Crown DC-300A - Operating Precautions; Load Protection Methods

The fuse inherently protects the power supplies against
overload. The AC line for 120, 128VAC is fused with a
10A 250V type AB fuse (on 240, 248VAC, 5A type 3AG
250V).
The use of any other size fuse will invalidate the
warranty.
Never change fuses with power applied!
On each heat sink (see Fig. 2-2) is mounted a thermal
switch which protects the amplifier against insufficient
ventilation. If either heat sink becomes too hot, the AC
line power will be interrupted until the temperature falls
to a safe level, whereupon power will be automatically
restored. When such an event occurs, the external
symptoms are: no indication of AC power (by the pilot),
and a warm front panel.
All of the amplifier’s voltage-amplifier circuitry is de-
signed to be inherently current-limited. Thereby, if any
of the devices should fail, (which is extremely unlikely),
no damage will occur to the rest of the stages.
The input stage is protected against overdrive damage
by a series limiting resistor should the input signal level
ever become very excessive.
The amplifier features a controlled slewing-rate which,
coupled with the SPACE controller, protects the ampli-
fier from blowups when fed large RF input signals.
3.3. OPERATING PRECAUTIONS
The following are a number of operating precautions
given as an aid to understanding proper and improper
amplifier usage:
1. Use care in making connections, selecting signal
sources, and controlling the output level. The loud-
speaker you save may be your own. CROWN is not
liable for any damage done to loads due to careless
amplifier usage or deliberate overpowering (CROWN
speakers excepted). For pointers on load protection
see Section 3.4.
2. Never parallel the two outputs by directly tying
them together or parallel them with any other
amp’s output. Such connection does not result in
increased power output. Damage incurred by such
operation is not covered by the warranty.
3. Never drive a transformer-coupled device or any
other device which appears as a low frequency short
(less than 3 ohm) without a series isolating capacitor.
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Such operation may damage the device and/or need-
lessly waste output power.
4. Do not short the ground lead of an output cable to
the input signal ground as oscillations may result
from forming such a ground loop.
5. Never remove fuses with power applied.
6. Operate and fuse the amplifier only as set forth in
Section 3.2.
7. Operate the amplifier from AC mains of not more
than 10% above the selected line voltage and only on
50, 60, or 400Hz AC. Failing to comply with these
limits will also invalidate the warranty.
8. Never connect the output to a power supply output,
battery, or power main. Damage incurred by such a
hookup is not covered by the warranty.
9. Do not expose the amplifier to corrosive chemicals
such as soft drinks, lye, salt water, etc.
10. The amplifier is not recommended for high power
industrial usage at frequencies above 20KHz.
11. Tampering in the circuit by unqualified personnel
or the making of unauthorized circuit modifications
invalidates the warranty.
12. Do not expose the output leads to areas likely to be
struck by lightning. Such an installation could in-
validate the amplifier.
3.4 LOAD PROTECTION METHODS
The most common of all load protection schemes is a
fuse in series with the load. The fuse may be single,
fusing the overall system. Or, in the case of a multi-
way speaker system, it may be multiple with one fuse
on each speaker.
Fuses help to prevent damage due to prolonged overload,
but provide essentially no protection against damage
that may be done by large transients and such. To min-
imize this problem, high-speed instrument fuses such
as Littlefuse 361000 series are most appropriate for
such applications. For a nomograph showing fuse size
vs. loudspeaker ratings refer to Fig. 3-3.