L-Acoustics LA15a User manual

Version 1.0
January 2003
LA15a
115V Version
OWNERS MANUAL

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 1
TABLE OF CONTENTS
LIST OF FIGURES ................................................................................................................................... 2
LIST OF TABLES ..................................................................................................................................... 2
1. APPROVALS........................................................................................................................................ 3
2. WARNINGS......................................................................................................................................... 3
2.1 Explanation of Graphical Symbols ................................................................................................. 3
2.2 WARNING .................................................................................................................................... 3
2.3 CAUTION..................................................................................................................................... 3
2.4 Important Safety Instructions ........................................................................................................ 3
2.5 User Responsibility ........................................................................................................................ 4
2.5.1 Speaker Damage..................................................................................................................... 4
2.5.2 Speaker Output Hazard ......................................................................................................... 4
2.5.3 Radio Interference .................................................................................................................. 4
3. INTRODUCTION .............................................................................................................................. 6
3.1 Unpacking...................................................................................................................................... 6
3.2 Front Panel .................................................................................................................................... 6
3.3 Rear Panel...................................................................................................................................... 7
4. REAR PANEL FEATURES ................................................................................................................... 8
4.1 Impedance Matching (MLS switch)................................................................................................ 8
4.2 Clip Limiter.................................................................................................................................... 9
5. INSTALLATION................................................................................................................................ 10
5.1 Mounting...................................................................................................................................... 10
5.2 Cooling ........................................................................................................................................ 10
5.3 Operating Voltage ....................................................................................................................... 10
5.4 Grounding.................................................................................................................................... 11
5.5 Power Consumption ................................................................................................................... 11
5.6 Heat Power Calculation .............................................................................................................. 12
5.7 Gain and Sensitivity...................................................................................................................... 13
6. CONNECTIONS .............................................................................................................................. 14
6.1 Input Connections ....................................................................................................................... 14
6.1.1 Balanced Inputs..................................................................................................................... 14
6.1.2 Unbalanced Inputs ................................................................................................................ 15
6.2 Connecting Speakers................................................................................................................... 15
7. OPERATION..................................................................................................................................... 16
7.1 Operation Modes ........................................................................................................................ 16
7.1.1 QUAD Mode........................................................................................................................ 16
7.1.2 Tandem Mode ...................................................................................................................... 17
7.1.3 Bridged Mono Mode ............................................................................................................ 17
7.1.4 Bridged Mono Mode Features.............................................................................................. 17
7.2 Operating Precautions................................................................................................................. 17
7.3 Powering Up – Soft Start............................................................................................................. 18
7.4 Input Attenuators ........................................................................................................................ 18
7.5 Indicators ..................................................................................................................................... 18
8. PROTECTION FEATURES............................................................................................................... 19
8.1 Clip Limiter.................................................................................................................................. 19
8.2 Thermal Protection ..................................................................................................................... 19
8.3 VHF Protection............................................................................................................................ 19
8.4 Short Circuit Protection .............................................................................................................. 19
8.5 AC Mains Voltage Protection ...................................................................................................... 20
8.6 DC Protection ............................................................................................................................. 20
9. MAINTENANCE............................................................................................................................... 20
9.1 Troubleshooting .......................................................................................................................... 20

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10. WARRANTY.................................................................................................................................... 21
11. Technical Assistance and Service..................................................................................................... 21
LIST OF FIGURES
Figure 1: LA15a Front Panel .................................................................................................................... 6
Figure 2: LA15a Rear Panel ..................................................................................................................... 7
Figure 3: LA15a Rear Panel MLS Switches .............................................................................................. 8
Figure 4: XLR Input Connector Wiring ................................................................................................. 14
Figure 5: Balanced XLR wiring .............................................................................................................. 14
Figure 6: ¼” TRS plug............................................................................................................................ 14
Figure 7: LA15a Rear Panel Detail......................................................................................................... 14
Figure 8: Unbalanced Line Connection ................................................................................................. 15
Figure 9: Balanced Line with Unbalanced Equipment ........................................................................... 15
Figure 10: Loudspeaker Connection Details ......................................................................................... 15
Figure 11: Reverse polarity operation for Channels B and D................................................................ 16
Figure 12: Bridged mono cabling........................................................................................................... 17
Figure 13: LA15a Front Panel Detail ..................................................................................................... 18
LIST OF TABLES
Table 1: Output Power Ratings and MLS Switch Settings for the LA15a................................................ 8
Table 2: MLS Switch Setting Applications................................................................................................ 9
Table 3: LA15a Current Draw @ 4 ohms............................................................................................. 11
Table 4: LA15a Maximum Output Power versus Mains Input Power .................................................. 12
Table 5: LA15a Output Power and Input Sensitivity versus MLS Switch Setting (32 dB gain).............. 13
Table 6: Maximum Recommended Length for Damping Factor > 20 ................................................. 16

