Lowell PA250A User manual

Optional Accessories (sold separately):
Lowell Manufacturing offers a full line of 19" equipment cabinets, wall mount shelves, rack mount AC power panels,
AC power strips, and cooling fans that can be used to install the Lowell Amplifier products.
See www.lowellmfg.com for product details.
Getting Started:
Please study carefully the “IMPORTANT SAFETY INSTRUCTIONS” that are given on page 2 before applying power
to your PA250A amplifier. Those experienced with the use of a power amplifier will find that the input jacks and
terminals, output terminals, and controls follow industry standard conventions and will be very intuitive. Standard
4W, 25V, and 70.7V speaker outputs are provided. Those that are not familiar with the speaker wiring required for
these industry standard outputs will find a very helpful section covering this topic on pages 6, 7, and 8 of this
“Installation Sheet and Owner’s Manual”.
1
REMOVABLE
PHOENIX
CONNECTOR
Instruction Sheet
IS-PA250A
Issued: 1-18-19
Lowell Manufacturing Company 100 Integram Drive Pacific, Missouri 63069 U.S.A.
Call: 800-325-9660 Fax: [phone] Click: www.lowellmfg.com
PA250A
INSTALLATION SHEET
AND OPERATORS MANUAL
PA250A General Description:
The PA250A power amplifier provides 250 watts of continuous RMS power directly into a minimum 4Wspeaker load
or uses the internal 25V/70V output transformer to feed 250W to distributed speaker systems. The PA250A was
designed to be used as a 250 watt booster amplifier to increase the power capability of Lowell’s MA Series mixer/
amplifiers, or as an all-in-one telephone system paging solution. The PA250A includes a balanced transformer
isolated telephone paging input and a 600Wline level balanced transformer isolated input with both inputs controlled
by a tamper-resistant volume control located on the rear panel. A Hi-Z unbalanced Aux input with parallel RCA
connectors is provided for a music program source input. A unique music ducking circuit mutes the aux music input
when a page signal is present at either the telephone paging input or the 600Wline level balanced input. The mute
circuit may be defeated by removing the chassis cover and changing a circuit board jumper. A paging horn
protection (also called “trumpet protection”) low-cut filter is available by cutting a separate circuit board jumper. A
master volume control, main power switch, and circuit breaker are provided on the rear panel.

2
REMOVABLE
PHOENIX
CONNECTOR
Lowell Manufacturing Company 100 Integram Drive Pacific, Missouri 63069 U.S.A.
Call: 800-325-9660 Fax: [phone] Click: www.lowellmfg.com
Read these instructions. Keep these instructions. Heed all warnings.
Follow all instructions. Do not use this apparatus near water.
Clean only with a dry cloth.
Do not block any ventilation openings. Install in accordance with manufacturer’s instructions.
Do not install near any heat sources such as radiators, registers, stoves, or other apparatus (including amplifiers)
that produce heat.
Do not defeat the safety purpose of the polarized or grounding-type plug. A polarized plug has two blades with
one wider than the other. A grounding-type plug has two blades and a third grounding prong. The wide blade or
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.
Protect the power cord and plug from being walked on or pinched particularly at plugs, convenience receptacles,
and the point where it exits from the apparatus.
Only use attachments and accessories specified by the manufacturer.
Use only with the cart, stand, tripod, bracket, or table specified by the manufacturer, or sold with the apparatus.
When a cart is used, use caution when moving the cart/apparatus combination to avoid injury from tip-over.
Unplug this apparatus during lightning storms or when unused for long periods of time.
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 or objects have fallen into the
apparatus, the apparatus has been exposed to rain or moisture, does not operate normally, or has been dropped.
The plug on the power cord is the AC mains disconnect device and must remain readily operable. To completely
disconnect this apparatus from the AC mains, disconnect the power supply cord plug from the AC receptacle.
This apparatus shall be connected to a mains socket outlet with a protective earthing connection.
When permanently connected, an all-pole mains switch with a contact separation of at least 3 mm in each pole
shall be incorporated in the electrical installation of the building.
If rack mounting, provide adequate ventilation. Equipment may be located above or below this apparatus, but
some equipment (like large power amplifiers) may cause an unacceptable amount of hum or may generate too
much heat and degrade the performance of this apparatus.
This apparatus may be installed in an industry standard equipment rack. Use screws through all mounting holes
to provide the best support.
WARNING: To reduce the risk of fire or electric shock, do not expose this apparatus to rain or moisture. Apparatus
shall not be exposed to dripping or splashing and no objects filled with liquids, such as vases, shall be placed on the
apparatus. This product may contain chemicals known to the State of California to cause cancer, or birth defects or
other reproductive harm.
NOTE: This equipment may generate, uses, and can radiate radio frequency energy and, if not installed and used in accordance
with the instructions, may cause harmful interference to radio communications. However, there is no guarantee that interference
will not occur in a particular installation. If this equipment does cause harmful interference to radio or television reception, which
can be determined by turning the equipment off and on, the user is encouraged to try to correct the interference by one or more
of the following measures: Reorient or relocate the receiving antenna.
Increase the separation between the equipment and receiver.
Connect the equipment into an outlet on a circuit different from that to which the receiver is connected.
Consult the dealer or an experienced radio/TV technician for help.
CAUTION: Changes or modifications not expressly approved by Lowell Manufacturing will void the manufacturers warranty.
IMPORTANT SAFETY INSTRUCTIONS
Instruction Sheet
IS-PA250A
Issued: 1-18-19

Instruction Sheet
IS-PA250A
Issued: 1-18-19
Lowell Manufacturing Company 100 Integram Drive Pacific, Missouri 63069 U.S.A.
Call: 800-325-9660 Fax: [phone] Click: www.lowellmfg.com
3
PA250A Power Amplifier Block Diagram
Technical Specifications
Dimensions: 19" wide x 3.5" high x 12.5"deep (482mm wide x 88mm high x 317.5mm deep) Color: Black
Frequency Response: 20Hz-20KHz + 3dB (any input to any output)
Mixing
And
Control
Section
“TEL”
and 600W
Inputs Mute
the Aux Input.
The mute
functionmay
be defeated by
changing an
internal circuit
board jumper.
Speaker Level Outputs: Screw terminals. 250W RMS @ 4W, 25V, 70.7V. 200W RMS @ 4Woutput into 8Wload
Total Harmonic Distortion: < 0.3% at 1kHz at rated power (4Wdirect output)
Hum and Noise: > 90dB below rated output (unweighted)
Weight: 18LBS. (8.2KGS) Shipping Weight: 20LBS. (9.1KGS)
Power
Amplifier
250W RMS
“TEL” Input
& 600WInput
Gain
Control
SPEAKER LEVEL OUTPUTS
TEL-PAGE Input: Removable Phoenix Connector. 50mV @ 600Wbalanced (required for full rated output power)
AUX Input: Parallel RCA Connectors, 500mV @ 10kWunbalanced (required for full rated output power)
Amplifier Muting-Priority Scheme
The amplifier includes unique voice-activated (VOX) ducking/muting
circuitry. When a page is made into the “TEL” input or 600Wline level
input, music that is feeding the Aux input will duck in volume (not be
completely muted). If the incoming page is strong enough in level, the
music volume will be completely muted. When the page has been
completed, the volume of the music will return to its normal level. The
muting circuit can be defeated by a qualified technician, by removing
the chassis cover and repositioning the X2 MUTE jumper that is
located behind the recessed gain control on the input circuit board.
Parallel
RCA Jacks
(Pin +, Sleeve )
+ TEL - GND + 600W-
Note: Both the TEL-PAGE input and the 600Winput
are 600Wbalanced transformer isolated linelevel
inputs, but the 600Winput has lower input sensitivity
than the TEL-PAGE input. In other words, the TEL-
PAGE input is a “hotter” input so it takes less input
signal to drive the amplifier tofull power when using
the TEL-PAGE input.
Red Peak LED Note: If the red peak LED flashes, it indicates the peaks are approaching
full power. A steady bright redglow couldindicatethat the amplifier is being
over-driven, has oscillationin the system, or has a shorted speaker line.
Peak LED (Red): If the red peak LED flashes, it indicates the peaks are approaching full power. A steady bright
red glow could indicate that the amplifier is being over-driven, there is sub-sonic or ultra-sonic
oscillation in the system, or the amplifier is loaded with a shorted speaker line.
“Power On” LED (Green): Lights when the power amplifier is switched on.
