Universal Audio 1178 User manual

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DUAL PEAK LIMITER
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TABLE OF CONTENTS PAGE
SECTION I; INTRODUCTION
1.1 DESCRIPTION 1
1.2 SPECIFICATIONS -ELECTRICAL 2
1.3 FRONT PANEL CONTROLS 3
1.4 PHYSICAL 4
1.5 CONNECTIONS 4
SECTION II: INSPECTION AND INSTALLATION
2.1 UNPACKING AND INSPECTION 5
2.2 ENVIRONMENTAL CONSIDERATIONS 5
2.3POWERING 5
2.4 LINE VOLTAGE SWITCH 6
2.5 EXTERNAL CONNECTIONS 6
2.6 IMPEDANCE AND TERMINATION 8
2.7 ACCESSORIES 8
2.7.1 MODEL 301 ADAPTER 9
2.7.2 MODEL SC-2 SECURITY COVER 9
SECTION III: OPERATING INSTRUCTIONS
3.1 GENERAL DEFINITIONS 10
3.2 INITIAL SETUP H
3.3 PERFORMANCE CHECK 11
3.3.1 INPUT-OUTPUT SIGNAL LEVEL 11
3.3.2 GAIN REDUCTION CHECK 12
3.3.3 STEREO TRACKING 12
3.4 APPLICATION 12
3.4.1 CONTROL SETTINGS 12
3.4.2 EFFECTS OF COMPRESSION RATIO 13
3.4.3 EFFECTS OF THRESHOLD 13
3.4.4 EFFECTS OF ATTACK TIME 13
3.4.5 EFFECTS OF RELEASE TIME 14
3.5 PLACEMENT OF THE 1178 LIMITER IN THE SIGNAL PATH 14
3.5.1 AFTER PREAMP 14
3.5.2 EQUALIZATION, BEFORE OR AFTER THE LIMITER? 15
3.5.3 FULL PROGRAM MATERIAL 15
3.5.4 PRIOR TO OUTPUT AMPLIFIER 16
3.6 STEREO OPERATION 16
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TABLE OF CONTENTS
(continued) PAGE
SECTION IV: THEORY OF OPERATION
4.1 GENERAL 17
4.2 INPUT AMPLIFIER 17
4.3 VOLTAGE-VARIABLE RESISTOR ATENUATOR 17
4.4 SIGNAL PREAMPLIFIER 17
4.5 LINE AMPLIFIER 17
4.6 GAIN REDUCTION AMPLIFIER 18
4.7 METER AMPLIFIER 19
4.8 POWER SUPPLY 19
SECTION Vi MAINTENANCE
5.1 GENERAL 20
5.2 REPAIRS AND WARRANTY 20
5.3 INTERNAL SERVICE ADJUSTMENTS 20
5.3.1 POWER SUPPLY 21
5.3.2 Q-BIAS ADJUSTMENT 21
5.3.3 GR METER ZERO AND TRACKING 22
5.3.4 COMMON MODE REJECTION 22
5.4 IN CASE OF DIFFICULTY 22
5.4.1 GENERAL 22
5.4.2 PRECAUTIONS FOR TROUBLESHOOTING 23
5.4.3 VISUAL TEST 24
TROUBLESHOOTING TABLE (FIG. 5-1) 24
5.5 PC BOARD CLEANING 26
5.6 CLEANING THE LIMITER 26
SECTION VI ;APPENDIX
FIGURE 6-1. MOUNTING INSTRUCTIONS FOR MODEL 301 27
FIGURE 6-2. MODEL 1178 DUAL PEAK LIMITER SCHEMATIC
DIAGRAM 28

SECTION I
INTRODUCTION
FIGURE 1-1. MODEL 1178 FRONT PANEL.
1.1 DESCRIPTION
The Model 1178 is atwo channel version of the very successful
1176LN Peak Limiter used in many recording studios, broadcast
stations, sound reinforcement installations, etc. around the world.
The Dual Peak Limiter was designed for those applications where two
channels of program material are processed, and the balance of these
audio channels must be maintained. The two limiters inside the
Model 1178 are matched to assure the high degree of tracking neces-
sary for critical stereo applications.
Single adjustments of attack and release time affect both chan-
nels simultaneously, while the input and output level controls are
adjusted independently. In the stereo mode, afront panel switch
allows the two limiters' gain reduction amplifiers to be linked
together such that the audio signal with the larger peak-to-peak
amplitude causes the same amount of gain reduction in both channels.
On the other hand, when switched to single mode operation, the two
limiters process the signals independently. This provides the user
with exceptional flexibility in operation.
The Model 1178 was designed to have performance characteristics
similar to the single channel 1176LN. Limiting is accomplished by
utilizing FET's as voltage variable resistors. Unique circuitry
permits severe limiting without added distortion and the compression
ratio may be optimized for various program materials. The gain
structure throughout the Model 1178 maintains an excellent signal-
to-noise ratio at normally used control setting.
Dual, backlighted front panel meters may be switched to read
either the amount of gain reduction, or the output signal level
referenced to +4 dBm or +8 dBm. Also switch selectable are the
meter reading characteristics: either indicating average signal
(VU ballistics) or peak.
-1-

