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Limiters
The FP24 has two built-in peak-responding limiters, one for each
input channel. Each limiter has a two-stage circuit: the first stage
keeps the input gain stage from clipping; the second stage limits the
variable gain stage according to the Limiter Threshold control set-
ting. This unique, two-stage topology limits the gain stage directly
after the mic input transformer without changing the input imped-
ance as other “at the mic” limiters do. The circuit enables the FP24
to limit in excess of 50 dB, making it very difficult to clip the unit re-
gardless of the gain setting.
The FP24’s limiters can operate in either either dual-mono or ste-
reo. Use the three-position switch on the input panel to set the limit-
ing mode:
• In the middle position, the limiters are turned off.
• In dual mono operation (ON switch position), the limiters
work independently, responding only to their input signals.
• In stereo-linked operation (LINK switch position), the
limiters control both channels identically. This maintains a
stable stereo image.
When the limiters activate, the Peak/Limiter LED on the front
panel illuminates amber in proportion to the amount of limiting for
each channel.
Tone Oscillator and Slate Microphone
The switch labeled 1 kHz/SLATE on the front panel controls two
functions. The 1 kHz position (left) mutes all inputs and sends a 1
kHz sine wave calibration tone at +4 dBu to all outputs. The tone
oscillator can be used to verify connections and set nominal levels
on recording and transmission equipment.
The SLATE position (right) is a momentary switch position that
activates the built-in slate microphone located behind the front pan-
el. When the switch is held to the right, all inputs are muted and
slate mic signal is sent to all outputs. The slate mic circuit contains
an AGC (automatic gain control) to keep the slate level relatively
constant regardless of the acoustic level. Use the slate microphone
to document takes right at the mixer location when microphones
are on talent or away from the production mixer.
Metering and Clip-Indicators
Three two-color clip-indicators monitor signal levels throughout
the FP24:
1. Above each input gain control, a two-color LED glows red 3
dB below clipping. This clip circuit monitors both the front end
gain stage and the variable gain stage.
2. A headphone-clip LED, located above the headphone level
control, indicates when either channel of the headphone cir-
cuit approaches clipping.
3. The headphone-clip LED also indicates when either channel
of the tape return audio is near clipping.
When the limiter is activated, these input LEDs glow amber in
proportion to the amount of limiting.
The FP24 has a seven segment LED output meter that shows
peak output level in dBu. From left to right, the first three LEDs are
separated by 15 dB and the remaining LEDs are separated by 4 dB.
This calibration provides a 46 dB-dynamic range with good resolu-
tion in the critical signal level range.
The meter uses premium Gallium Nitride (GaN) LEDs which are
visible even in direct sunlight. A three-position switch on the front
panel controls the brightness of the output meter. Choose low, nor-
mal, or super-bright.
CAUTION – When set to super-bright intensity, the LEDs in the
output meter can irritate or damage eyes with prolonged direct ex-
posure.
Power Sources and Battery-Life
The FP24 operates from either two internal AA alkaline cells or
an external 5–14 VDC battery supply or AC to DC transformer
(such as the Shure PS20). The power connector is a locking,
threaded coaxial connector. Center pin = positive, sleeve = nega-
tive. Use the supplied threaded power connector if necessary (see
Figure 5).
The external DC input supply is galvanically isolated (floating)
from the chassis and the rest of the circuitry. The isolation provides
trouble-free interconnection to other pieces of equipment sharing
the same DC power source. The center pin of the locking DC con-
nector is positive, and the sleeve is negative. Since the external DC
supply is floating, the positive or the negative can be connected to
the chassis with no adverse effects.
Two AA, 1.5 Volt lithium cells can power the FP24 for up to
approximately 11 hours. However, several factors affect how long
a given pair of batteries will last. These factors include battery
chemistry, ambient temperature, use of phantom power, output
meter brightness, and the output drive level of the FP24. The chart
below provides estimates of battery life in different kinds of circum-
stances. Experimentation can determine expected battery life for
specific setups and environments.
NOTE: AA carbon cells and AA nickel-cadmium cells are not rec-
ommended for use in the FP24 since they have lower power capac-
ity than other types and will result in very short service life.
Battery
Type
Microphone Type and
FP24 Settings
Battery
Life
Eveready AA
No. L91 (lithium)
2 dynamic handheld microphones, low
meter brightness, no headphones
11 hrs.
2 dynamic handheld microphones, low
meter, low headphone level
6 hrs.
Duracell AA MN
1500 (alkaline)
2 condenser, 15-Volt phantom pow-
ered microphones, normal meter in-
tensity, normal headphone level
4 hrs.
2 studio condenser, 48-Volt phantom
powered microphones, super bright
meter intensity, high headphone level
2 hrs.
Test conditions: 70 degrees F, 42 dB of gain with an acoustic music
source, 600 ohm load, +4 dBu output
Interconnecting the FP24 and FP33
You can connect the balanced, line level XLR outputs of the
FP24 to the mix bus of the FP33 using the Shure A33LK accessory
kit (or construct a cable assembly as shown in Figure 4 on the fol-
lowing page).
Combining an FP24 and a Shure FP33 (or FP32A) mixer yields
a flexible, cost-effective, 5 X 2 field mixing setup with the following
features:
• Simultaneously accepts a stereo microphone, hard-wired
shotgun, and two RF lavalier microphones.
• 48-volt phantom power for all five microphone inputs
• Limiters on the FP24 inputs
• Input assignment switches on the FP24 and pan pots on the
FP33 inputs
• Two output buses—assignable to mic or line level output.
• Master volume and headphone monitoring for all five inputs
controlled from the FP33.
• Compact—FP24 adds little bulk to an existing FP33
package.
• The FP24’s isolated external DC input jack eliminates
ground related interference problems.