600WInput: Removable Phoenix Connector. 500mV @ 600Wbalanced (required for full rated output power)
AC Supply Voltage: 115/230VAC 60Hz + 10%, Power Consumption: 480W @ Rated Power
Jumper X2 in Mute-Active
Position as set at the Factory Jumper X2 shown in
Mute-Defeated Position
4W
GND 25V70.7VC
AUX
Input
_
Paging Horn Protection Low-Cut Filter
The PA250A includes a low-cut filter (also called a “trumpet protect” filter) that is
not activated when the amplifier is shipped from the factory. When the amplifier is to be
used to power paging horns that can be damaged by the low frequency energy that is often
present in paging/music signals, we recommend that a qualified technician activate the low-
cut filter by removing the chassis cover and cutting the red jumper that is located directly
behind the recessed gain control on the input circuit board. The red jumper is labeled as
shown in the picture to the right. The low-cut filter has a 3dB down point at 400 Hz.
A low-cut filter may be activated by cuttinga
circuit board jumper. Activating thelow-cut
filter is recommended when using the PA250A
to power paging horns (for trumpet protection).
External Circuit Breaker: 5A Internal Fuse: 5X20mm 5A Slow Blow

4
REMOVABLE
PHOENIX
CONNECTOR
Instruction Sheet
IS-PA250A
Issued: 1-18-19
Lowell Manufacturing Company 100 Integram Drive Pacific, Missouri 63069 U.S.A.
Call: 800-325-9660 Fax: [phone] Click: www.lowellmfg.com
PA250A Rear Panel
5
3
6
PA250A Front Panel
21
PASSIVE CONVECTION COOLING AND VENTILATION
This unit is cooled via passive convection and therefore designed for continuous operation. Excessive heat due to
poor ventilation can shorten the lifespan of electronic equipment and could void the manufacturer’s warranty. Do
not block the amplifier’s vent slots located on the top and bottom of the chassis.
Amplifier Installation and Connection
This unit was carefully checked and packed before leaving the factory. However, it is always a good idea to inspect
the shipping container and unit for indications of improper handling. If the unit has been damaged, make an
immediate claim to the dealer or distributor from whom it was purchased. If the unit was shipped to you, notify the
transportation company without delay, saving all packing materials, in order to process the claim.
INSPECTION
Before installing the amplifier, continue the inspection by running a quick performance check. Connect an 8Wtest
speaker to the 4Wdirect output terminals and a tuner, tape deck, or CD player to one of the aux inputs and set the
controls for operation. CAUTION: TO PROTECT THE SPEAKER FROM DAMAGE, DO NOT TURN THE UNIT
ON UNTIL ALL CONNECTIONS HAVE BEEN MADE. IN ADDITION, MAKE SURE THE UNIT IS GROUNDED
BEFORE TESTING. In this way you can check the basic operation of the amplifier before actually installing the
unit and making all of the final terminations required for the permanent installation. If shipping damage has
resulted in the amplifier being inoperable out of the box, call Lowell Customer Service to arrange for a replacement.
INITIAL PERFORMANCE CHECK
Never run microphone cables near AC power wires, data cables, telephone cables, or speaker wiring to avoid pick-
up of extraneous signals. Do not run high impedance unbalanced wiring more than 20' to avoid picking up noise
and affecting the frequency response of the system. Avoid running speaker cables near data or telephone cables,
or other low voltage wiring that could pick up the signal from the high level speaker cables. Always use shielded
cable for microphone and line level cables except the balanced 600Winput line from a telephone system may be
typical telephone twisted pair cable. Make sure that all speakers are wired in phase with each other and always
test the speaker lines to make sure there are no short circuits before connecting them to the amplifier output.
SOUND SYSTEM WIRING AND INSTALLATION GUIDELINES
7
4
8910 11 12
LED Peak Indicator (Red)
1
LED Power-On Indicator (Green)
2
3IEC Power Cord Connector
Amplifier Circuit Breaker
4
Main Power Switch
5
Input Voltage Selector Switch
6
4W/25V/70.7V Speaker Outputs
7
Master Volume Control
8
TEL-PAGE & 600WGain Control
10
TEL (Telephone Page) Input
1
1
600W(Line Level) Input
12
Parallel Hi-Z Aux Input Jacks
9

Instruction Sheet
IS-PA250A
Issued: 1-18-19
Lowell Manufacturing Company 100 Integram Drive Pacific, Missouri 63069 U.S.A.
