Low-Frequency Protection Circuit For Inductive Loads - Crown Micro-Tech 600 Reference Manual

Micro-Tech 600/1200/2400 Power Amplifiers
1. Note the load resistance of the loudspeakers connected
to each channel of the amplifier. Mark this value on the
"Load Resistance" line of the nomograph.
2. Select an acceptable damping factor and mark it on the
"Damping Factor" line. Your amplifier can provide an excel-
lent damping factor of 1,000 from 10 to 400 Hz in Stereo
mode with an 8 ohm load. In contrast, typical damping fac-
tors are 50 or lower at these frequencies. Higher damping
factors yield lower distortion and greater motion control over
the loudspeakers. A common damping factor for commer-
cial applications is between 50 and 100. Higher damping
factors may be desirable for live sound, but long cable
lengths often limit the highest damping factor that can be
achieved practically. (Under these circumstances, Crown's
IQ System is often used so amplifiers can be easily moni-
tored and controlled when they are located very near the
loudspeakers.) In recording studios and home hi-fi, a damp-
ing factor of 500 or more is very desirable.
3. Draw a line through the two points with a pencil, and con-
tinue until it intersects the "Source Resistance" line.
4. On the "Two Conductor Cable" line, mark the length of
the cable run.
5. Draw a pencil line from the mark on the "Source Resis-
tance" line through the mark on the "Two Conductor Cable"
line and intersect the "Copper Wire" line.
6. The required wire gauge for the selected wire length and
damping factor is the value on the right-hand scale of the
"Copper Wire" line. For metric wire sizes, find the recom-
mended resistance in ohms per 305 meters (1000 feet) and
use this information to reference the correct wire size. Note:
Wire size increases as the AWG gets smaller .
7. If the size of the cable exceeds what you want to use,
(1) find a way to use shorter cables, like using the IQ Sys-
tem , (2) settle for a lower damping factor, or (3) use more
than one cable for each line. Options 1 and 2 will require the
substitution of new values for cable length or damping factor
in the nomograph. For option 3, doubling the number of
conductors of equal thickness will reduce the resistance in
ohms per 1000 feet (305 meters) by half. When using AWG
standards, you can estimate the effective wire gauge by
subtracting 3 from the given wire gauge every time the num-
ber of conductors of equal gauge is doubled. So, if #10 wire
is too large, two #13 wires can be substituted, or four #16
wires can be used for the same effect.
SOLVING OUTPUT PROBLEMS
High-frequency oscillations can cause your ampli-
fier to prematurely activate its protection circuitry. The
effects of this problem are similar to the effects of the
RF problems described in Section 3.3.4. To prevent
high-frequency oscillations, follow these guidelines:
1. When using long cable runs, or when different
amplifiers share a common cable tray or jacket,
use tie-wraps to bundle individual conductors
so the wires for each loudspeaker are kept
close together. Do not bundle wires from differ-
ent amplifiers. This reduces the chance of con-
ductors acting like antennas that transmit or
receive the high frequencies that can cause os-
cillations.
2. Avoid using shielded loudspeaker cable.
3. Never tie together input and output grounds.
4. Never tie together different amplifier outputs.
5. Keep output cables separated from input
cables.
6. Install an RF filter in series with each input (see
Section 3.3.4).
7. Install input wiring according to the instructions
in Section 3.3.4.
Another problem to avoid is the presence of large sub-
sonic currents when primarily inductive loads are
used. Examples of inductive loads are 70-volt step-up
transformers and electrostatic loudspeakers.
Inductive loads can act like a short circuit at low fre-
quencies. This can cause the amplifier to produce
large low-frequency currents and activate its protec-
tion circuitry. Always take the precaution of installing a
subsonic filter in series with each of the amplifier's in-
puts when inductive loads are used. A three-pole, 18
dB per octave filter with a –3 dB frequency of 50 Hz is
recommended (some applications may benefit from an
even higher –3 dB frequency). See Section 3.3.4 for
some examples.
Another way to protect inductive loads from large low-
frequency currents and to prevent the amplifier from
prematurely activating its protective systems is to par-
allel a 590 to 708 mF nonpolarized motor start capaci-
tor and a 4-ohm, 20-watt resistor in series with the
amplifier's output and the positive (+) lead of the trans-
former. This circuit is shown in Figure 3.14. It uses com-
ponents that are available from most electrical supply
stores.
4-ohm, 20-watt
Resistor
+
590 to 708 f Capacitor
120 VAC, N.P.
From
Amplifier
Output
–
Fig. 3.14 Low-Frequency Protection
Circuit for Inductive Loads
+
Inductive
Load
–
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