SPA100 1 / 2 - R a c k P o w e r A m p l i f i e r
Speaker Impedances And Power Ratings:
When connecting speaker cabinets, you must observe proper load impedances. Whenever connecting multiple cabinets to an amplifier, the total load impedance must be calculated to insure proper performance from the amplifier. The impedance chart which follows shows the total load impedances of many common parallel speaker combinations:
IMPEDANCE CHART: | SYSTEM ONE | |
| SINGLE SPEAKER | TWO SPEAKERS |
| IN PARALLEL | |
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| 8 | 16 | 8:16 | 16:16 |
| ONE | 8 | 4 | 5.3 | 3.2 | 4 |
SYSTEM | SPKR | 16 | 5.3 | 8 | 4 | 5.3 |
TWO | TWO IN | 8:16 | 3.2 | 4 | 2.7 | 3.2 |
| PARALLEL | 16:16 | 4 | 5.3 | 3.2 | 4 |
Note that some combinations result in impedances less than 4 ohms. (the shaded areas) These combi- nations are not recommended for the SPA100 and should be avoided. They are included on the chart for reference only. Too low an impedance will activate the amplifier’s protection circuit.
The formula for this chart is:
1
1/R1 + 1/R2 + ... + 1/RN
Where “R” is the speaker impedance. So, using two 16 ohm speakers in combination with one 8 ohm speaker, the imped- ance would be:
1 | = | 1 | = | 1 |
1/8 +1/16+1/16 |
| .125+.0625+.0625 |
| .250 |
= 4 ohm load impedance
You can also determine the combined impedance by means of the Impedance Chart. The combined impedance appears at the intersection of the 16:16 column (System One) and 8Ω row (System Two).
Power Output:
The power output of an amplifier changes in accordance with the load impedance or mode of operation. The total output power of the SPA100 is as follows:
total load impedance: | 8 ohms | 4 ohms |
total output power: | 75 watts | 100 watts |
Output Power Per Speaker Cabinet:
If each parallel connected speaker cabinet has the same rated impedance, divide the total power output by the number of cab- inets used. For example: Four 16 ohm cabinets have a total load impedance of 4 ohms (see the impedance chart above), which allows a total output power of 100 watts RMS: 100/4 = 25 watts RMS for each cabinet.
Speaker cabinets with different rated impedances will draw different amounts of power. To calculate the power output per cabinet, obtain the total load impedances from the impedance chart above and divide it by each speaker impedance. For exam- ple: Three speaker cabinets, one with a rated impedance of 8 ohms and two at 16 ohms are connected in parallel. According to the impedance chart, the total load impedance is 4 ohms, which means a total power output of 100 watts as above. Divide the total impedance by the impedance of each speaker and multiply the results by 100.
Rtotal = | 4/8 = 1/2 x 100 = 50 watts for the 8 ohm cabinet |
RN | 4/16 = 1/4 x 100 = 25 watts for each of the 16 ohm cabinets |
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Joining Two Amplifiers For Rack Mounting:
The SPA100 amplifier is shipped with a heavy duty steel joining plate (not attached to the unit). This allows two amplifiers to be mounted side by side in a single rack space (19”W x
(1)Remove one rack mount flange from each amplifier: the flange on the right side of the amplifier on the left and the flange on the left side of the amplifier on the right.
REMOVE THESE 2 | REMOVE THE 6 | REMOVE THESE 2 |
SIDE SCREWS | ||
FRONT SCREWS | (3 PER UNIT) | FRONT SCREWS |
Power |
| Power |
On |
| On |
Off |
| Off |
Power Signal Limit Protect |
| Power Signal Limit Protect |
REMOVE THESE 2
RACK MOUNT FLANGES
(2)Remove two front panel screws from each amplifier: the two screws on the right side of the amplifier on the left and the two screws on the left side of the amplifier on the right.
PLACE UNITS SIDE BY SIDE - ALIGN THROUGH HOLES | |
IN JOINING PLATE OVER SCREW HOLES IN UNITS | |
Power | Power |
On | On |
Off | Off |
Power Signal Limit Protect | Power Signal Limit Protect |
| INSERT 4 |
| FRONT SCREWS |
(3)Position the amplifers side by side, align the holes in the joining plate with the holes in the front panel of the adjacent amplifier. Insert the screws through the joining plate into the front panel of the adjacent amplifier and tighten securely.
TWO UNITS, JOINED AND READY FOR RACK MOUNTING:
Power |
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| Power |
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On |
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| On |
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Off |
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| Off |
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Power | Signal | Limit | Protect | Power | Signal | Limit | Protect |
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