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 3
1. APPROVALS
This equipment is tested and approved according to the U.S. safety standard ANSI/ UL 6500 and
Canadian safety standard CSA E60065-00. Tests were conducted by ETL and ETL is a Nationally
Recognized Testing Laboratory (NRTL).
The ETL Listed Mark is recognized by local inspectors and Authorities Having Jurisdiction (AHJs)
throughout North America and also in some areas of South America. ITS ETL SEMKO is NRTL-
recognized by OSHA (Occupational Safety and Health Administration), the ETL Listed Mark is an
accepted alternative to UL and, as such, inspectors and AHJs are required by law to recognize,
acknowledge, and accept the mark as proof of product compliance. See more on the ETL Website:
www.etlsemko.com/S1OurMarks/index.html
2. WARNINGS
2.1 EXPLANATION OF GRAPHICAL SYMBOLS
The lightning symbol within a triangle is
intended to alert the user to the
presence of un-insulated “dangerous voltage”
within the amplifier’s enclosure that may be of
sufficient magnitude to constitute a risk of
electric shock to humans.
The exclamation point within a triangle is
intended to alert the user to important
operating and service instructions in the
literature accompanying the product.
2.2 WARNING
To reduce risk of fire or electric shock, do not
expose this apparatus to rain or moisture.
2.3 CAUTION
To reduce the risk of fire or electric shock, do
not remove screws. No user-serviceable parts
inside. Refer servicing to qualified service
personnel.
2.4 IMPORTANT SAFETY INSTRUCTIONS
Before using your amplifier, be sure to carefully read these operating instructions and
follow the safety suggestions outlined below:
1. Keep this manual for future reference.
2. Heed all warnings.
3. Follow all instructions.
4. Do not use this unit near water. Do not spill water or other liquids into or on the unit. Do not
operate the amplifier while wet or standing in liquid.
5. Clean only with a dry cloth.
6. Do not block the air intake or exhaust ports. Install the unit in accordance with the
instructions outlined in this manual.
!
!

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 4
!
7. Do not operate the amplifier near heat producing devices such as radiators, heat registers,
stoves or any other apparatus that produces heat.
8. Always operate the unit with the chassis ground wire connected to the electrical safety earth.
Do not defeat the safety purpose of the grounding-type plug. A grounding-type plug has two
pins and a third grounding prong. The third prong is provided for your safety. If the provided
plug does not fit into your outlet, consult an electrician for replacement of the obsolete outlet.
9. Connect only to AC power outlets rated 100-120 V, 50-60 Hz.
10. Do not use this amplifier if the power cord is broken or frayed. Protect the power cord from
being walked upon or pinched - particularly at the plugs and the point where the power cord
exits from the apparatus.
11. Only use accessories specified by the manufacturer.
12. The unit is intended to be mounted in a 19” rack. Follow the mounting instructions outlined in
this manual and always rear support the unit when used in portable applications. When a rack
on wheels is used for portable applications, use caution when moving the loaded rack in order
to avoid injury from tipping over.
13. Unplug this apparatus during lightning storms or when unused for long periods of time.
14. Do not connect an amplifier output in parallel or series with any other amplifier’s output. Do
not connect the amplifier output to any other voltage source, such as a battery, mains source,
or power supply, regardless of whether the amplifier is turned on or off.
15. Do not run the output of any amplifier back into another channel's input.
16. Refer all servicing to qualified service personnel. Servicing is required when the apparatus has
been damaged in any way such as:
• Power supply cord or plug is damaged
• Liquid has been spilled into the unit
• An object has fallen into the unit
• The unit has been exposed to rain or moisture
• The unit does not operate normally
• The unit was dropped or the enclosure is damaged
17. Do not remove top or bottom covers. Removal of the cover will expose hazardous voltages.
There are no serviceable parts inside and removal may void the warranty.
18. An experienced user shall always supervise the use of professional audio equipment, especially
if inexperienced adults or minors are using the equipment.
2.5 USER RESPONSIBILITY
2.5.1 SPEAKER DAMAGE
Your amplifier is very powerful and can be potentially dangerous to both loudspeakers and
humans alike. Loudspeakers can be damaged or destroyed by overpowering, especially with
the high power available from a bridged amplifier. Always check the loudspeaker’s continuous
and peak power handling capabilities. Note: even when using the amplifier’s front panel attenuator to
reduce the gain, it is still possible to reach full output power if the input signal level is high enough.
2.5.2 SPEAKER OUTPUT HAZARD
Power amplifiers are capable of producing hazardous output voltages. To avoid electrical
shock, do not touch any exposed speaker wiring while the amplifier is operating. Refer to
Section 6.2 concerning proper connection of loudspeakers.
2.5.3 RADIO INTERFERENCE
This equipment has been tested and found to comply with the limits for a Class B digital device,
pursuant to Part 15 of the FCC Rules. These limits are designed to provide reasonable protection
!

