Skip Navigation

Rockford Fosgate Power 851X

110W x 4 Car Amplifier

Item # 575851X

This item is no longer available.

Shop all Car Amplifiers


Our take on the Rockford Fosgate Power 851X

When you install a Power series amplifier, you get the biggest, beefiest power Rockford Fosgate has to offer. The 4-channel Power 851X amplifier incorporates innovative circuitry, precision controls, and MOSFET power to ensure that your music sounds absolutely pristine... and rocks hard!

When you install a Power series amplifier, you get the biggest, beefiest power Rockford Fosgate has to offer. The 4-channel Power 851X amplifier incorporates innovative circuitry, precision controls, and MOSFET power to ensure that your music sounds absolutely pristine...and rocks hard!

Use the razor-sharp (24 dB per octave slope) variable front and rear crossovers to tailor the 851X for different tasks. Switch to high-pass mode to eliminate extreme low bass from full-range speakers or components. Or bridge the amp to 2-channel mode, activate the low-pass crossovers, and drive a pair of subwoofers with 425 watts per side!

The variable Punch Bass boost (up to 18 dB at 45 Hz) enhances low-frequency response from a slight bump to a ride-rockin' roar. A remote control is included so you can boost the thump right from the driver's seat. Convenient preamp outputs pass full-range signal to another amplifier in your system.


  • 110 watts RMS x 4 at 4 ohms
  • 215 watts RMS x 4 at 2 ohms
  • 425 watts RMS x 2 in bridged mode at 4 ohms
  • 2-, 3-, or 4-channel output
  • 4-ohm stable in bridged mode
  • Tri-way capable (Tri-Way crossover required)
  • requires 4-gauge power and ground leads and a 60-amp fuse — wiring and hardware not included with amplifier
  • MOSFET power supply and output devices
  • front and rear 50-250 Hz high-pass/low-pass crossovers, 24 dB/octave slopes
  • MEHSA mounting technology increases the heat transfer from the MOSFET output devices to the heat sink, for higher power output, lower operating temperatures, and greater reliability
  • TOPAZ — noise-killing differential inputs that eliminate 80% of ground loop noise
  • Trans•ana circuitry keeps preamp voltages low and signal paths simple for outstanding audio quality
  • NOMAD — advanced protection circuitry for "intelligent" shutdown protection
  • wired remote control included for fingertip control of Punch Bass (for 0-18 dB of bass boost at 45 Hz)
  • preamp-level (RCA) inputs
  • preamp (RCA) outputs
  • 17-1/16"W x 2-3/8"H x 9-7/8"D
  • 2-year warranty

What's in the Box:

  • 4-Channel amplifier
  • 2 End caps (attached to amplifier)
  • 1 Remote Punch Bass control
  • 15' Remote Punch Bass cable
  • 1 Mounting bracket for Remote Punch Bass control
  • Two 3/8" self-tapping screws
  • 1 Self-adhesive Remote Punch Bass control decal
  • 1 Remote Punch Bass control knob
  • Four 3/4" self-tapping screws
  • One 1/8" Allen wrench
  • One 7/64" Allen wrench
  • One 3/32" Allen wrench
  • One 3/32" set screw (extra)
  • One 1/8" set screw (extra)
  • 1 Female quick-slide
  • Installation and Operation manual
  • Punch Verification Certificate
  • "Team RF" sticker

Hands-on research

Product Research

Inputs and Outputs Notes


Cast Aluminum Heatsink: The cast aluminum heatsink dissipates heat generated by the amplifier's circuitry. The inherent advantage of casting provides a 30% improvement of cooling over conventional extrusion heatsink designs.

Trans-ANA (TRANSconductance Active Nodal Amplifier): The trans-ANA is a circuit that allows the audio signal to pass through the amplifier at low voltage. The signal is directly level-shifted to the fixed high voltage rails via a pair of drive transistors. Signal linearity is assured by an active node formed  by the drive transistors at ultrasonic frequencies. The result is crystal clear sound quality.

MEHSA (Maximum Efficiency Heat Sink Application): MEHSA is a Rockford Fosgate proprietary process that increases the heat transfer from the MOSFET devices to the heatsink. The multi-layer insulated metal MEHSA substrate operates with minimal thermal resistance, spreading heat both downward and outward to quickly dissipate heat from each device across the heatsink. This allows greater power output, enhanced thermal stability, and maximum reliability.

TOPAZ (Tracking Operation Pre-Amplifier Zone): TOPAZ is a circuit that simply eliminates noise. The TOPAZ circuit allows the amplifier ground to move with the source unit ground, creating a "noise elimination" zone just before the preamp section. If there is any noise in the ground coming down the RCA line to the amplifier, TOPAZ amazingly tracks it down and eliminates it. The result is a 30-40dB improvement of the isolation between input ground and chassis ground. 

NOMAD (NOn-Multiplying Advanced Decision): NOMAD is an intelligent protection circuit that uses an analog computer to make real time decisions regarding temperature, voltage, current and impedance. The unit is protected without limiting sound output unless needed.

DSM (Discrete Surface Mount): In the manufacturing process high speed robotics are used to place 0% tolerance resistors and large board traces providing greater reliability and efficiency.

INDIGO (INDexed Ideal Gain Operation): INDIGO is a circuit that maximizes the output power of the amplifier by making better use of the power supplied by the car. As the battery voltage drops, INDIGO tightens up so the amplifier delivers the rated power, resulting in more consistent power coming from the amp.

Internal Crossovers: The front and rear channels have independent crossovers. Each crossover is a 24dB/octave Butterworth filter selectable for high-pass, all-pass (full range), or low-pass operation variable from 50Hz to 250Hz.

Remote Punch Bass: The Punch Bass helps correct for acoustical deficiencies in the listening environment by helping reproduce full-range sound without adding excessive boost. The Punch Bass control operates for the rear channels only, and has a narrow band adjustment centered at 45Hz, variable from 0dB to +18dB. Using the mounting bracket, the supplied Punch Bass wired remote can be mounted in any convenient location under your vehicle's dash. The wired remote is 2.317" wide, 1.523" tall and 2.035" deep. The included cable is 15' in length.

RCA Pass-Thru Jacks: The pass-thru jacks provide a convenient source for daisy-chaining an additional amplifier without running an extra set of RCA cables from the front of the vehicle to the amplifier location. The pass-thru jacks output a full-range signal.

Power LED: The LED power indicator is located under the RF Power center badge on top of the amplifier. The center badge illuminates when power is supplied to the amp.

Fuse Requirements: The amplifier requires one external 60-amp fuse (not supplied).

Inputs and Outputs Notes

Left Side of Amplifier:

  • Remote Punch Bass: RJ-14 jack for use with supplied Remote Punch Bass wired remote

Right Side of Amplifier:

  • B+ (Battery Power): set screw terminal that will accept up to 4-gauge power wire
  • GND (Ground): set screw terminal that will accept up to 4-gauge ground wire
  • Speaker Outputs: 4 pairs of set screw terminals that will accept up to 8-gauge speaker wire
  • REM (Remote Turn-on): 0.250" male quick-slide for remote turn-on lead (a female quick-slide is supplied)

Our Product Research Team

At Crutchfield, you'll get detailed, accurate information that's hard to find elsewhere. That's because we have our own in-house Product Research team. They verify what’s in the box, check the owner's manual, and record dimensions, features and specs. We stay on top of new products and technologies to help people make informed choices.

Staff Reviews

Product reviews from the Crutchfield Labs

What are the Labs?