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 ªat the micº 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 | Microphone Type and | Battery |
| Type | FP24 Settings | Life |
| | | |
| Eveready AA | 2 dynamic handheld microphones, low | 11 hrs. |
| No. L91 (lithium) | meter brightness, no headphones |
| |
| | | |
| | 2 dynamic handheld microphones, low | 6 hrs. |
| | meter, low headphone level |
| | |
| | | |
| Duracell AA MN | 2 condenser, 15-Volt phantom pow- | |
| ered microphones, normal meter in- | 4 hrs. |
| 1500 (alkaline) |
| tensity, normal headphone level | |
| | |
| | | |
| | 2 studio condenser, 48-Volt phantom | |
| | powered microphones, super bright | 2 hrs. |
| | meter intensity, high headphone level | |
| | | |
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 busesÐassignable to mic or line level output.
•Master volume and headphone monitoring for all five inputs controlled from the FP33.
•CompactÐFP24 adds little bulk to an existing FP33 package.
•The FP24's isolated external DC input jack eliminates ground related interference problems.