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 5
against harmful interference from electrical equipment. This product uses radio frequency energy and
if not used or installed in accordance with these operating instructions, may cause interference to
other equipment such as radio receivers. However, there is no guarantee that interference will not
occur in a particular installation.
If this equipment does appear to cause harmful interference to radio or television reception, verify by
turning the equipment on and off. The user is then encouraged to try to correct the interference by
one or more of the following measures:
• Reorient or relocate the antenna.
• Increase the separation between the equipment and receiver.
• Connect the equipment to an outlet on a circuit different from that to which the receiver
is connected.
• Consult the dealer or an experienced radio/TV technician for further assistance.

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 6
3. INTRODUCTION
Thank you for purchasing the L-ACOUSTICS LA15a power amplifier. This manual contains important
information on operating your power amplifier correctly and safely. Please take time to read this
manual in order to familiarize yourself with the many advanced features of this amplifier.
3.1 UNPACKING
Carefully open the shipping carton and check for any noticeable damage. Every L-ACOUSTICS
amplifier is tested and inspected before leaving the factory and should arrive in perfect condition. If
found to be damaged, notify the shipping company immediately. Only the consignee may institute a
claim with the carrier for damage incurred during shipping. Be sure to save the carton and packing
materials for the carrier's inspection.
It is also advisable to save the carton and packing material even if the amplifier is undamaged. Should
you ever need to ship the amplifier, always use the original packing.
3.2 FRONT PANEL
Figure 1: LA15a Front Panel
1. Carry/Protection Handle
Both handles can be used to carry the amplifier and the handles also provide protection for the front
panel. For fixed installation (or when rack front covers are too shallow) the handles may be removed
by unscrewing the retaining bolts behind the front panel.
2. Input Level Attenuators
These controls are used to alter the signal level entering the amplifier. They are calibrated in dB to assist
in the setup of active loudspeaker systems. Input attenuation can also be used to reduce unwanted noise
from the input signal. See Section 7.4 for further details.
3. Over Temperature Protect Indicator
This indicator illuminates if the amplifier tries to operate above its maximum operating temperature
(90°°C). The indicator first comes on as a warning to either turn down the input level or check
amplifier ventilation. Beyond the maximum temperature the amplifier will mute the input signal. When
cooling fans have returned the output heat sinks back to normal operating temperature, the input
signal is un-muted.
4. Protect Indicator (VHF and short circuit)
This indicator illuminates when signals above 12 kHz are continuously present at full power at the output
terminals. If this occurs the input signal is muted and the process cycles until the VHF signal is no longer
present.
7
22
1
6
345
6

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 7
5. Clip/Limit Indicator
This indicator signals when the amplifier output is clipping or limiting. There are two different indication
states depending on whether the rear panel clip limiter is engaged (see Sections 4.2, 8.1):
• If the clip limiter is engaged, the Clip/Limit Indicator has a short time constant and illuminates
briefly.
• If the clip limiter is not engaged, the Clip/Limit Indicator has an increased time constant and
illuminates for a longer period.
6. Fan Grill Filters
Two grilles with foam filters are located on the front panel in order to prevent dust from entering the
amplifier. The grilles are removed by simply pulling them off, allowing for convenient cleaning of the
filters. Foam filters should be used at all times.
7. Power Switch
Turns the mains power on or off.
3.3 REAR PANEL
Figure 2: LA15a Rear Panel
1. Output Speaker Connector
The Neutrik® Speakon connector may be unfamiliar to some users. See Section 6.2 for details.
2. Minimum Load Selector (MLS™) Switch
These switches are used to select the maximum output power. See Section 4.1.
3. Clip Limiter Switch
Turns the clip limiter on (switch IN position) or off (switch OUT position). See Sections 4.2, 8.1
4. Input Signal Connector
The Neutrik® Combojack features XLR and ¼” TRS phone jacks. (Pin 2 is “hot”, see Section 6.1.1).
5. Link Switch
Allows a single input to drive channels B and C simultaneously.
6. AC Power Cable
7. Link (Bridge) Switch
Allows a single input to drive channels A and B and/or C and D simultaneously. Also used for bridged
operation of channel pairs (see Section 7.1.3).

