Macro-Tech 3600VZ Power Amplifier
Page 18
Reference Manual
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 subsonic 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 output and
the positive (+) lead of the transformer. The circuit
shown below uses components that are available from
most electronic supply stores.
Fig. 3.14 Inductive Load (Transformer) Network
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 ex-
cellent damping factor of 1,000 from 10 to 400 Hz in Ste-
reo mode with an 8-ohm load. In contrast, typical damp-
ing factors are 50 or lower. Higher damping factors yield
lower distortion and greater motion control over the loud-
speakers. A common damping factor for commercial ap-
plications is between 50 and 100. Higher damping fac-
tors 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 desirable.
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 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 damp-
ing 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.
4-ohm, 20-watt
Resistor
590 to 708
µ
F Capacitor
120 VAC, N.P.
+
–
Inductive
Load
+
–
From
Amplifier
Output