Yamaha P-2200 Owner's Manual - Page 27

approximately -50dB (2.45mV) for a microphone, +4dB (1.23 volts) for a line level signal. The exact voltage is not critical, and 1000Hz is a standard reference frequency, but any other appropriate frequency can be used. 2. Set the input channel level control on the mixer at its rated "nominal" setting, and adjust the master level control so that the output level is 20dB below the rated maximum output level for the mixer. For the Yamaha PM-180 Mixer used in the example, the maximum rated output level is +24dB (12.3 volts), so the output level should be adjusted to +4dB (1.23 volts), as indicated either on an external voltmeter, or on the mixer's VU meter (0VU). 3. Assume that the rated maximum input level for the graphic equalizer in the example is +14dB (3.88 volts). Subtracting +4dB from +14dB leaves only 10dB of headroom, so a 10dB resistive pad must be inserted between the mixer output and equalizer input. Now, the signal level at the input to the equalizer should be -6dB (388mV), which can be confirmed with a voltmeter. 4. Assume that the maximum rated output level of the equalizer in this example is +18dB (6.16 volts). Adjust the master level control on the equalizer so that the output level is 20dB below this rated maximum, or -2dB (616mV). Since the equalizer has no VU meter, you need an external voltmeter to confirm this level. 5. Finally, starting with the attenuators on the P-2200 at maximum attenuation (maximum counter clockwise rotation), slowly rotate them clockwise, watching the peak reading meters. When the peak reading meters indicate 2.3 watts output from the P-2200, there is 20dB headroom left before clipping. To operate this system, use only the controls on the mixer, and avoid levels that consistantly peak the mixer's VU meter above the "zero" mark on its scale, or that peak the P-2200's meters above a safe power level for the speaker system. Any adjustments of the other devices in the system will upset the headroom balance. However, the P-2200's calibrated attenuators allow easy setups and quick changes, if you decide to change the headroom figure. They also allow you to momentarily fade the entire program or a single channel and to later bring it back up to exactly the same level. To use this technique with any system, first design the required speaker system, and calculate the number of power amplifiers needed to safely operate the speaker system with adequate headroom. Then, choose the mixer, and other devices that feed the power amplifiers, and set up the system according to the above instructions. In some cases, it may be useful to set up different headroom figures in different parts of a complex system. For example, background music and paging should be severely compressed in a noisy lobby area, but the same program material would sound more natural in less noisy office and auditorium areas of the same installation if the headroom figure were increased. By placing a compressor/limiter in the circuit just before the P-2200 that feeds the lobby areas, the headroom figure can be lowered for that section only, without affecting other parts of the system. Cabling the System Audio circuits may be divided into the following classifications (by signal level): 1. Low level circuits: any circuit carrying signals of -80dB (77.5 microvolts) to -20dB (77.5 millivolts), example: microphone lines. 2. Medium or line level circuits carrying signals of -20dB (77.5mV) to +30dB (24.5 volts), example: mixer outputs. 3. High level circuits carrying signals above +30dB (24.5 volts), example: speaker lines. 4. AC power circuits, including lighting circuits. 5. DC control (or supply) cables to relays, from batteries, etc. Generally, each of these categories should be physically separated from the others to avoid crosstalk, oscillation, and noise spikes. One possible exception is that DC control or supply cables and line level signal cables can be routed together if the DC signal is adequately filtered. Figure 45 shows the undesirable results that can occur if line or speaker cables are placed near microphone cables. This situation occurs in concert sound when mixer outputs and mic inputs feed through the same "snake" cable. Fig. 45 - Example of Crosstalk Figure 46 shows an equipment rack with a good cable layout. Note that the different categories of cable are carefully separated, and that where it is necessary to cross two categories, they cross perpendicular to each other. These suggestions apply to all types of systems, portable as well as permanent. Fig. 46 - Cable Routing in Equipment Rack. (Reprinted from Sound System Engineering by Don & Carolyn Davis published by H. W. Sams Co.) Figure 47 shows the rear of a P-2200 amplifier with its two inputs "chained" using a phone-to-phone cable. In this mode, the signal fed to the first side is also fed to the second side of the amplifier. This could also be accomplished with an XLR-to-XLR cable. For low and medium level balanced signal cables, use good quality twisted pair shielded cable. For portable