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 8
4. REAR PANEL FEATURES
4.1 IMPEDANCE MATCHING (MLS SWITCH)
The MLS™ (Minimum Load Select) switches located on the rear panel (2) offer power/impedance
matching, allowing the LA15a to power 2 ohm loads without increased heat loss. Please refer to Table
1 to determine the power delivered into a specific load for various MLS switch settings. Note that
higher powers can be achieved utilising higher MLS positions when connected to lower speaker
impedances and as can be seen in Table 1, the LA15a can produce output power in excess of 380 watts.
Essentially, the LA15a amplifier is a power converter and by selecting MLS positions you are allocating a
portion of the available 1200 watts power.
Figure 3: LA15a Rear Panel MLS Switches
Table 1: Output Power Ratings and MLS Switch Settings for the LA15a
LA15a
LOAD CONFIGURATION -3 dB 0 dB
16 ohms Quad (4 channel) 95 W 200 W
8 ohms Quad (4 channel) 200 W 370 W
4 ohms Quad (4 channel) 380 W 600 W
2 ohms Quad (4 channel) 500 W n/a
16 ohms Stereo (2 channels bridged) 380 W 740 W
8 ohms Stereo (2 channels bridged) 760 W 1200 W
4 ohms Stereo (2 channels bridged) 1000 W n/a
Power in watts (EIA 1 kHz, 1% THD)

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MLS positions can be different for each channel and a variety of combinations can be selected as seen in
Table 2. There is no risk for amplifier damage since the LA15a is very well protected (even down to 0.3
ohms). However, if uncertain or where time does not allow, use the default position (MLS = 0 dB).
Table 2: MLS Switch Setting Applications
Channel Impedance MLS Power Comments
A
8 -3dB 200W Power
B 8 -3dB 200W reduction
A
4 0dB 600W High pea
k
B 4 0dB 600W power
A
8 0dB 370W Power
B 8 -3dB 200W matching
A
8 -3dB 200W Power
B 4 0dB 600W sharing
4.2 CLIP LIMITER
The LA15a Channel A, B, C, and D clip limiters prevent dangerous clipped signals from reaching the
loudspeaker and causing damage. When an amplifier is severely overdriven, the output waveform is
clipped and peaks become squared off, causing a reduction in crest factor. In extreme cases, the
waveform can approach that of a square wave. Normally an amplifier is capable of producing far more
power under these conditions than its normal, undistorted rated output power.
The LA15a clip limiter works by monitoring the output and comparing the distortion produced between
the input and output of the amplifier. If the distortion exceeds 1% THD for any reason (voltage or
current clipping), the limiter reduces the input signal proportionally. Once the amplifier comes out of a
protect condition, the output level has a slow rise time (the effect is like turning the gain up slowly).
Under normal operation, clip limiting circuitry is inaudible.
Note: if the signal is distorted or clipped before it reaches the amplifier, the clip limiter cannot detect
this and will not be activated.
Individual channel clip limiters can be turned on (or off) by depressing (or pressing) the relevant clip limit
switch. L-ACOUSTICS recommends leaving clip limiters switched ON (button depressed) at all times.
Clip Limiter ON = switch depressed (IN position)
Clip Limiter OFF = switch not depressed (OUT position)

