OPERATING INSTRUCTIONS
To give you a better understanding about the operation of
the FPL 44, a quick course in its design should be useful.
Included as a part of this manual is the data sheet covering
this product. This sheet includes a block diagram of one
channel of the unit. You may want to consult this document
to aid you in understanding the following.
The limiter circuit used in this module is what Rane refers
to as a “servo-locked limiter.““’ This term derives from the
similarity of this design to that of a servo motor such as is
used in robotics and precision control systems. When the
output of the circuit is below the setting of the THRESH-
OLD control, the FPL 44 channel acts only as a unity gain
amplifier. The output is monitored at all times by a differen-
tial amplifer. One input of this differential amplifier has a
reference voltage connected to it. The other input is driven
by a full-wave rectifier whose level is a product of the
RMS output voltage of the voltage controlled amplifier
(hereafter VCA) and the THRESHOLD setting. The output
of this circuit produces an error voltage any time the
rectifier’s output exceeds the fixed reference. By nature, a
differential amplifier’s output travels wherever necessary in
terms of output voltage to keep its two inputs equal. Since
the output of this amplifier connects to the control pin of
the VCA, we have provided a means for this to occur. This
scheme creates a servo-loop which will not allow the output
of the VCA to increase above the THRESHOLD setting.
Any time the FPL 44 is limiting, the error signal created
by the servo amplifier will be proportional to the amount of
overdrive on its VCA’s output. This control voltage may
be connected to other VCAs in the FPL 44 by setting the
switch on the front panel to the AUTO-SLAVE position.
Doing so connects the VCA’s control pin to the slave bus
in the unit. In this way, when one channel limits, all chan-
nels on the bus attenuate the same amount. This preserves
the spectral distribution of the signal.
This is a rather brief comment on the internal workings of
each of the limiters in the module. Certain details regarding
time-constants and other witchcraft necessary to make a lim-
iter sound good have been eliminated in the interest of clar-
ity.
The AUTO-SLAVE feature is most useful when the FPL
44 is being utilized to protect amplifiers and drivers con-
nected to the outputs of an active crossover. Many feel
when one driver is protected by limiting action, all drivers
should be equally attenuated to preserve the spectral bal-
ance of the original program material. There are others
who feel the opposite is the case. In an attempt to satisfy
all, we include the option for either mode.
Operating the FPL 44 is quite simple. Simply set the
THRESHOLD control to the level you never wish to ex-
ceed at the output. This may be accomplished either scien-
tifically or empirically (what we used to call “trial and
error”). The trial and error method is the simplest and also
the most dangerous if the FPL 44 is being employed for
system protection. Finding the point at which a driver or
amplifier should be limited to prevent damage may produce
the damage you are trying to avoid.
If you know that a driver can safely handle, say, 100
watts, and the limiter is to be connected directly to the
input of the amplifier, the specifications of the amplifier
may be consulted to determine the appropriate threshold set-
ting on a given limiter channel. One must be certain that all
variables are taken into account such as level control set-
tings on amplifiers and so on. Failure to do so could result
in disaster. The FPL 44 is also an invaluable signal proces-
sor for recording. Tape saturation on an analog machine
can be tolerable within limits. Overloading the input of a
digital recorder is quite another matter. The sonic character-
istics of the FPL 44 make it a very suitable protection de-
vice in the studio. In this environment, a bit of trial and
error in setting the THRESHOLD is far safer. Excessive
levels during recording only damage the take, not the hard-
ware.
CHASSIS GROUNDING
Rane Flex Series modules are supplied with either a
rear, or a bottom/side mounted ground-lift switch. The
unit is shipped with this switch in the “grounded” posi-
tion, tying circuit ground to chassis ground. If after hook-
ing up your system it exhibits excessive hum or buzzing,
I there is an incompatibility in the grounding configuration
between units somewhere. Your mission, should you ac-
cept 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 unit
and available on request at no charge if you lost your first
one) for further information on system grounding.
Copyright 1991, Rane Corporation 10802 47th Ave. W. Mukilteo, WA 98275 (425)355-6000
All features & specifications subject to change without notice 520-177 291