1.2 SPECIFICATIONS
ELECTRICAL (Each Channel):
INPUT: Balanced bridging, differential amplifier.
INPUT IMPEDANCE: 20 kohms, used as balanced input.
10 kohms, used as unbalanced
(single-ended) input.
MAXIMUM INPUT LEVEL: +20 dB maximum (Ref. 0.775 volts rms).
MAXIMUM GAIN: 45 dB, ±1 dB.
FREQUENCY RESPONSE: ±1 dB, 20-20,000 Hz.
OUTPUT: Floating, transformer isolated.
OUTPUT LOAD: 150 ohms or greater.
MAXIMUM OUTPUT LEVEL: +24 dBm into 600 ohm load (12.28 volts).
+20 dB into 150 ohm load (7.75 volts).
DISTORTION: Less than 0.5% THD, 50 Hz to 15 kHz with
limiting, at 1.1 seconds release setting;
(as with all limiting devices, distortion
of low frequency peaks increases with
faster release time)
.
SIGNAL TO NOISE RATIO: Greater than 81 dB at threshold of limiting,
30 Hz to 15 kHz.
ATTACK TIME: Less than 20 microseconds for 100% recovery;
adjustable to 800 microseconds with front-
panel control
.
RELEASE TIME: 50 milliseconds minimum, 1.1 seconds
maximum (for 63% recovery). Adjustable
with front-panel control.
Input Level at Relative
Minimum Limiting Output at
Threshold ±2 dB Threshold*
-24 dB +10 dBm
-25 dB +9 dBm
-26 dB +8 dBm
-30 dB +7 dBm
*with output gain control set to provide
areserve of approximately 10 dB.
NOTE: Throughout this manual, where the expression "dB" is used
to denote asignal level, it is referenced to 0dB -0.775 Vrms.
THRESHOLD vs
OUTPUT LEVEL: Compression
Ratio
Setting
20:1
12:1
8:1
4:1
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POWER REQUIREMENTS: 100 -125 VAC, or 200 -250 VAC, 50/60 Hz,
switch selectable, less than 10 W.
ENVIRONMENT: Operating, 0*C to +50 ®C;
Storage, —20“C to +60®C.
1.3 FRONT PANEL CONTROLS
INPUT AND OUTPUT LEVEL: Continuously adjustable.
ATTACK :Continuously adjustable, 20 microseconds
to 800 microseconds. (Fastest attack is in
the maximum clockwise (CW) position.)
LIMITING ON/OFF: This switch is coupled
with the Attack Time control. In the
full counterclockwise position (CCW) the
limiting function is disabled.
RELEASE: Continuously adjustable from 50 milli-
seconds to 1.1 seconds. (Fastest release
in obtained in the maximumum CW position.)
A/B -STEREO SWITCH :In the A/B position the Model 1178 may be
operated as two independent limiters. In
the STEREO position the gain reduction
amplifiers of the two channels are linked
together, and the maximum limiting in one
channel causes an equal amount of limiting
in the other channel. (The compression
ratio, attack time, and release time are
always identical in both channels .
)
COMPRESSION RATIO: Switch selectable with four interlocking
pushbuttons
.
METER FUNCTION: Three interlocked pushbuttons switch
the meter to read either the amount of
gain reduction or the output signal level
referenced to +4 dBm or +8 dBm. The
fourth pushbutton selects the response
characteristic of the meter when reading
the output level. "Average" is typical
VU indication as selected with the +4 or
+8 dBm pushbutton. In the "Peak" mode the
meter reacts faster, and 0VU corresponds
to alevel which is 6dB higher than the
selected output signal reference.
(Gain reduction indication is always
average responding.)
-3-