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5. INSTALLATION
5.1 MOUNTING
The LA15a is two rack units high (2U) and will mount in an EIA-standard 19'' rack. Four mount points
are provided on the front panel of the amplifier for rack mounting.
For permanent installation, the amplifier can be mounted using standard 19'' rack mount rails and/or
slides within the amplifier rack using the four front mount points. Amplifiers may be stacked directly
on top of each other and there is no need for spacing between units. If it is the intention to fill a rack
with amplifiers, we recommend racking is started from the bottom of the rack. It is also
recommended that rear supports are used for amplifiers mounted in the middle of the rack.
For touring or portable sound system applications, it is essential that all LA15a amplifiers are rear
supported in addition to front panel mounting.
Any mechanical damage to LA15A amplifiers used in portable applications without rear support are not
covered under warranty.
For all portable applications, the amplifier should be fixed in the back with a rear support kit (L-
ACOUSTICS part code: APSUP). The amplifier support kit APSUP is a universal rear support kit for
any 19'' rack since it is adjustable to accommodate varying rack depths. Two rear support brackets,
two amplifier support brackets, four M4 hex button head screws and four washers are included in
each APSUP kit. The kit integrates the amplifier support bracket with the amplifier and the rack
support bracket with the rack. Depth is adjustable and amplifiers can be removed from the rack by
simply removing the 4 button head screws. APSUP is suitable for use with all LA amplifiers.
5.2 COOLING
Your amplifier uses a forced air cooling system to maintain a low and even operating temperature. All
fan cooled L-ACOUSTICS amplifiers have front to rear airflow. There are several reasons for this, one
being that there is usually cooler air outside the rack than inside and therefore the amplifiers can run
at higher continuous power levels without thermal problems. Never try to reverse the airflow since
the Intercooler® needs a pressure chamber between the fans and heat sink and this only works in
one direction of the airflow.
Should a heat sink get too hot, its sensing circuit will mute the overheated channel. If the power
supply overheats, another sensing circuit will mute all output channels until the power supply cools
down to a safe operating temperature.
Make sure that there is an adequate air supply in front of the amplifier, and that the rear of the
amplifier has sufficient space to allow the exhaust to escape. If the amplifier is rack-mounted, do not
use covers or doors on the front or rear of the rack.
For installations with a central cooling system (usually found in fixed installations with a dedicated rack
room) it may be necessary to calculate the maximum heat emission. Please refer to Sections 5.6 and
5.7 concerning Power Consumption and Heat Power Calculation.
5.3 OPERATING VOLTAGE
WARNING!
A label just below the mains cable on the rear of the amplifier indicates the AC mains
voltage for which the amplifier is wired. Connect the power cable only to the AC source
referred to on the label. The warranty will not cover damage caused by connecting to the
wrong type of AC mains.

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 11
L-ACOUSTICS switch mode amplifiers use primary switching, i.e. the mains power is rectified directly
before the transformer, which means that the power supply is insensitive to the mains frequency and
will operate from DC to 400 Hz.
If the supplied power plug is not appropriate for your country, it can be cut off and wired to a suitable
connector in the following manner:
BLACK or BROWN LIVE
WHITE or BLUE NEUTRAL
GREEN or GREEN/YELLOW EARTH
Once the AC connector is connected to a suitable AC supply, the amplifier can be started with the
power switch. When you power up the amplifier it takes a few seconds to check its circuits (this is
known as the soft or slow start sequence), the fans blow at high speed before going into idle, then the
two bottom green LED’s come on to indicate that output circuits are receiving the correct rail voltage.
5.4 GROUNDING
There is no ground lift switch or terminal on this amplifier. The signal ground is always floating via a
resistor to chassis and the grounding system is automatic. If a potential above 0.6 V presents itself
between signal ground and chassis ground, a short circuit is introduced between the two, thereby
enabling electrical protection. If a unit in the system is faulty, its mains fuse will blow due to this
automatic ground system.
If you wish to tie the signal ground to chassis, connect the XLR-connector’s shell lug to pin 1. In the
interest of safety never disconnect the earth pin on the AC power cord.
For all units that are FCC-approved (radio interference), there is an AC mains filter. This filter needs the
chassis ground for reference, otherwise a current loop is formed via the signal ground.
Use the balanced inputs to avoid hum and interference.
5.5 POWER CONSUMPTION
There are three ways to define the power/current consumption of a power amplifier:
First, the peak current draw at full output power. Under this condition the power amplifier will trip
the mains breaker within 30 seconds and the amplifier will operate for less than 2 minutes before
thermally limiting. During this time, the temperature of the power supply will be stabilized at a
temperature that will have no effect on the insulation rating of the AC line cord.
It is more useful to state the current draw for different loads and output power levels. The current
draw is stated in Amperes (rms) and this figure corresponds to the value of the mains fuse required.
Table 3: LA15a Current Draw @ 4 ohms
Operating Condition Current Draw in Amperes (rms)
Idle state (no load) 2.8 A (rms)
1/8 of full power (- 9 dB) 12 A (rms)
1/3 of full power (-5 dB) 18 A (rms)
Full power (0 dB) at 1 kHz, 1% THD 32 A (rms)

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 12
Second, the maximum expected average current under worst case conditions, corresponding to 1/3
full power program material according to the FTC standard. At this level the music will be in a state of
constant clip and is therefore the highest power level one can obtain without completely obliterating
the program material.
Third, the normal operating power, as defined by the safety standard IEC 65/ANSI/UL 6500 and used
by a majority of safety agencies. The normal operating power is measured using pink noise, with an
average output power equal to 1/8 of full power. The 1/8 full power specification is as loud as you can
play music while making some attempt to avoid obvious clipping and corresponds to 9 dB of
headroom (which is very low for normal audio program).
Table 4: LA15a Maximum Output Power versus Mains Input Power
MAXIMUM OUTPUT POWER MAINS INPUT POWER
Load # of ch Power 1/3 Power 1/8 Power Idle
(watts) (see note 1) (see note 2)
8 ohms 4X 370 1140 820 175
4 ohms 4X 380* 1325 840 175
2 ohms 4X 500* 2125 1440 175
note 1 Mean power with music as program source (amplifier driven to clip level)
note 2 Normal music power with 9 dB headroom (IEC standard power rating)
*MLS switch set at -3 dB
It is recommended that you specify your power distribution service to ensure sufficient current for at
least 1/8 power operation and to upgrade service to meet 1/3 power requirements for heavy duty
demands like discos etc.
5.6 HEAT POWER CALCULATION
The heat power can be calculated according to the following example:
Consider headroom of at least 9 dB (1/8 of full power) and a 4 ohm load on an amplifier producing
380 watts per channel. The 1/8 power per channel is then: 380 / 8 = 47.5 watts; and the total output
power is : 4 x 47.5 = 190 watts.
The power consumption according to Table 4 is 840 watts (Table 4 gives the active power
consumption of the amplifier for different loads and power levels).
The heat power produced is the difference between the power consumption and the output power:
840 - 190 = 650 watts per amplifier.

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5.7 GAIN AND SENSITIVITY
As standard, the LA15a is delivered with 32 dB gain. For most professional applications, mixing consoles
operate at a nominal level of +4 to +6 dBu, therefore the 32 dB position provides optimal gain
structure with plenty of fader movement. In addition, all L-ACOUSTICS LLC analog controllers and
OEM factory presets for approved digital processors are optimized for use with 32 dB gain amplifiers.
Sensitivity is defined as how many volts (rms) or dBu (referred to 0.775Vrms) are required at the
amplifier input to obtain full output power. Since the output power delivered varies with the load
impedance, 4 ohms is generally taken as the reference. For the LA15a, peak output power into 4
ohms is 600 W (MLS switch = 0 dB) and standard gain is 32 dB. This corresponds to an input
sensitivity of 1.22 Vrms or 4.4 dBu.
For the LA15a, the input sensitivity will depend on the specific MLS switch setting. Input sensitivity is
summarized below in Table 5.
Table 5: LA15a Output Power and Input Sensitivity versus MLS Switch Setting (32 dB gain)
LA15a
LOAD CONFIGURATION -3 dB 0 dB
16 ohms Quad (2 channel) 95 W 200 W
(2.0 dBu) (5.3 dBu)
8 ohms Quad (2 channel) 200 W 370 W
(2.3 dBu) (4.9 dBu)
4 ohms Quad (2 channel) 380 W 600 W
(2.0 dBu) (4.4 dBu)
2 ohms Quad (2 channel) 500 W
(1.2 dBu)
16 ohms Stereo (2 channels bridged) 380 W 740 W
(2.0 dBu) (4.9 dBu)
8 ohms Stereo (2 channels bridged) 760 W 1200 W
(2.0 dBu) (4.0 dBu)
4 ohms Stereo (2 channels bridged) 1000 W
(1.2 dBu)
Power in watts (EIA 1 kHz, 1% THD)
Sensitivity in dBu

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 14
6. CONNECTIONS
6.1 INPUT CONNECTIONS
6.1.1 BALANCED INPUTS
XLR Input connectors are active balanced and wired according to IEC 268 (i.e., pin 2 hot) in the
following way:
PIN 1 GROUND/SHIELD
PIN 2 HOT (+)
PIN 3 COLD (-)
Figure 4: XLR Input Connector Wiring
Figure 5: Balanced XLR wiring
The Neutrik® Combojack includes a ¼” (6.3 mm) phone jack, which is wired in parallel with the XLR
in the following manner:
TIP HOT
RING COLD
SLEEVE SHIELD/GROUND
Figure 6: ¼” TRS plug
The input impedance is high enough (20 kOhms, balanced) to allow ”daisy-chaining”, or multiple parallel
input connections. The headroom of the input circuits is also high enough to accept the maximum output
level from virtually any low-level signal source. Balanced signals are less sensitive to AC hum and radio
interference. The source impedance should be less than 1 kOhms in order to avoid high frequency loss in
long cables.
Figure 7: LA15a Rear Panel Detail

