Rane FAC 28 - Manual (Page 4)

All features & specifications subject to change without notice. 520-187 OCT94
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OPERATING INSTRUCTIONS
Before attempting a complete run-down of operating
guidelines for the FAC 28 crossover, a few words about state-
variable Linkwitz-Riley filters are in order. Rane implements
this alignment using four cascaded two pole butterworth
filters in the FAC 28 to produce the eighth order characteris-
tic. The FAC 28 delivers a steep 48dB per octave. At the
crossover frequency the High and Low Outputs are 6dB down
from unity and in phase with each other. The result is a
combined output which is completely flat from one end of the
audio spectrum to the other. See Rane Note 119 for greater
detail. The important element here is that the outputs be in
phase. This is relatively easy to accomplish electrically, and
much more difficult acoustically where it really counts.
Therein lies the reason for the phase correction capabilities of
the crossovers. They are able to alter their phase response to
compensate for phase problems in drivers and their cabinetry.
CROSSOVER FREQUENCY should be set primarily
based on the driver manufacturer’s recommendations. Some
fine tuning may be necessary depending on the specifics of
the system. Caution should be used when straying too far
away from factory guidelines. Crossing over a horn or other
high frequency component too low may result in permanent
damage. Low frequency drivers generally can be driven with
high frequencies with no ill results other than poor sound. It is
better to err on the high side than the low.
INPUT AND OUTPUT LEVELS should generally be set
at or near the unity gain marks. There should be no reason to
take gain in a crossover other than to make up for efficiency
differences between the high and low drivers. Taking too
much gain or loss in a crossover usually indicates inappropri-
ate gain structure elsewhere in the system. For best noise
performance, system gain should be accomplished at the
mixer or source. Taking gain in components throughout the
system usually yields poor noise performance. We recom-
mend that a realtime analyzer or other measurement system
be used to set the relative levels between the low and high
outputs. Most speaker systems can be made relatively flat
using only the level and phase controls on the crossover
before equalization is applied.
PHASE CONTROLS as well as the Polarity switches on
the Outputs of the crossover may be adjusted using the
aforementioned test equipment. Starting with the Invert
switches in their POS position and the PHASE control full
CCW, look at the crossover frequency area with your test
equipment. If you are using pink noise as a source, be sure to
observe only the crossover region. Room acoustics and
imperfect driver response can throw off everything else to the
point of total frustration. Mute the High end and set the High
LEVEL control for a 0dB indication on the test equipment at
the crossover frequency. Now Mute the Low and unMute the
High. Set the High out for 0dB at crossover. Release the Mute
on the Low and you should see a +3dB indication at cross-
over. If you do not, first try to achieve it by setting the
PHASE IN/OUT switch to the IN position and slowly rotate
Printed in the U.S.A. on Recycled Paper
the PHASE control clockwise. If unable to achieve + 3dB at
the crossover frequency, return the PHASE control to its full
CCW position and change only one of the Invert switches to
its IN position. Again, rotate the PHASE control slowly
clockwise. With one of these combinations you should be
able to optimize the combining of the two drivers.
3 WAY AND BEYOND. The FAC 28 modules may be
combined to produce 3, 4, and 5 way systems. Have a look at
the diagram below for a typica1 app1ication and consu1t the
F1ex User’s Guide.
IMPORTANT NOTE
CHASSIS GROUNDING
Rane Flex Series modules are supplied with a rear-
mounted ground-lift switch. The unit is shipped with this
switch in the “grounded” position, tying circuit ground
to chassis ground. If after hooking up your system it
exhibits excessive hum or buzzing, there is an incompat-
ibility in the grounding configuration between units
somewhere. Your mission, should you accept it, is to
discover how your particular system wants to be
grounded. Here are some things to try:
1. Try combinations of lifting grounds on units that
are supplied with ground lift switches or links.
2. If your equipment is in a rack, verify that all
chassis are tied to a good earth ground, either through
the line cord grounding pin or the rack screws to another
grounded chassis.
3. Units with outboard power supplies do not ground
the chassis through the line cord. Make sure that these
units are grounded either to another chassis which is
earth grounded, or directly to the grounding screw on an
AC outlet cover by means of a wire connected to a
screw on the chassis with a star washer to guarantee
proper contact.
Please refer to Rane Note 110 (supplied with your
product and available on request at no charge if you lost
your first one) for further information on system
grounding.
Application Diagram (3,4 Way System)