Crown Audio 133472-1A manual 1.3.5Solving Output Problems, High-FrequencyOscillations

16 Chapter 1: Crown Amplifiers In-Depth

Two different types of fuses are required for thermal protection and voltage protection. Slow-blow fuses are usually selected to protect loudspeakers from thermal damage because they are similar to loudspeakers in the way they respond to thermal conditions over time. In contrast, high-speed instrument fuses like the Littlefuse 361000 series are used to protect loudspeakers from large transient voltages. The nomograph in Figure 1.24 can be used to select the properly rated fuse for either type of loudspeaker protection.

There are basically two approaches that can be taken when installing fuses for loud- speaker protection. A common approach is to put a single fuse

in series with the output of each channel. This makes installa-

tion convenient because there is only one fuse protecting the loads on each output. The main disadvantage of this approach becomes obvious if the fuse blows because none of the loads will receive any power.

A better approach is to fuse each driver independently. This allows you to apply the most appropriate protection for the type of driver being used. In general, low-frequency drivers (woofers) are most susceptible to thermal damage and high-frequency drivers (tweeters) are usually damaged by large transient voltages. This means that your loudspeakers will tend to have better protection when the woofers are protected by slow-blow fuses and high-fre- quency drivers are protected by high-speed instrument fuses.

1.3.5Solving Output Problems

High-Frequency Oscillations

Sometimes high-frequency oscillations occur which can cause your amplifi er to prematurely activate its protection circuitry and result in ineffi cient opera- tion. The effects of this problem are similar to the effects of the RF problem described in Section 1.2.2. To prevent high-frequency oscillations:

1.Lace together the loudspeaker conductors for each channel; do not lace together the conductors from different channels. This minimizes the chance that cables will act like antennas and transmit or receive high fre- quencies that can cause oscillation.

2.Avoid using shielded loudspeaker cable.

3.Avoid long cable runs where the loudspeaker cables from different ampli- fi ers share a common cable tray or cable jacket.

Amplifier Application Guide

Figure 1.24

Loudspeaker Fuse Nomograph