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6.1.2 UNBALANCED INPUTS
To connect an unbalanced source, tie pin 3 (ring on TRS jack) to the shield of the connector. Note: if you
leave pin 3 disconnected, you will lose 6 dB.
A better method for using unbalanced sources is shown in Figure 8. This is similar to the connection for
balanced lines, but pin 3 is connected to the shield at the source. The hum and noise rejection for the
cable is equivalent to that for a balanced line. To minimize hum, use balanced inputs whenever possible.
Figure 8: Unbalanced Line Connection
Figure 9: Balanced Line with Unbalanced
Equipment
6.2 CONNECTING SPEAKERS
Speaker connections are made via the two Neutrik® NL4FC Speakon connectors. The Speakon
connector is designed for high power speaker connections and assures correct polarity, locks securely in
place and prevents from shock hazard.
The LA15a power amplifier Speakon connectors are wired in the following manner:
Channel A/B Channel C/D
Pin +1 Channel A+ Channel C+
Pin -1 Channel A- Channel C-
Pin +2 Channel B+ Channel D+
Pin -2 Channel B- Channel D-
Since the Channel A/B Speakon is bi-wired, it is possible to access channels A and B on a single
connector (similarly, the Channel C/D Speakon provides access to channels C and D). This is useful
for bridged or bi-amped operation. For bi-amped operation, be careful to ensure that Channel A or B
outputs are correctly assigned to the LF or HF component of your active loudspeaker system. As
standard, L-ACOUSTICS active 2-way loudspeakers are cabled as follows: LF +/- = 1+/1- (Ch A) ;
HF +/- = 2+/2- (Ch B). The same convention holds for the Channel C/D output connector, i.e., LF
+/- = 1+/1- (Ch C) ; HF +/- = 2+/2- (Ch D). Pay attention to loudspeaker polarity: loudspeakers
connected out of phase degrade sound quality and may be damaged as a consequence.
Figure 10: Loudspeaker Connection Details
Channel A and B (or C and D) connected
via one Speakon (for quad or bi-amp
applications)
Bridged mono connection via Channel A/B (or C/D)
output connector (pins 1+/2-) to loudspeaker (+/-)

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 16
In order to preserve high damping factor (essential to the sonic qualities of your loudspeaker system
and to prevent overshoot of cone displacement which can result in mechanical damage), it is desirable
to keep loudspeaker cables as short as possible and with a gauge offering low resistance per unit
length. Do not use shielded wire, such as microphone or guitar cable and remember that the speaker
cable robs the power of the amplifier by increasing the load impedance and introducing resistive
power losses (so called I2R losses). The following table provides information regarding the minimum
wire cross-section versus length:
Table 6: Maximum Recommended Length for Damping Factor > 20
Cross Section Gauge 8 ohms 4 ohms
Metric (mm2) Imperial Metric Imperial Metric Imperial
2.5 13 30 m 100 ft 15 m 45 ft
4 11 50 m 150 ft 25 m 75 ft
6 8 75 m 225 ft 37 m 110 ft
10 6 120 m 360 ft 60 m 180 ft
7. OPERATION
7.1 OPERATION MODES
7.1.1 QUAD MODE
In this mode, all channels operate independently of each other and the level attenuators on the front
panel control the respective channel levels. All link switches (i.e., Link A+B, Link B+C, Link C+D)
are in the OUT position. This is used for stereo amplification of 2-way active enclosures or 4-channel
amplification of passive enclosures. Please refer to Section 6.2 for details concerning Speakon pin
assignments.
Never connect either Channel A/B or C/D output Speakon terminals to ground or in
parallel.
The recommended minimum nominal impedance for quad or stereo operation is 2 ohms per channel
and the MLS switch should be in the -3 dB position.
Note Concerning Bench Test Procedures:
Channels B and D are always polarity reversed at the input, but this polarity reversal is internally
compensated for by feeding the minus pin on the Channel B or D output with the positive output
voltage. Channels A and C are operated with normal polarity. With channel A and B (C and D)
operating in opposite or reverse polarity, the energy storage in the power supply is more efficient.
This is significant for signals below 100 Hz (sub bass etc) and improves the power bandwidth. Note:
Formerly, this was referred to as reverse stereo operation for the LA15.
For this reason, be sure to use balanced inputs on all measurement equipment (also
oscilloscope probes) if you are bench testing.
Figure 11: Reverse polarity operation for Channels B and D

