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M2B
Version 2
Crossover
Configuration Suggestions
The M2B Version 2 loudspeaker offers
user flexibility by allowing both full-range and bi-amplified operation.
A switch on the unit’s input panel allows the operating mode to be
selected. In the full-range mode a single amplifier channel will
correctly drive the unit. An internal crossover divides the frequency
band so as to correctly drive the low-frequency (LF) and high-frequency
(HF) components. When selected to operate in the bi-amplified mode, the
M2B Version 2 loudspeaker requires separate amplifier channels for the
LF and HF sections. Through performance testing and field listening
experience, suggested crossover and equalization parameters were
established.
Crossover:
A symmetrical crossover configuration provides good performance. The
slope of both the low-pass and high-pass filters should be 24 dB per
octave, with a Linkwitz-Riley filter implementation preferred. For both
the LF and HF channels the –6 dB crossover point should be selected for
1.5 kHz.
Equalization:
For best performance one "notch" filter in the LF output path is
suggested. The center frequency of the filter should be approximately
460 Hz. The Q should be 5 with a depth of –3 dB. For additional LF impact
a 1/3 octave “bass bump” at approximately 80 Hz can be implemented.
Depending on the taste of the listener, anywhere from +2 to +6 dB of
equalization (bump) can be effective. Listeners who liked "mongo" bass
selected +6 dB as their preference. The author feels that +3 dB is more
reasonable
The HF output would also benefit from
including a notch filter. The center frequency of the filter should be
approximately 4 kHz. The filter’s Q should be 3 and the depth –2 dB. No
high-frequency horn compensation (“CD horn EQ”) is required. Adding some
if desired should not pose a problem.
Sensitivity Compensation: The
difference in relative output sensitivities of the LF and HF components
make a level compensation adjustment useful. If this is to be done in
the electronic crossover then the level of the HF output should to be
–9 dB relative to the LF output. (LF output at unity gain; HF output
attenuated 9 dB.) This then allows the LF and HF amplifier channels to be
set to identical input sensitivities. It may also be desirable to
operate the electronic crossover channels at unity gain. In this case
sensitivity adjustment can also be made using the input attenuator
control on the audio amplifier’s HF channel.
Additional Filtering for Loudspeaker
Protection: Applying a high-pass filter to the LF channel can
provide significant loudspeaker protection. When using a filter whose
slope is 24 dB per octave, a –6 dB point in the range of 50 to 65 Hz would
be appropriate. This will prevent the M2B Version 2’s woofer from
receiving energy in a frequency band where it is not capable of
generating significant acoustic output. Almost all of the energy at this
low a frequency would simply be dissipated in the voice coil as heat.
Give your amplifier and loudspeaker a break and apply a high-pass
filter!
Practical
Implementation: This paragraph describes how to configure an XTA
Electronics Ltd. DP224 Speaker Management System for good M2B Version 2
bi-amp operation. The unit was set for 2 x 2 way crossover mode. A
low-pass setting of 1.47 kHz for the LF output and a high-pass setting of
1.53 kHz for the HF output worked very well. The filter types were set
for Linkwitz-Riley at 24 dB per octave. A notch in the HF output of –2 dB,
4 kHz, Q=3 was implemented. In the LF output a notch of –3 dB, 463 Hz, Q=5
was included. Setting the LF output to have a high-pass filter frequency
of 60 Hz provides protection for the woofer. In the LF output, a “bass
bump” of +4 dB at 80 Hz, bandwidth 0.32 (Q=3.125), created heightened LF
performance. With these settings the overall sonic performance of the
M2B Version 2 was quite good. Other fine loudspeaker management systems,
such as from BSS or Ashly Audio, can be similarly configured.
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