Mackie M1200/M1400 Owner's Manual page 22

3. Set the LOW CUT FILTER
control to either 63Hz or 125Hz, depending
on the cutoff frequency selected in the
subwoofer amplifier
Since these exact frequencies are not la-
beled on the amplifier, you can guesstimate the
position of the knob. 63Hz is located at about
1 o'clock and 125Hz is about 4 o'clock.
If you want to be really accurate, you will
need to use a variable-frequency signal genera-
tor along with a frequency counter and a volt-
meter or oscilloscope to find the exact position
for the knob. The voltmeter or o'scope should
be capable of measuring low frequencies accu-
rately (down to 60Hz).
1. Connect the signal generator to the INPUT
of the amplifier and monitor the
SPEAKER OUTPUT
or o'scope.
22
frequency 2. Adjust the signal generator level so you
.
3. Change the frequency of the signal genera-
4. Adjust the LOW CUT FILTER
After you've completed this exercise, you
with the voltmeter
should mark the position of the control with
a stick-on arrow or label so you don't have
to repeat it should the control accidentally
get moved.
measure 1V rms output at 1kHz. (You
should turn down the GAIN knobs on the
amplifier about halfway.)
tor to 63Hz or 125Hz, whichever you're
using as your subwoofer crossover fre-
quency. You may need to use a frequency
counter for accuracy.
control until the voltmeter or o'scope reads
0.707VAC rms. This is the –3 dB point, and
means you have now matched the LOW
CUT FILTER frequency cut-off point to the
SUBWOOFER FREQUENCY switch setting
(either 63Hz or 125Hz).
frequency