POWER SWITCH:
1.4 PHYSICAL
DIMENSIONS:
FINISH:
WEIGHT:
SHIPPING WEIGHT:
ACCESSORIES:
1.5 CONNECTIONS
Toggle switch with LED to indicate when the
Model 1178 is powered.
483 X89 mm rack panel .Depth behind
panel 203 mm (19" x3-1/2" x8").
Panel is 3.18 mm (1/8") brushed clear
anodized aluminum in 2shades. Chassis
is cadmium plated steel.
5kg (11 lb)
.
6.6 kg (14.5 lb)
.
Model 301 XLR/QG Adapter for signal input
and output
.
Model SC-2 Security Cover.
Rear chassis barrier strip for input and
output. Power through 3wire lEC-style
connector
.
-4-

SECTION II
INSPECTION AND INSTALLATION
2.
1
UNPACKING AND INSPECTION
Your Model 1178 was carefully packed at the factory, and
the container was designed to protect the unit from rough handling.
Nevertheless, we recommend careful examination of the shipping
carton and its contents for any sign of physical damage which could
have occurred in transit.
If damage is evident, do not destroy any of the packing mater-
ial or the carton, and immediately notify the carrier of apossible
claim for damage. Shipping claims must be made by the consignee.
The shipment should include:
Model 1178 Dual Peak Limiter
UREI Instruction Manual (this book)
Two-part Warranty Card bearing the
same serial number as the Model 1178.
Rack mounting hardware.
2.2
ENVIRONMENTAL CONSIDERATIONS
The system will operate satisfactorily over arange of ambient
temperatures from 0*C to +50-0 (+32*F to 122*F), and up to 80% non-
condensing relative humidity.
If the system is installed in an equipment rack with high heat
producing equipment (such as power euntiplifiers ),adequate ventilation
should be provided in order to assure longest component life. Also,
while circuitry susceptible to hum pick-up is sufficiently shielded
from moderate electromagnetic fields, installation should be planned
to avoid mounting the system immediately adjacent to large power
tr ans formers ,motors ,etc
.
2.3
POWERING
The 1178 may be operated from either 100 -125 VAC or 200 -
250 VAC mains (50 Hz or 60 Hz, single phase.) As indicated in
Section 2.4, the nominal line voltage may be selected with arear
panel switch. BE SURE TO VERIFY BOTH THE ACTUAL LINE VOLTAGE, AND
THE SETTING OF THE VOLTAGE SELECTOR SWITCH BEFORE CONNECTING
THE 1178 TO THE MAINS.
-5-

To comply with most Electrical Codes, the 1178 is supplied
with athree-wire lEC style connector, the grounding pin of which
is connected to the chassis* In some installations this may create
ground—loop problems* Ground loops can result in hum and buzz if
asignificant potential difference exists between the AC conduit
ground and the grounded metal enclosure in which the chassis is
installed* If hum is experienced, one may check for the possibility
of ground loops by using a3—prong to 2—prong AC adapter between
the power cord and the mains supply, ungrounding the AC plug
temporarily* This ungrounds the Model 1178, and may cure the hum
or buzz, but is not asubstitute for proper system grounding* Be
aware that unless the Model 1178 Dual Peak Limiter is AC grounded,
asafety hazard can exist* UREI accepts no responsibility for legal
actions or for direct, incidental or consequential damages that may
result from violation of any electrical codes*
2*4 LINE VOLTAGE SWITCH
Unless atag on the line cord specifies otherwise, the
Model 1178 was shipped ready for operation with nominal 115 VAC
power mains* In order to change this for nominal 230 V(50 Hz or
60 Hz), slide the VOLTAGE SELECTOR switch on the rear panel to the
230 position* The voltage is visible in awindow next to the switch
slot* Be sure to change the fuse to the correct value: 1/8-amp slo-
blo when changing to 230 Voperation or 1/4-amp slo-blo for 115 V
operation* Asmall screwdriver should be used to move the recessed
switch*
2*5 EXTERNAL CONNECTIONS
Permanent input and output signal wires should be shielded
cable, and connected in accordance with standard wiring practice, as
indicated on the rear panel barrier strip* If the XLR/QG connectors
are to be used, install the accessory Model 301 according to the
instructions suppplied with the adapter* (See also "Mounting
Instructions," Page 9, and Section VI*)
If the Model 1178 output is connected to ahigh impedance
circuit, we recommend shunting the "±" and "COM" output terminals
with a620 ohm, 1/2 watt resistor* This assures optimum loading*
(See Section 2*6 regarding input termination)*
(See Figures 2-1 and 2-2 on the following page
for recommended connection procedures*)
-6-