Call: 800-325-9660 Fax: [phone] Click: www.lowellmfg.com
5
MOUNTING THE AMPLIFIER: The PA250A may be placed on a wooden or metal wall-mount shelf. Do not
remove the rubber feet from the bottom of the amplifier chassis. When the amplifier is placed on a countertop or is
shelf-mounted, the rubber feet maintain the proper spacing under the amplifier for passive convection cooling. Do
not place anything on top of the amplifier where the vents on the top of the chassis would be blocked. Rack mount
ears are built into the front panel of the PA250A. If the amplifier will be rack mounted, we recommend that you
provide a minimum of 1.75” of open space above and below the unit to assure proper ventilation. Provide
ventilation fans in the equipment cabinet if that is required to maintain an amplifier operating temperature of no
higher than 92oF. To mount the amplifier in a Lowell equipment cabinet, use standard 10-32 Phillips-head machine
screws (like the Lowell model RS or RSP rack screws) with integral plastic washers to protect the finish of the front
panel of the amplifier. The PA250A is very heavy, so we recommend that you provide rear support for the amplifier
when rack mounting it to remove some of the load from the built-in rack mount ears.
LINE LEVEL AUX INPUT: The amplifier includes a mono high-impedance unbalanced auxiliary input which may be
used for connecting signal sources such as an AMFM tuner, CD player, Cassette deck, or an unbalanced feed from
an equalizer or mixer/preamplifier. Two (2) parallel RCA-phono connectors are provided for the aux input so stereo
input cables can be connected to this mono input. Mono inputs need only use one or the other of the RCA-phono
connectors. Note: Connecting 2 pieces of equipment that have grounded AC plugs with a standard unbalanced
RCA cord can cause a ground loop that will cause a “hum” or “buzz” in the speakers. If this occurs, feed the
amplifier using the “Tel” or “600W”inputs because the built-in isolation transformer will eliminate the ground loop.
POWER WIRING: The AC power cord is equipped with a North American style NEMA 5-15P plug and the
“VOLTAGE SELECT” switch is set to operate on a 115 VAC 50/60 Hz. power source. If this unit is to be used
where the “VOLTAGE SELECT” must be switched to 230 VAC, remove the plug from the end of the cord and
replace it with one appropriate for the local power grid receptacle standards. The internal wire color code for the
cord supplied with this unit is: Green: Earth “E” or Safety Ground, White: Neutral “N”, and Black: Line “L” or Hot.
Note: If a British Standard BS1363 plug is installed, it must be provided with a 5 Amp fuse. Since electrical color
codes vary around the world, make sure that the correct connections are made to the cord even if the local color
code is different. If in doubt, obtain the services of a locally qualified electrical professional. NOTE: THIS UNIT
MUST BE EARTH GROUNDED.
600WCHANNEL INPUT WIRING
From an
External
Device
Balanced
Line-level
600W
Output
600WStranded Twisted-Pair Wiring
Shieldedwiring from anexternal device feeding the 600W
balanced line level would typically be required as shownbelow.
BUTT SHIELD
(No connection) DRAIN
SHIELD
The amplifier includes a 600Wline level input channel. This input accepts a low impedance balanced 600Wline
level signal that would typically be fed from an equalizer, from another system, or from some other outboard device.
Note: The “TEL” (telephone page) input and the 600WLine Level Input are both 600Wbalanced line level inputs,
but the “TEL” input is a “Hotter” input requiring less input signal to drive the amplifier to full power. If distortion
results from feeding the “TEL” input with a high level signal, use the 600WLine Level input instead.
600WINPUT CHANNEL
+TEL- GND +600W-
TEL-PAGE INPUT CHANNEL: The amplifier includes a “TEL” telephone page input. This input accepts a low
impedance balanced 600Wline level signal that would typically be available as a page port output from a telephone
system. Note: The “TEL” Input and the 600WLine Level Input are both 600Wbalanced transformer isolated line
level inputs, but the “TEL” Input is a “Hotter” input requiring less input signal to drive the amplifier to full power. If
distortion results from feeding the “TEL” input with a high level signal, use the 600WLine Level Input instead.
