Lowell MA250 - Manual (Page 11)

Instruction Sheet
IS-MA60/125/250
Issued: 5-1-14
Lowell Manufacturing Company 100 Integram Drive Pacific, Missouri 63069 U.S.A.
Call: 800-325-9660 Fax: 636-257-6606 Click: www.lowellmfg.com
11
5W
2W
1W
.5W
.25W
COM
5W
2W
1W
.5W
.25W
COM
5W
2W
1W
.5W
.25W
COM
AMPLIFIER
70V
COM
To More
Speakers
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 10, illustrates why industry engineers developed the
“Constant Voltage” concept. Series/Parallel configurations are manageable for small localized systems using up to
8 speakers and when no expansion is anticipated. Even with only 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 8
W secondary of the
transformer is connected directly to the 8
W input 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.
Lowell R1810-72
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:
SPEAKER
+
-
8 W
SPEAKER
+
-
8 W
SPEAKER
+
-
8 W