LOAD IMPEDANCE (OHMS)
FIGURE 6. TYPICAL OUTPUT POWER VS. LOAD IMPEDANCE
FROM 2 OHMS TO 32 OHMS
I 1 KHz. 2% TOTAL HARMONIC DISTORTION)
(DIRECT OUTPUT ONLY)
tection circuitry and cause low-frequency breakup or dis- tortion of the Amplifier audio output.
The 70-volt output of the SRlO5A Amplifier is trans- former-coupled to facilitate the use of balanced lines in the long cable runs associated with distributed speaker systems. A center-tap connection to the secondary of the output transformer (CT) is provided on the 70-VOLT OUT- PUT terminal strip (11). This terminal may be grounded to provide speaker lines which are balanced to ground. This is done by connecting a wire from the center-tap terminal to the ground terminal. This arrangement can be effective in eliminating hum induced in the speaker lines by strong external fields. It can also be effective in eliminating cross- talk and a tendency for system oscillation in applications where the 70-volt speaker output and low-level audio input are run in the same raceway.
The full-rated output of 150 watts from the SRI05A Ampli- fier at the 70-volt output terminals (11) will be realized with a 33-ohm load. The output voltage under these circum- stances will be 70.7 volts. In order to activate the 70-volt output of the Amplifier, turn on the 70V-OFF Switch (10). When using this output, care should be taken to see that the sum of the individual loads (i.e., each speaker in a dis- tributed speaker system) does not exceed the 150-watt power output capability of the Amplifier.
It is possible to utilize both the direct-coupled speaker output and the 70-volt output of the SR105A Amplifier simultaneously, if desired. In this case, reference should be made to Figure 7, Page 7 to determine the power avail- able to the 70-volt output as a function of the load on the direct speaker output. As an example, assume an 8-ohm speaker load is to be connected to the direct-coupled speaker output, and the 70-volt output is also to be used. Following the 8-ohm vertical line on the graph (Figure 7, Page 7) to the point where it intersects the curve, note that the horizontal line also passing through this point is 80 watts. This is the power available on the 70-volt output with a simultaneous 8-ohm direct output load.
CAUTION
Parallel connection of the DIRECT OUTPUT Con- nectors (8,9), or the speaker cables between two SR105 Power Amplifiers, or the SR105 Amplifier and any other amplifier, should not be attempted. Bal- ancing paralleled amplifiers is virtually impossible, and the circuitry mismatch cannot be balanced. The same precaution should be observed with regard to the 70V OUTPUT Connectors (11) of two SR105A Amplifiers, or the SR105A Amplifier and any other amplifier. Such a connection may result in damage to one or both amplifiers and is not covered by the Guarantee. This restriction does not extend to the series amplifier connections described in Bridging Connections, Page 11.
As indicated on the rear panel of the Amplifier, Class 2 wiring* may be used for speaker connections to the DIRECT OUTPUT terminals (8,9) of the Amplifier as well as the interconnection between speakers in the direct-coupled distribution circuit. Class 2 wiring may be used for speaker connection to the 70V OUTPUT terminals (11) of the Ampli- fier or interconnection of speakers within this circuit.
In order to avoid appreciable power loss in the cables used for speaker connection, it is necessary to consider the length of cable required, the total impedance of the speak- ers being connected, and the Amplifier output (direct or 70-volt) being used. The graph presented in Figure 8, Page 8, for direct speaker output indicates the recommended wire gauge for various speaker loads and amplifier-to- speaker distances. As an example, assume two 16-ohm speakers are to be connected in parallel at a distance of 18.3m (60 ft) from the amplifier, and the direct output of the Amplifier is to be used. The total impedance of the speakers connected in parallel is eight ohms. Following the 8-ohm horizontal line on the graph (Figure 8) to the point where it intersects a vertical line representing 18.3m (60 ft), note that the nearest solid diagonal line to this point is labeled AWG 18. This is the recommended wire
*A s defined by U.S. National Electrical Code.
160
140
E120
I-
-III 0 0
f, 8 0
Z
g 60
I-
2 40
I-
s 2 0
0
I | 2 | 3 | 4 5 6 8 1 0 | 2 0 | 4 0 | 6 0 8 0 1 0 0 |
DIRECT SPEAKER OUTPUT LOAD (OHMS)
FIGURE 7. 70-VOLT OUTPUT POWER VS. DIRECT OUTPUT
LOAD IMPEDANCE FROM 4 OHMS TO 100 OHMS
(SRIOSA ONLY)