“TEL” TELEPHONE PAGE CHANNEL INPUT WIRING
From the
Telephone
System
Page-Port
Output
600WTwisted-Pair Wiring
Shieldedwiring from the telephone system is typically not required, but if
shielded wiring is used, drain theshield at the telephone input and butt
the shield (no connection) at the telephonesystem page port output.
BUTT SHIELD
(No connection)
DRAIN
SHIELD
+TEL- GND +600W-

AMPLIFIER
OUTPUT
4W
70.7V
25V
C
GND
AMPLIFIER
OUTPUT
4W
70.7V
25V
C
GND
Instruction Sheet
IS-PA250A
Issued: 1-18-19
Lowell Manufacturing Company 100 Integram Drive Pacific, Missouri 63069 U.S.A.
Call: 800-325-9660 Fax: [phone] Click: www.lowellmfg.com
6
Speaker System Connections
The PA250A includes a speaker output screw terminal strip. Connect a minimum 4W
speaker line to the 4Woutput. Note: A speaker line with an impedance greater than
4Wcan safely be connected to the 4Woutput, but it will draw less than full amplifier
output power. For 25V or 70V operation, connect the low side of the speaker line to
“C” and the hot side to the 25V or 70.7V terminal. Never connect a 4Wor 8Wspeaker
system (that has no 25V or 70V matching transformer) to the 25V or 70.7V output.
Four 8WSpeakers
wired in Series/Parallel
8 Ohm Series/Parallel Speaker System Wiring
The Lowell PA250A power amplifier includes a direct 4Woutput that can drive a minimum 4Wspeaker line without
the use of a line matching transformer. The impedance of the speaker load must always be equal to or greater than
the 4Wimpedance of the amplifier output. For example, it is safe for the amplifier if the 4Woutput is used to drive an
8Wload or a 16Wload, but it is not safe for the 4Woutput to drive a 2Wload. Overloading the amplifier output can
cause distortion, damage the amplifier, and void the manufacturer’s warranty. It is also important to pay attention to
the power rating of the speakers used. If a 4Wspeaker is fed from the 4Woutput of an amplifier and the amplifier is
turned all the way up, the speaker will receive the amplifier’s maximum output. A speaker must be chosen with
sufficient power handling capacity for the amplifier used. For example, if a 4Wspeaker can handle 250 watts and it
will be fed from the 4Woutput of the amplifier, an amplifier with a power output of 250 watts or less should be
chosen so that if the amplifier is turned all the way up, the speaker will be able to handle the amplifier’s full power
output. All of the Series/Parallel configurations shown below will result in load impedances that will be safe for the
PA250A amplifier when connected as shown. Verify that the 250W maximum amplifier power when divided
between the speakers, will not exceed the power rating of the speaker chosen.
Speaker Signal Polarity
In a paging amplifier, making sure that a positive going input signal
results in a positive going speaker output signal, is often not critical.
If, however, multiple amplifiers feed speakers covering the same
area, observing correct polarity can be important so all speakers are
in phase with each other. The picture to the right shows how inputs
should be wired to result in no polarity change in the PA250A.
Balanced Inputs
RCAPhonoPlug Input
+
Sleeve
_+
Pin TEL PAGE
Input
_
+
600W
Input
_
+
GND 25V70.7V
C
4W
SPEAK ER
+
_8W
SPEAK ER
+
_8W
SP EAKER
+
_8W
SPEAK ER
+
_8W
Eight 8WSpeakers
wired in Series/Parallel
Result:
Each speaker receives
1/8 of the
total amplifier power.
(1/8 of 250W RMS @ 4W)
SPEAK ER
+
_8W
SP EAKER
+
_8W
SP EAKER
+
_8W
SP EAKER
+
_8W
SP EAKER
+
_8W
SP EAKER
+
_8W
SP EAKER
+
_8W
SP EAKER
+
_8W
AMPLIFIER
OUTPUT
4W
70.7V
25V
C
GND SPEAKER
+
_8W
Result:
One speaker
receives less than the
total amplifier power.
(200W RMS @ 8W)
One 8WSpeaker
AMPLIFIER
OUTPUT
4W
70.7V
25V
C
GND SPEAKER
+
_8W
SP EAKER
+
_8W
Result:
Each speaker receives
1/2 of the
total amplifier power.