OUTPUT QRD INPUT
FIGURE 2-1. CONNECTING THE MODEL 1178 WITH BALANCED
INPUT AND BALANCED OUTPUT CIRCUITS.*
FIGURE 2-2. CONNECTING THE MODEL 1178 WITH UNBALANCED
INPUT AND UNBALANCED OUTPUT CIRCUITS.*
*With abalanced input and unbalanced output, or vice-versa,
use the appropriate conhections suggested by each of the above
diagrams. No special switching or transformers are needed.

2.6 IMPEDANCE AND TERMINATION
Audio engineering had its roots in the telephone industry, and
"600 ohm circuits" (together with their predecessors, "500 ohm
circuits") are carry-overs from telephone transmission practices.
Long audio transmission lines, like their video counterparts, must
be properly sourced from and terminated in equipment which matches
their characteristic impedance, if optimum frequency response and
noise rejection are to be achieved.
However, transmission line theory and techniques are not
only unnecessary but impractical within modern recording studios,
broadcast studios and other local audio systems where transmission
circuits are seldom more than several hundred feet in length. The
advent of negative feedback circuitry and solid-state electronics
has spawned modern audio amplifiers and other signal processing
devices having source impedances of only afew ohms. They are
essentially indifferent to load impedances and by varying their
output current inversely to changes in load impedance, maintain
the same output voltage into any load impedance above arated
minimum, with no change in frequency response. Most new designs
(all at UREI) have high input impedance to allow use with various
source impedances.
Most modern audio systems, therefore, utilize amplifiers and
other active devices which have very low output impedances and high
(lOK to 50K) input impedances. These products may thus be cascaded
(operated in tandem), or many inputs may be connected to asingle
output of aproceeding device, without regard to impedance "matching".
Switching, patching, etc. is simplified because "double loads" and
"unterminated" bugaboos are essentially eliminated. "Floating"
(ungrounded) transformer outputs minimize ground loop problems, and
differential transformerless input circuitry (or input transformers)
minimize common mode noise or interference which may be induced
into the interconnecting wires or cables, or produced by different
ground potentials
.
Where audio must be transmitted through cables or wire pairs
of more than several hundred feet in length, however, transmission
line termination practices should still be observed.
The Model 1178 has an input impedances of 20,000 ohms when
used in abalanced, differential input configuration, and 10,000
ohms when used unbalanced (one side grounded). This makes the
device suitable for use with any normal source impedance, low or
high. Only when it is used from asource which requires alow
impedance termination (such as a 600-ohm transmission line or older
vacuum tube equipment) is asource termination resistor required at
the 1178 input.
-8-

2.7ACCESSORIES
2.7.1 MODEL 301 ADAPTER
Two holes are provided adjacent to the ends of the IN/OUT bar-
rier strip. This allows simple mounting of the Model 301 and adapts
the limiter to signal cables fitted with XLR/QG termination.
2.7.2 SECURITY COVER SC-2
An optional Security Cover is available to cover all operating
controls of the Model 1178, and to protect against any inadvertent
misad justment of critical settings.
-9-

SECTION III
OPERATING INSTRUCTIONS
3.1 GENERAL DEFINITIONS
Before operating the 1178 Peak Limiter with program material,
it may be helpful to become familiar with the terminology used in
this manual
.
Compressors and Limiters are typically used in applications
where the dynamic range of program material is too large to be
processed by succeeding equipment, or where the peak—to—peak
amplitude is too large for the headroom of the following equipment.
The two different names generally refer to the degree to which
the dynamic range is restricted. The relationship of input level
change versus output level change is called the Compression Ratio .
If, for example, an increase of 8dB input signal level should cause
the output to increase by 2dB, this would represent a4:1 compression
ratio.
Although no strict standards exist, amplifiers with compression
ratios of up to 8:1 are typically considered to be Compressors ,
while those with ratios higher than 8:1 are called Limiters .By
this definition, the following graph shows that the Model 1178 can
function as acompressor or alimiter since it has compression
ratios selectable from 4:1 to 20:1.
FIGURE 3-1. MODEL 1178 TRANSFER CHARACTERISTICS.
-10-