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 17
7.1.2 TANDEM MODE
In tandem mode, two, three or four channel inputs are linked (A+B, B+C or C+D) and receive the
same signal. Linking is performed by depressing the rear panel LINK switches to the IN position. All level
attenuators are active, allowing you to set different levels for each channel. Remaining input
connectors are available for daisy chaining the signal to other amplifiers.
Note: only the inputs are connected in parallel and this is NOT a parallel output mode.
Never connect either output terminal to ground or in parallel
Note: Always disable linking when using the amplifier for bi-amplification of 2-way active
enclosures
7.1.3 BRIDGED MONO MODE
Figure 12: Bridged mono cabling
Bridged mono mode is used to deliver two channels' power to a
single load. The nominal impedance of the load must be more
than 4 ohms.
Set the Link switch (A+B or C+D, as desired) to the IN
position and use one of the input connectors (A or B, C or D, as
desired). The remaining input connector can be used for daisy
chaining to other amplifiers. Both Channel A and B (or C and D)
level attenuators must be at the same position and it is
recommended that they are operated in the 0 dB (full) position.
Connect the loudspeaker as shown in Figure 12.
7.1.4 BRIDGED MONO MODE FEATURES
Bridged mono mode combines the power of two channels into one speaker. This results in twice the
voltage swing, four times the peak power and just less than three times the full power of a single
channel.
One way to understand the load and power from the amplifier’s perspective in bridged mode is that it
is zero voltage at the center of the voice-coil winding. This is because the coil is driven with positive
voltage at one pole and an equivalent negative voltage at the other pole. If an 8 ohm load is connected
in bridged mode, one channel shares one 4 ohm part of the load and the other channel shares the
other 4 ohm part. The power into 4 ohms from an LA15a (MLS at -3 dB) is 380 W. Therefore, the
total bridged power into the 8 ohm load will be 2 x 380 = 760 W.
7.2 OPERATING PRECAUTIONS
Never connect either Channel A/B or C/D output terminals to ground or to some other
output or input terminal (see also warnings in Section 2.4).
For normal quad operation, connect each speaker load across its respective output channel’s positive
and negative terminals. Pay attention to speaker polarity - loudspeakers connected out of
polarity degrade sound quality and may be damaged as a consequence.
WARNING: To prevent electrical shock, do not operate the amplifier with any of
the conductor portion of the loudspeaker cable exposed.

L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003 18
• Refer to Sections 5.1 and 5.2 concerning amplifier mounting and cooling considerations.
• Make sure that the AC mains voltage is correct and the same as the one printed on the rear
panel of the amplifier. See Sections 5.3-5.7 concerning operating voltage, grounding and
power consumption for more details.
• Make sure that the power switch is set to “off” before making any input or output
connections or operating the switches on rear panel.
• Make sure that the switches on the rear panel for clip-limiters and the MLS™ switches are in
the correct position. See Section 7 concerning operation modes, Section 8.1 about clip
limiters and Section 4.1 concerning the MLS switches.
• It is always a good idea to turn down the gain controls during power-up to prevent speaker
damage in case a high signal is present at the input.
• Remember the old saying: “amps on last, amps off first”
7.3 POWERING UP – SOFT START
When you power up the amplifier it takes a few seconds to check its circuits (this is known as the soft or
slow start sequence). The fans blow at high speed before going into idle and the two bottom green
LEDs illuminate to show the amplifier is operational.
7.4 INPUT ATTENUATORS
The four input level attenuators on the front panel adjust the signal level for their respective amplifier
channel in all modes. Input level attenuators are calibrated in dB to assist in setting up active loudspeaker
systems or cutting down unwanted noise from the input signal.
In bridged mode, both controls must be in the same position so that the speaker load will be shared
equally between the channels.
7.5 INDICATORS
Figure 13: LA15a Front Panel Detail
The two bottom green ”ON” LED’s indicate that the output circuits are receiving the correct rail
voltage.
The ”-25 dB” LED’s illuminate if the output signal is greater than -25dB (with 0 dB referenced to full
output power). These LED’s also act as signal present indicators.
The rest of the green LED indicates -10 dB below clip.