(1/2 of 250W RMS @ 4W)
Two 8WSpeakers
wired in Parallel
Result:
Each speaker
Receives 1/4 of the reduced
total amplifier power.
(¼ of 200W RMS @ 8W)
8WLOAD
8WLOAD
4WLOAD
4WLOAD
+
_
+
_
Speaker Outputs
Direct
Out 25V/70V
Out

Instruction Sheet
IS-PA250A
Issued: 1-18-19
Lowell Manufacturing Company 100 Integram Drive Pacific, Missouri 63069 U.S.A.
Call: 800-325-9660 Fax: [phone] Click: www.lowellmfg.com
7
70 volt and 25 volt “Constant Voltage Distribution Systems” have been a source of confusion for people for a long
time. Our Series/Parallel speaker wiring discussion on page 6, illustrates why industry engineers developed the
“Constant Voltage” concept. Series/Parallel configurations are manageable for small localized systems using up to
4 to 8 speakers and when no expansion is anticipated. Even with only 4 to 8 speakers, wiring a Series/Parallel
configuration can be confusing. Nobody would ever want to attempt Series/Parallel wiring for 8000 speakers spread
over an entire office complex. The 70V and 25V speaker systems (and the 100V system which is used in Europe)
rely on inexpensive line matching transformers to simplify the “impedance matching” and “amplifier loading” issues.
The line matching transformers allow all of the speakers in a 70V or 25V speaker system to be wired in parallel
regardless if there are 8 speakers or 8000 speakers in the system, as long as the total of all transformer taps
used does not exceed the power rating of the amplifier. Parallel connections make field wiring easy and it
makes it possible to add speakers anywhere along the speaker string without any concern about Series/Parallel
load impedance. The term “Constant Voltage” can be misleading. “Constant Voltage” means whether you have a
10W 70V amplifier, 100W 70V amplifier, or a 1000W 70V amplifier, the output will be at 70.7V when the amplifier is
turned all the way up. When a 70V amplifier is turned down, it will be running at less that 70V. The key thing to
remember is that the transformer power tap values are calculated for when the amplifier is running at a full 70V. A
70V speaker transformer tapped at 5 watts, will only draw a full 5 watts, when the amplifier is at it’s full 70V output.
70V / 25V Speaker System Wiring
Individual Speaker Transformer Tap Settings:
A 70V or 25V speaker that is chosen by the system designer, has a matching transformer
with a maximum power rating . For example, the Lowell R1810-72 (shown to the right) has
a Lowell TLM-572 5-watt 70V/25V dual voltage transformer. That means that this speaker
can be used on both 70V systems and 25V systems. The 8Wsecondary of the
transformer is connected directly to the 8Winput terminals on the 810 speaker. The
primary has transformer taps for either 70V or 25V at 0.25W, 0.5W, 1W, 2W, and 5W.
When a designer is mapping out the coverage of these ceiling speakers, he/she will
determined how many speakers are required and in what locations, and what power tap
will be used for the speakers to be loud enough for this application. Let’s say for example
that the designer has determined that 100 speakers will be required tapped at 1W each at
70 volts. That means that our total speaker load is 100 watts. The speakers will be wired
in the field in parallel as shown below making sure the connections are made with the
proper polarity to keep all of the speakers operating in phase with each other.
A 70V or 25V speaker system is easy to design and easy to wire in the field because all speakers can be wired in
parallel. The higher transmission voltages used greatly reduce the power lost in the speaker lines so smaller less
expensive speaker wiring can be used. The volume of a single speaker on the line can be adjusted louder or softer
by adjusting the power tap used on that speaker’s matching transformer. That adjustment does not affect the
volume of the other speakers in the system. The volume adjustment of a single speaker on the line or the volume
adjustment of groups of speakers in a zone can be made accessible to the user by way of inexpensive wall-mount
autoformer-based volume controls. The settings on wall-mount volume controls can be easily bypassed during
pages or during emergency announcements. (See Lowell “priority attenuator” volume controls that include a “-PA”
in the model number). It is easy to add speakers to the system in the future without having to change any of the
existing wiring. If adding additional speakers in the future will require a more powerful amplifier at the head-end, an
amplifier with a higher output power capability can replace the existing unit without changing any of the existing
speaker wiring or any of the existing speaker transformer tap settings. For example: A Lowell MA60 could be
replaced with a Lowell MA125 so that more speakers could be added to the system, with no other changes required
to the existing speaker wiring or transformer taps.