The graph also indicates the region of the Threshold »Signals
at levels below the threshold will not be affected by the compres-
sion/limiting action. Higher signal levels will cause acontrolled
decrease in amplifier gain resulting in achange of dyncimic range.
The 1178 Input Level control adjusts the eimount of signal to be
processed above the threshold, and hence the degree of compression
or limiting
.
The Attack Time of the 1178 is the time it takes for the
device to respond to asignal which exceeds the threshold. It is
variable, and since the adjustment may have asignificant effect
on the sound quality of the program material, amore detailed
description is given in the section on application.
The Release Time may be defined as the time for the limiter
to return to its normal gain, after the signal which caused the
gain reduction has dropped below threshold. The release time is
variable and can also alter the sound of the program material
considerably. The adjustments are discussed in more detail in
the section on application.
3.2 INITIAL SET-UP
After the Model 1178 has been installed and is connected to
both the signal source and the succeeding equipment according to
Section II, power may be turned ON. Set the controls as follows:
Input
Output
Attack
Release
Compression Ratio
Meter Function
Peak/Avg
CCW
Half rotation
Fully CCW, switch to OFF position
Half rotation
8:1
+4 dB
Avg
3.3PERFORMANCE CHECK
The following steps will verify the general performance of the
limiter and indicate the operation of all functions. If the unit
fails to pass these tests, malfunction is indicated. Unless other-
wise indicated, these tests should be performed for both channels.
3.3.1 INPUT-OUTPUT SIGNAL LEVEL
Apply asine wave signal with alevel of approximately 0dB
(0.775 Vrms) to the input terminal. Turn the Input control CW
to approximately half-rotation. There should be an indication on
the VU meter. Adjust the Output control until the VU meter reads
"0". Measured with an external meter, the output signal will
read about +4 dBm. Push the +8 dB meter function switch. The VU
indication will drop by 4dB. The actual output level will not
-11-

change. Push the +4 dB meter function switch and select the Peak
characteristic. The indication on the meter will be -6 dB. Again,
the actual output level does not change. Return the meter character-
istic to AVG (average) reading.
3.3.2 GAIN REDUCTION CHECK (Allow 15 minutes warmup).
Push the GR meter function switch. The VU meter should read
0VU since the limiting function is disabled [the Attack control
is turned fully counterclockwise (CCW)3.
Turn the Attack control ON; gain reduction should be indicated.
Observe the tracking between the indicated amount of gain reduction
and the actual change in output (VU )level as measured with an ex-
ternal meter. The tracking should be within ±1 dB.
Selecting other compression ratios of the limiter will indi-
cate some gain reduction in each setting ,however the actual amount
will vary due to the different threshold sensitivities.
3.3.3 STEREO TRACKING
Apply the same signal to both inputs. Switch the meter func-
tion to GR, and, with the mode switch in the A/B position, adjust
the Input controls until the same amount of GR is indicated on each
VU meter —for example -5 dB.
Select the +4 dB meter function and adjust the Output level
controls for the same indication on both meters —for example 0VU.
Switch the mode switch to STEREO and check the tracking between the
two meters to remain within 1dB.
3.4 APPLICATION
3.4.1 CONTROL SETTINGS
Since all important paremieters of the Model 1178 are variable,
the instrument is flexible enough to be used in virtually all phases
of sound processing, from recording of asingle track to reproduc-
tion of the full program material. The limiter finds application
as acreative instrument during production, as well as adevice for
automatic level control and protection against high amplitude
problems
.
There is no single correct setting of the controls; rather
experimentation is necessary to find the best combination of
compression ratio, attack and release time, and the amount of
gain reduction for the job at hand. Experience will show that
the dynammic parameters (attack and release time) have astronger
effect on the character of the program material than the static
parameters (compression ratio and threshold).
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3.4.2 EFFECTS OF COMPRESSION RATIO
Amoderate form of gain reduction is achieved with ratios up
to about 8:1. The dynamic range of the program material is controlled
without obvious alteration. The average signal level will be
increased and the softer passages will be made louder.
Amore drastic gain reduction results from compression ratios
of 12:1 and 20:1. The output signal level is limited, and for
practical purposes cannot exceed apreset level. Dynamic range is
significantly reduced, and high level peaks are minimized.
3.4.3 EFFECTS OF THRESHOLD
Changing the relationship of the average input level to
the threshold will determine the amount of gain reduction, or how
much of the signal envelope is affected by the limiter action.
If only occasional large-amplitude peaks are allowed to exceed
this threshold, the program material remains very much the same.
However, overload problems which would be caused by these peaks are
eliminated. The result can be an increase in loudness because the
operator can raise the signal level without adding distortion due to
headroom limitations.
When the input is adjusted so that the average signal level is
above the threshold all the time, then the entire program material
will be compressed. Monitoring the amount of gain reduction is
made easy when the meter function is switched to the GR mode.
3.4.4 EFFECTS OF ATTACK TIME
Fast transients and high frequencies with large peak-to-peak
amplitudes can only be limited or compressed if afast attack time
is chosen. Then any overshoot, and with it overload problems,
are kept to aminimum. However, limiting with afast attack time
alters the musical quality of some instruments which are character-
ized by sharp percussive attacks.
Aslow attack time will allow the first segment of fast ^pli-
tude changes to pass through the limiter unaffected. The limiter
reduces gain after the initial envelope attack of the program
material. Thus, slow attack maintains the "punch" of such percus-
sive instruments as drum, bass, piano, guitar, etc. with some
sacrifice of amplitude control.
The aim of the user in adjusting the limiter for the correct
attack time then is to find the best setting for the task at hand.
It may be acompromise between apermissible eimount of initial
peak-to-peak amplitude before full compression, and how much alter-
ation of the program material is acceptable.
-13-

