From: Patrick Jankowiak (eccm@swbell.net)
Date: Wed Jun 25 2003 - 23:20:15 PDT
Explanation:
The "0" "250" "500" "1000" "2500" are impedances in ohms. A home/car
stereo uses 4 or 8 ohms as we are aware. Such a unit will drive the
speaker directly but at much reduced power.
0 is common. you can interface the unit with a compatible amp which
has an output at one of these impedances, or use a transformer to
match it. The match does not have to be perfect and the transformer
need not be expensive or special. Be sure to observe the wattage
rating. Although the unit is large, the wattage may be only 10-20
watts. It will be plenty loud none the less.
Here is how to interface using a common old surplus power transformer.
Contrary to popular belief, power transformers are fairly good for
audio in communications applications.
(P.A., not hi-fi).
(non-technical folks may wish to skip to the worked out solutions at
the end)
HOW TO pick a transformer. use a slide rule if you must:
1. get the load impedance of your amp.
example = 8 ohms.
2. pick an impedance of the speaker.
example= 2500 ohms.
3. Note the wattage.
example = 20 watts.
4. Figure the impedance ratio betwen the speaker and the amp.
It is 2500 to 8. 2500 / 8 = 312.5 to one (312.5/1)
5. Figure the voltage ratio.
The impedance ratio is the square of the voltage ratio.
Therefore the voltage ratio is the square root of the impedance ratio.
SQRT 312.5 = 17.677
The 2500 ohm side of the arrangement will have 17.677 times more
voltage than the 8 ohm side.
6. select a transformer with the right voltage ratio.
The example transformer has a 120VAC primary winding.
Seek a transformer with a secondary voltage of 120 / 17.677 which
would be 6.788 volts.
There are no transformers in the example surplus store with this
voltage, but some are close.
7. compromise slightly and check your results.
In the example, you find some old transformers with 120V input and 6.3
volt output windings.
120 / 6.3 = 19.04.
The voltage ratio is 19.04 to 1.
The impedance ratio is the square of this.
It is 362.5 to one.
8 ohms will then be transformed to 2900 ohms.
8. Find out what actual load ths arrangement will present to your 8
ohm amplifier output.
The 2500 ohm load provided by your speaker's 2500 ohm connection will
present a load of (2500/2900)*8, which is 6.89 ohms.
This will work quite well.
9. Consider the power.
A 20 watt power level will pass through the transformer.
The transformer has a 6.3 volt winding.
Use the figures to determine how much current in AMPs will flow
throught the transformer's 120V winding when 20 watts is delivered to
the speaker. From this, figure the current rating needed for the 6.3V
winding.
9a. Power/load impedance=current squared. 20W / 2500 ohms = 0.08
9b. SQRT 0.08 = 0.089 current = 0.089 amps in the 120V winding and
speaker.
9c. current ratio of the transformer is the inverse of voltage ratio.
9c1. voltage ratio was 19.04 to one. (step 7)
9c2. current ratio is therefore one to 19.04
9d. 0.089 * 19.4 = 1.7266 amps on the 6.3V winding.
9d1. The amplifier will supply this current. If you check the work,
you note that 1.7266 amps flowing through a 6.89 ohm load requires
13.81 volts. (1.7266*6.89=13.81)
9d2. Wattage verification: 13.81 volts * 1.7266 amps = 20.54 watts.
9e. Select a transformer with a current rating of 2 to 3 amps.
10. Connect it so that the 6.3 volt winding is connected to the amp
and the 120V winding is connected to the speaker.
11. play bagpipe music loudly when the social defect with rap music
shows up next to you.
SGT PJ's commo NOTES:
1. Placing two speakers in parallel gives HALF the impedance. (1250
ohms)
2. Placing two speakers in series gives TWICE the impedance. (5000
ohms)
3. Add the total wattage of all the speakers together when choosing
the power handling capacity of the matching transformer.
4. Careful, the voltage at the 2500 ohm level is 223VAC at 20 watts!
This makes 220V and other higher voltage transformers useful for this
application.
4a. The target voltage rating of the high voltage side of the
transformer should be no less than 1/3 of the actual speaker voltage,
and no more than twice the speaker voltage, for most efficient
operation. (a 120V transformer has 223V across it in the example).
4a1. Farther outside these limits, there will be increasing losses due
to insufficient turns per volt on the '1/3' side of this, and too much
winding resistance on the 'twice' side of this.
4a2. Farther outside these limits, there will be loss of bass and
increasing amplifier loading problems at very low frequencies due to
insufficient turns per volt on the '1/3' side of this, and loss of
treble due to too much winding inductance and reduced amplifier
loading at high frequencies on the 'twice' side of this.
4a3. These issues can be mitigated somewhat by using a lower powered
transformer when approaching the "1/3" side of this, and a higher
powered transformer when approacing the "twice" side of this.
Explanation of the testing procedure and calculations for such
deviations is outside the scope of this posting. Experiment.
5. The example SUBSITUTE transformer for identical function could have
a 240VAC primary and a 12.6V secondary with a 1.7266 amp rating. It is
the VOLTAGE RATIO and POWER HANDLING that matters. Such a transformer
would be larger and have twice the power rating as the 120V to 6.3V
model, but that is not a bad thing.
6. examples using 120V transformers at 20 watts:
FIND SECONDARY VOLTS AND CURRENT RATINGS:
6.. speaker to amp ohms: PRI to SEC V, SEC A.
6a. 500 ohms to 8 ohms: 120V to 15.1V, 1.6A
6b. 1000 ohms to 8 ohms: 120V to 10.7V, 1.58A
6c. 2500 ohms to 8 ohms: 120V to 6.788V, 1.7266A
6d. 500 ohms to 4 ohms: 120V to 10.7V, 2.24A
6e. 1000 ohms to 4 ohms: 120V to 7.58V, 2.24A
6f. 2500 ohms to 4 ohms: 120V to 4.8V, 2.236A
Just get 'em close (voltage ratio within 5 to 7%)
SGT PJ
Bobby Joe Pendleton II wrote:
>
> A friend and I have come across a Military Loudspeaker LS-10/TIQ it is 19
> inches across the large end of the horn and 15.0 inches OAL and has five
> wire connectors in a circular pattern on the backend. They are numbered "0"
> "250" "500" "1000" "2500"
> Any Idea what is goes to, how to hook it up?
> How does this fit in with Military Vehicles, you ask?? Well, I was wanting
> to mount it on a Truck and play music through it in Parades
>
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