70V / 25V Constant Voltage Speaker System Advantages:
Lowell R1810-72
Note: Always insulate the
transformer taps that are
not being used sothey
don’t short together.
5W
2W
1W
.5W
.25W
COM
5W
2W
1W
.5W
.25W
COM
5W
2W
1W
.5W
.25W
COM
AMPLIFIER
70V
COM
To More
Speakers
SP EAKER
+
-
8W
SP EAKER
+
-
8W
SP EAKER
+
-
8W

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Instruction Sheet
IS-PA250A
Issued: 1-18-19
Lowell Manufacturing Company 100 Integram Drive Pacific, Missouri 63069 U.S.A.
Call: 800-325-9660 Fax: [phone] Click: www.lowellmfg.com
8
Cable distance should be kept as short as possible to minimize power loss. The chart below can be used as a
general guide showing various wire sizes and the maximum distance related to cable loading that speakers can be
placed from the unit for an approximate 0.5dB loss (-12.5% power).
Wiring Size and Length:
70V and 25V Amplifier Load Calculations and Amplifier Selection:
In fact, it is good design practice to limit the speaker load to 85% of the amplifiers power rating. That designed-in
headroom allows for future expansion (adding a few speakers) and also the reduced load will increase the lifetime of
the amplifier by allowing it to run cooler. Continuing with the example from page 7, the designer knows that the
expected speaker load is 100 watts. What would be an appropriately sized amplifier for that system? If the
designer considers a 125W amplifier, 125 X .85 = 106.25 watts. It would be good design practice to limit the
speaker load to no more than 106.25 watts, so since the speaker load in the example is 100W, the 125W watt
amplifier would be a good choice. Sometimes the amplifier has already been purchased and the number of
speakers required is known, but the question becomes what transformer tap setting to use. Desired speaker SPL
calculations to determine the speaker transformer tap required are beyond the scope of this paper, but there are
some practical considerations when choosing the power tap. If a 125 watt amplifier has already been purchased,
using the 85% loading rule of thumb, 125W X .85 = 106.25 watts. It would be good design practice to limit the
speaker load to no more than 106.25W. Since we know from our example that 100 speakers would be required, it is
obvious that tapping the speakers at 1W is the best choice.
Some system designers feel it is best to tap the speaker transformers as high as possible. Consider an example
where a sound system that has a 125W 70V amplifier, only has 10 speakers. If the transformers are tapped at
0.25W each, the total speaker load will be 0.25W X 10 speakers = 2.5W. If the amplifier is turned all the way up so
its output is 70 volts, those speakers will still only draw a maximum of 0.25W each and it would not be possible to
get the speakers any louder. That would result in tons of headroom but you wouldn’t have much volume out of the
speakers. Some designers would suggest that the speakers in this example should be tapped at 5W each so the
speaker load would be 5W X 10 speakers = 50W. Again, plenty of headroom for the amplifier but more than 12dB
more sound pressure level out of the speakers. Other designers would prefer that the speakers be tapped at 1W
each (if they are sure that the 1W tap would provide sufficient SPL) because the 1W tap provides 2 higher powered
taps that are available if this particular speaker needs to be louder, and 2 lower-powered taps if this speaker is too
loud. The designer needs to determine the appropriate tap setting after considering the requirements of the system
design. The question might be asked, what happens when the designer has a 125 watt amplifier, and needs 25
speakers tapped at 5 watts each? That results in a speaker load of 125 watts with no headroom. Sometimes a
designer is confident that adding speakers in the future will not be necessary, and competitive pressure in a bid
situation will not make it possible to use a larger amplifier to provide the desired headroom. Although headroom is
always desirable, be assured that the Lowell amplifiers will survive and operate safely when fully loaded.
Never use a speaker load (the sum of all speaker transformer taps) that exceeds the amplifier power rating.
Overloading the amplifier can cause distortion and could damage the amplifier and void the warranty.
WIRE LENGTHS INFEETWIRE LENGTHS INFEET