These general guidelines are mainly applicable when the 1178
is used for the limiting of individual signal sources. For mixed
program material* amiddle—of“the—range setting will usually be a
good starting point for additional experimentation.
3.4.5 EFFECTS OF RELEASE TIME
This control* too* may be set to accommodate different program
material requirements. The best adjustment maintains aquick
response of the limiter to dynamic changes of the signal while
avoiding such affects as "pumping" or "breathing." Again* let us
examine the behavior with some extreme settings.
If the release time is set very short* the limiter will return
the gain reduction to zero every time the signal drops below the
threshold. For low frequency solo passages* such as bass and
drum* this can occur between cycles or individual notes and beats.
We would hear the gain come up each time* and* with it* the softer
passages or background noise. Extremely short release times may
even introduce harmonic distortion if the gain reduction changes
between cycles
.
Setting the release time to avery long duration will maintain
the initial gain reduction caused by alarge amplitude* and the
dynamic range of the passage which follows is strongly affected.
If the gain reduction continues through asoft program section* it
may make this section inaudible because it* too* will be reduced in
gain. Also* short dynamic changes will be ignored due to the
continuous high level of gain reduction.
The most desirable release time setting for particular program
material can usually be found quickly by experimentation. Here,
too* the effects of varying the adjustment are more obvious with
individual signals* and less apparent with afull mix. As with the
Attack control* agood starting point is half rotation.
3.5 PLACEMENT OF THE 1178 LIMITER IN THE SIGNAL PATH
3.5.1 AFTER PREAMP
Small signals need to be amplified before they will cause proper
operation of the 1178 Limiter. Only signal levels which are above
the threshold of the limiter can cause gain reduction. For example:
In the 4:1 compression ratio* the lowest threshold is -30 dB.
If the signal applied to the input is about -20 dB* then the
1178 will provide amaximum of 10 dB compression with the Input
Level control set to maximum. The sensitivity is high enough
that it should not be difficult to obtain sufficient signal
level from most sources which supply the audio to be processed
by the 1178.
-14-

