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16 Chapter 1: Crown Amplifi ers In-Depth
Amplifi er Application Guide
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.
1.3.5 Solving 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.
Figure 1.24
Loudspeaker Fuse
Nomograph