjustment
are swept out-of-phase, and their outputs are summed. The channel 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 input of
the reference channel such that the hot output is connected to the reference
channel's (+) input, and its (-) input is grounded.
2) Connect another output from the sweep generator to the channel under
adjustment. Connect the hot output to this channel's (-) input; ground its (+)
input.
3) Sum the outputs of the two channels by connecting each (+) output to one
side of a22K 5% 1/4 watt carbon resistor (the value is not particularly
critical). Connect the other side of both resistors to the vertical input of an
oscilloscope.
4) Sweep the channels from 20-20,000Hz, and adjust the scope sensitivity to
easily see the swept component.
5) Switch aj] IN/OUT switches on both channels OUT. Adjust the GAIN control
on the channel 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 channel).
7) Slightly adjust the various controls on the channel under adjustment to
minimize the amplitude of the differential component as observed on the scope.
2: SOUND The 674A is an extremely versatile sound reinforcement equalizer. It can function
PPIMFORrPMFNT as anotch filter>broadband equalizer, filter, and electronic crossover. Many of
these functions are simultaneously available.

f
Many reinforcement systems are mono. In these installations, particularly powerful
results can be obtained by connecting both 674A channels in series .Atotal of
sixteen bands are then available for notch filtering or equalization. In addition, the
first set of filters can be used to set the passband of the system, while the second
set can be used as an electronic crossover.
House Tuning: In an economy installation, the 674A 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 bands 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 a single 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 674A which covers it. Turn the BANDWIDTH control to broad, and pull
the EQ control down to -1 6dB. 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 4dB. 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 674A 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 a whole third-octave set!
The filters on the 674A 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 674A may be used solely as a narrowband notch filter set to eliminate up to
eight ring modes per channel. The notches obtainable are considerably sharper than
those created by athird-octave equalizer. The 674A's highpass and lowpass filters
may also be employed to roll-off the system in acontrolled way at low and high
frequencies.
12

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 674A. 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 each individual driver in asound
reinforcement system is assigned its own equalizer. Some practitioners feel that
this is the most powerful way of equalizing asystem. The 674A is ideal for this
application, because its lowpass and highpass filters can also be used as a
12dB/octave electronic crossover, and because it is cost-effective. (Each channel of
the 674A will ordinarily be used for adifferent driver, as the added power
obtained by connecting the two 674A channels in series is not needed in this
application.)
For this application, use the MA1N/LOWPASS output on each 674A.
Ordinarily, using l2dB/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 674A, exploiting
the balanced input feature.
To use a674A 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 +he 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 a674A as a low 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 belowthe
cutoff frequency can be used for EQ. The highpass filter is still available to shape
the lower cutoff frequency of the system.
To use a674A 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 674A is the ideal stage monitor equalizer because of its low
cost per channel 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 674A 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.
13

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 674A can
also be used as an electronic crossover.
In all cases, the setting of the 674A 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.
We do not recommend that the D.J. be permitted to operate the 674A in the
course of his normal activities. The 674A 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.
If athird-octave analyzer is available to monitor the acoustic response of the
system, it is not necessary or desirable to exactly match the channels in the
frequency range below 200Hz according to Stereo Matching above. This is because
different loudspeaker locations will ordinarily produce different bass balances jn
the room unless the room has been acoustically designed and is symmetrical (like a
good recording studio control room). In adance bar, the chances of this occuring
are slight, to say the least!
It is quite acceptable to adjust the two 674A channels substantially differently
below 200Hz to smooth out standing waves, resonances, and the like. Above 200Hz,
equalization is much more useful in correcting the response of the loudspeaker than
in correcting room acoustics. If the loudspeakers are reasonably well-matched, the
equalizer channels should therefore be adjusted to also be well-matched above
200Hz. If the "A" and "B" channel curves differ greatly, this often results in the
system's being well-equalized at only one place in the room (where the measuring
microphone was located) at the expense of other locations which may have their
problems exaggerated!
4: RECORDING The 674A can be used in the recording studio just about anywhere that a
\C 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 674A can
be used in its narrowband notching mode (BANDWIDTH full CCW; EQ at -l6dB) 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 a separate band.
14

The 674A 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 l2dB/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 674A beyond the point where the OVERLOAD lamp flashes.
The 674A 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.
5: MOTION PICTURE The 674A can be used to particular advantage as adialog equalizer because of its
many versatile features, such as:
I) 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 astandard 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 674A.
The 674A 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
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.
6: BROADCASTING in AM, FM, and TV broadcasting applications, the integral RF suppression will
greatly facilitate installation of the 674A. The optional output transformers are
particularly recommended for stations with studio and transmitter at the same site.
Normal precautions regarding grounding and shielding should be followed when the
674A is installed.
(Because of slight high frequency crosstalk (because of the proximity of "A" and
"B" channel controls), it is recommended that the two 674A channels process
stereophonically-related program material only.Do not use each channel for a
totally different program.)
15

The 674A is invaluable in the production studio. It can be used to "sweeten"
records, and to equalize the announce microphone for maximum punch.
If sibilance is aproblem, the best solution is the use of an Orban Dynamic
Sibilance Controller. If one is unavailable, tuning a674A 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 674A with the Orban
Stereo Compressor/Limiter, which assures clean carts under all EQ conditions.
The 674A 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 674A is amost effective phone line equalizer. Unlike simpler phone company
equalizers, the 674A's flexibility can effectively deal with minor response "glitches".
In the main studio, the 674A 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 674A 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
OPTIMOD-AM, which is acomplete signal processor including adjustable
equalization.
7.' ELECTRONIC MUSIC The 674A is ahighly valuable adjunct to any electronic music synthesizer. It is
particularly useful as aformant filter, and can be used as aresonator to simulate
certain instrument body sounds. (If both channels are connected in series, up to
sixteen high "Q" resonances can be simulated.)
The synthesist 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.
This concludes the SPECIFIC APPLICATIONS section.
16