Crown Audio Macro-Tech Series (600, 1200 & 2400) - Manual (Page 17)

Macro-Tech 600/1200/2400 Power Amplifiers
Page 17 Reference Manual
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. Higher damping factors yield lower dis-
tortion and greater motion control over the loudspeakers. A
common damping factor for commercial applications is be-
tween 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 monitored and controlled when they are
located very near the loudspeakers.) In recording studios
and home hi-fi, a damping factor of 500 or more is very desir-
able.
3. Draw a line through the two points with a pencil, and
continue until it intersects the “Source Resistance” line.
4. On the “2-Cond. 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 “2-Cond. Cable” line,
and on to intersect the “Annealed Copper Wire” line.
6. The required wire gauge for the selected wire length and
damping factor is the value on the “Annealed Copper Wire”
line.
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 System,
(2) settle for a lower damping factor, or (3) use more than one
cable for each line. Options 1 and 2 will require the substitu-
tion of new values for cable length or damping factor in the
nomograph. For option 3, estimate the effective wire gauge
by subtracting 3 from the apparent wire gauge every time
the number 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
Sometimes high-frequency oscillations occur which
can cause your amplifier to prematurely activate its pro-
tection circuitry and result in inefficient operation. The
effects of this problem are similar to the effects of the
RF problem described in Section 3.3.4. To prevent high-
frequency oscillations:
1. Lace together the loudspeaker conductors for
each channel; do not lace together the conduc-
tors from different channels. This minimizes the
chance that cables will act like antennas and
transmit or receive high frequencies that can
cause oscillation.
2. Avoid using shielded loudspeaker cable.
3. Avoid long cable runs where the loudspeaker
cables from different amplifiers share a common
cable tray or cable jacket.
4. Never connect the amplifier’s input and output
grounds together.
5. Never tie the outputs of multiple amplifiers to-
gether.
6. Keep loudspeaker cables well separated from
input cables.
7. Install a low-pass filter on each input line (similar
to the RF filters described in Section 3.3.4).
8. 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 trans-
formers and electrostatic loudspeakers.
Inductive loads can appear as a short circuit at low fre-
quencies. This can cause the amplifier to produce large
low-frequency currents and activate its protection cir-
cuitry. Always take the precaution of installing a high-
pass filter in series with the amplifier’s input when
inductive loads are used. A 3-pole, 18-dB-per-octave
filter with a –3 dB frequency of 50 Hz is recommended
(depending on the application, an even higher –3 dB
frequency may be desirable). Such a filter is described
with infrasonic frequency problems in Section 3.3.4.
Another way to prevent the amplifier from prematurely
activating its protection systems and to protect induc-
tive loads from large low-frequency currents is to con-
nect a 590 to 708 µF nonpolarized capacitor and
4-ohm, 20-watt resistor in series with the amplifier’s out-
put and the positive (+) lead of the transformer. The
circuit shown below uses components that are avail-
able from most electronic supply stores.
Fig. 3.14 Inductive Load (Transformer) Network
4-ohm, 20-watt
Resistor
590 to 708 µF Capacitor
120 VAC, N.P.
+
–
Inductive
Load
+
–
From
Amplifier
Output