This consideration is appropriate when the limiter is patched into
the signal path at apoint where individual inputs are to be con-
trolled artistically before mixing (solo tracks).
3.5.2 EQUALIZATION, BEFORE OR AFTER THE LIMITER?
In most cases, better results are obtained if one places the
limiter after the equalizer. This becomes evident when one consi-
ders that the equalizer could eliminate the amplitude balance so
carefully achieved with the limiter. An exception to this would
be in the case of large dynamic changes in program material that
are too difficult for the equalizer to handle, and that would thus
result in overload or apoor signal—to—noise ratio. Then the 1178
should be used to condition the audio signal before it is applied
to the equalizer or other device.
3.5.3 FULL PROGRAM MATERIAL
It is good to recall that the action of alimiter is most
obvious when applied to the full program material because at this
point one does not benefit from the masking that occurs when
individually limited sources are mixed. Therefore, it is even
more important to carefully select the dynamic parameters, like
attack and release time, to achieve an optimum result. Here are
some additional tips
:
On live program material ,the fastest attack time possible will
afford the maximum protection against overload, but will cause
some alteration of the sound. On pre-recorded tape, disc or
film, slower attack time may be used to maintain ahigh average
level
.
For program material containing relatively little low frequency
energy, the release time may be fast, thereby increasing
the short interval average level. The highest apparent average
level may be attained with the shortest release time before
"pumping" becomes objectionable. For program material contain-
ing more than average low frequency energy ,the release time
must be increased to the point that no low frequency distortion
is apparent. Afurther compromise may be made by slightly
reducing the amount of limiting action in favor of slightly
faster release time: the result —ahigher average level.
Adjustment of the Attack Time control also affects the amount
of sibilant speech sound .These high frequency components in
speech are normally at much lower level than the low frequency
components, and the control voltage derived from sibilants
alone is much less than from the "vowel" or low frequency
sounds. In cases where attack time is extremely slow, the
rise time and amplitude of the high frequency sibilants will
not cause control voltage to be developed until the "vowel"
-15-

energy envelope occurs. This results in the full gain applied
to the sibilants and reduced gain to the vowels, creating an
exaggerated sibilant sound. Again, experimentation is the
best tool.
3.5.4 PRIOR TO OUTPUT AMPLIFIER
Here the limiter fulfills mainly the function of protection.
Most equipment which is fed from the low level signal pathos output
terminal is sensitive to amplitude overload; the power amp's peak-
to-peak headroom, the saturation point of recording tape, the modu-
lation allowed in atransmitter, the amplitude handling capability
of film recording devices, etc.
If one were to summarize the ideal operation of awell adjusted
limiter, one could say:
"It should afford infallible protection against overload, while
maintaining the highest level, contribute no apparent degrada-
tion to the dynamic range which might subjectively affect the
artistic value of the program, and, at the same time, maintain
the individual quality of all sounds, regardless of their
starting transients."^
3.6 STEREO OPERATION
When stereo signals are processed the balance of the two audio
channels is important. To maintain agood balance between channel A
and channel B, the Model 1178 is simply switched into the STEREO
mode. Then the audio signal with the larger peak-to-peak amplitude
will cause the same amount of gain reduction in both channels.
The tracking of the GR circuitry is factory calibrated and does
not need further adjustment. However, it is important to balance
the signal levels with the Input controls since these determine how
much signal is within the threshold of limiting. The Output level
controls are used for make-up gain setting.
IT, Ref. M.T. Putnam, Audio Engineering Society paper. Spring, 1967.
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SECTION IV
THEORY OF OPERATION
4.1
General
This section provides ageneralized description of the 1178
circuitry and should be used in conjunction with the schematic at
the end of the manual. Where used, component designations in
parenthesis are those of channel B.
4.2
INPUT AMPLIFIER
The audio signal is applied through the rear panel barrier
strip to adifferential input eutnplifier IC 1, section A&B(IC 2,
A&B). This input can accept either balanced or unbalanced signal
sources. When used in an unbalanced mode, either the COM terminal
or the "i" terminal is connected to ground, the latter causing a
polarity reversal (180® out of phase) of input versus output signal.
(See Installation, Section 2.5).
Common mode rejection is factory trimmed with R1 (RlOl), and
is typically better than 60 dB.
4.3
VOLTAGE-VARIABLE RESISTOR ATENUATOR
The input signal is coupled from the differential amplifier
through the Input Level control to an "L" section consisting of R3
(R103) as the series element and field-effect transistor Q7A (Q7b)
as the voltage-variable shunt element. Below the threshold of
limiting the FET has avery high resistance. During limiting the
gain reduction amplifier causes the FET to conduct, decreasing its
resistance proportionately. The Q-bias network R12, R13, R14 is
adjusted to bring the FET into the region of conduction.
4.4
SIGNAL PREAMPLIFIER
This low noise amplifier stage provides approximately 28 dB of
gain. Trimpot R18 (R118) in the feedback loop is used to adjust
the circuit for aminimum of distortion products due to channel
non-linearity in the FET. The preamplifier output signal is
applied simultaneously to the Output Level control and to the
ratio switch for use in the gain reduction amplifier.
4.5
LINE AMPLIFIER
The signal from the output control is coupled to the output
amplifer consisting of IC 2A, Ql, Q2 (IC 4B, Q4, Q5 )and associated
components, and hence to the output transformer T2. This circuit
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