dance may be calculated by the formula:
1 | 1 | 1 | 1 |
- | = - | f - + - . . . .etc. |
ZT | ZI | 2 2 | Z, |
where ZTis the total impedance, Z,is the impedance of the first speaker, Z, is the impedance of the second speaker, and so forth, with a total number of fractional terms equal to the total number of speakers in the group. Before at- tempting to add the individual fractions, a common denom- inator for all fractions must be determined. For example, if an 8-ohm and a 16-ohm speaker are connected in parallel, the expression is written:
Examination indicates that 16 would be an appropriate common denominator, allowing the expression to be re- written as follows:
and simplifying,
Taking the reciprocal of (inverting) the final expression gives:
16
ZT = - = 5.3 ohms 3
For a series-parallel interconnection of different speaker impedances, determine the total parallel-connected speak- er impedance, and add it to the total of series-connected impedances. The sum is the total load impedance.
In planning a multiple speaker installation, the maximum power ratings of the individual speakers being used should be kept in mind. This is particularly important where speak- ers of different impedance values are connected to the DI- RECT OUTPUT Connectors (8,9). For example, assume an 8-ohm and a 16-ohm speaker are connected in parallel across the Amplifier output and the Amplifier is operated at full-rated output. Figure 6, Page 7, shows that approxi- mately 120 watts are developed across the 8-ohm speaker, while only 60 watts are produced across the l g o h m speaker. The power ratings of the speakers employed must be evaluated accordingly.
70-VOLT OUTPUT
When the 70-volt output of the SR105A Amplifier is uti- lized, matching the speaker load to the Amplifier output impedance is not necessary. The only requirement is that the total wattage of the speaker load connected to the 70- volt output does not exceed the power rating of the Ampli- fier.
In a 70-volt speaker system it is standard practice to cou- ple each speaker to the 70-volt line with a line transformer. These transformers are equipped with tapped secondary windings to accommodate various speaker impedances. The primary winding is also tapped and labeled with the
wattage rating applicable to each tap. For example, the Shure A102A, a 50-watt transformer, has taps at 50, 25, 12 and 6 watts. (Transformers are available at various power ratings to suit the requirements of the application.) The pri- mary taps allow adjustment of the power level delivered to the speaker when the transformer is connected to the 70- volt line. Connection of the 50-watt tap to the 70-volt line produces 50 watts across the speaker when the audio signal level on the line is 70 volts. Similarly, use of the 25- watt tap produces 25 watts across the speaker.
The total load in watts presented by a 70-volt multiple speaker system is the sum of the individual speaker loads. The value of each individual speaker load is indicated by the wattage rating of the transformer primary tap con- nected to the 70-volt line. For example, assume only two speakers are connected to a 70-volt line, the first using a 50-watt primary tap and the second using a 25-watt tap. The total load across the 70-volt line is therefore 50 plus 25, or 75 watts.
In the design of sound reinforcement installations, good engineering practice allows for change or growth. An ex- ample of this is a 70-volt installation requiring 300 watts. This system could be handled by two 70-volt lines of 150 watts each, connected individually to two SR105A Ampli- fiers. This system would be operating at full capacity with no room for expansion. The preferred installation would be three 70-volt lines of 100 watts each, connected individually to three SR105A Amplifiers. This arrangement allows addi- tions or changes in each 70-volt line of up to 50 watts.
OUTPUT VOLTAGE METER
The OUTPUT VOLTAGE Meter (1) provides a convenient aid for adjusting and monitoring the Amplifier operating level. The meter scale is calibrated in percent, with 100% being equivalent to the maximum output voltage of the Amplifier, prior to clipping. The performance of the meter is similar to that of a VU meter and it responds to program material in a similar manner.
It should be noted that a 100% meter indication repre- sents a signal level of 28.3 volts rms across the DIRECT OUTPUT (8,9). The maximum rated amplifier power output of 200 watts is obtained only when the total speaker load is 4 ohms across the DIRECT OUTPUT. With loads other than 4 ohms, a 100% meter indication represents a lower power output, e.g., for a total speaker load of 8 ohms, a 100% reading indicates a power output of 100vvatts. Note that it is not possible to obtain a greater power output with this load.
Figure 6 may be used as an approximate guide to the power indicated by a 100% meter indication for various di- rect speaker loads. Note that while the meter is calibrated for 28.3 volts output, the typical output power available is somewhat higher and is indicated by meter readings greater than 100%.
In the SR105A Amplifier, a 100% indication represents a signal level greater than 70.7 volts rms across the 70V OUTPUT Terminals (11). The OUTPUT VOLTAGE Meter
(1)may be recalibrated for a 100% indication with 70.7V output. Maximum 70-volt amplifier power output is ob- tained with a 150-watt load (33 ohms) across the 70V OUTPUT Terminals. Similarly, maximum power output may be obtained with a combination of direct and 70-volt loads (see Figure 7, Page 7).
The relationship between the OUTPUT VOLTAGE Meter
(1)reading and the Amplifier direct-coupled output power is illustrated in the following table:
