J. W. Davis & Company DA-120M User Manual - page 11
The transformer taps are marked in W=watts instead of Ohms (usually 4, 2, 1,
¹⁄₂). Again, it should be
born in mind that these levels of power output are only achieved when the transformer is working at 70.7
Volts. The transformers are connected in parallel. A good match is obtained by ensuring that the total of
all tap settings fits into the range of 40-80% of rated amplifier output. 80% is chosen to allow for trans-
former insertion loss. Also, It is good practice not to drive the amplifier to 100% of its capacity.
NEW TECHNOLOGY
OLD RELIABILITY
J.W. Davis & Company
3030 Canton Street • P.O. Box 710219 • Dallas, Texas 75371-0219
Sales 800-527-5705 • Fax 800-388-9106 • Corp 214-651-7341 • Fax 214-939-0328 • sales@jwd.com • www.jwd.com
•
The Mark of the Professional...
The following is an example using 16 8-Ohm speakers. Each
schematic is an impedance equivalent to its predecessor but
has been simplified.
(1)
Reduced line losses and ability to use smaller wire gauges. This is due to the higher voltage
and reduced current in the speaker lines.
(2)
Much simpler impedance matching procedures and connections.
Examples are shown below. For simplicity, it is assumed that all tap settings are the same at each
speaker. For a 120-Watt amplifier, the range 40-80% is equivalent to 48-96 Watts. Therefore:
A problem arises if one more speaker must be added at some
future date, as all the connections must be changed. This is
not a significant problem if only a few speakers are involved.
But if the network is extensive, the problem will be magnified.
Futhermore, failure of one speaker can take out a number of
associated units.
HIGH IMPEDANCE OF CONSTANT VOLTAGE (25 AND 70v)
SYSTEMS
The high impedance or constant voltage method of impedance
matching uses a high impedance amplifier output which is
transformed down to 8 Ohms by an impedance matching
transformer at each individual speaker. The big advantages of
this approach as compared to low impedance are:
Constant voltage is a misnomer in that the amplifier does not always produce 70V. Rather, the amplifier
output impedance is set at such a level that, irrespective of its rated power, it will produce70.7 Volts
output at full power. Thus a 10 Watt amplifier optimum load would have an impedance of 500 Ohms
(P = 70.7²/500), a 40-Watt amp would be 126 Ohms and 120-Watt amp, 42 Ohms.
Multiple transformer taps allow the impedance at each speaker to be adjusted individually to give a total
matched load. Because of the high impedance arrangement, the system is easier to impedance match
and is also inherently less susceptible to problems caused by mismatching.
11
18 speakers x 4 Watt taps each = 72W . . . . . . . . . . . Good match
12 speakers x 1 Watt taps each = 12W . . . . . . . . . . . Poor match
60 speakers x 1 Watt taps each = 60W . . . . . . . . . . . Good match
60 speakers x 4 Watt taps each = 240W . . . . . . . . . . Very poor match - overload
90 speakers x 1 Watt taps each = 90W . . . . . . . . . . . Good match
NOTE: None of the above tap settings guarantee the actual sound levels through each speaker.
This is a function of the master volume control as well as the tap setting. The tap setting simply defines
the maximum power consumed by an 8-Ohm speaker if presented with 70.7 Volt input. In systems with
a small number of speakers, it is always preferable to use a high tap setting and reduce the sound level by
turning down the master volume control. In calculating the amplifier rating needed for a typical music/
paging system using speakers distributed to an office environment, a good rule of thumb is to allow about
one watt per speaker and space speakers at 1
¹⁄₂ x ceiling height. For noisy areas, or where the volume level
required is higher, more power is required.