Macro-Tech 24x6 Reference Manual - page 15
Page 15
Macro-Tech 24x6 & 36x12 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 excellent damping factor of
1,000 from 10 to 400 Hz in Stereo mode with an
8-ohm load. In contrast, typical damping factors
are 50 or lower. Higher damping factors yield
lower distortion and greater motion control over
the loudspeakers. A common damping factor
for commercial 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 circum-
stances, Crown’s
IQ System
®
is often used so
amplifiers can be monitored and controlled
when they are located very near the loudspeak-
ers.) 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 Re-
sistance
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
Resistance
line through the mark on the
2-
Cond. Cable
line, and on to intersect the
An-
nealed 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 in-
creases as the AWG value becomes 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 damp-
ing factor, or (3) use more than one cable for
each line. Options 1 and 2 will require the sub-
stitution 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 that
can cause your amplifier to prematurely activate its pro-
tection circuitry which can result in inefficient opera-
tion. The effects of this problem are similar to the effects
of the RF interference described in Section 3.3.1. To
prevent high frequency oscillations from occurring:
1. Lace the loudspeaker conductors together. (Do
NOT lace cables together from different amplifi-
ers.) This minimizes the chance of them acting
like an antenna to 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 amplifiers share a com-
mon cable tray or 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 separated from input
cables.
7. Install a low-pass filter on each input line (similar
to the RF filters described in Section 3.3.1).
8. Install the input wiring according to the instruc-
tions in Section 3.3.1.
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 appear as a “short” at low frequen-
cies, causing the amplifier to produce large low fre-
quency currents and unnecessarily activate its
protection circuitry. Always take the precaution of in-
stalling a high-pass filter at the amplifier inputs when a
predominantly inductive load is used. A three-pole (18
dB per octave) filter with a –3 dB frequency of 50 Hz is
recommended. (Depending on your application, it
might be desirable to use a filter with more than a –3
dB frequency.) Such a filter should eliminate the sub-
sonic frequency problems mentioned in Section 3.3.1.
Another way to prevent the amplifier from activating its
protection systems early and also protect the inductive
load from large low-frequency currents is to connect a
590 to 708 µF nonpolarized capacitor and a 4 ohm,
20 watt resistor at the output of the amplifier and in
series with the positive (+) lead of the transformer. This
is depicted in Figure 3.12 on the next page.