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The Road Thunder RT4240 amp gives you MTX's rugged construction and dynamic power at a price that fits your budget. After undergoing stringent testing at the MTX factory, each amplifier receives its Certified Performance Certificate — a guarantee of reliability and performance. And with features like isolated preamp outputs, built-in crossovers, bass boost, and speaker-level inputs (with automatic turn-on), this American-built amp is an incredible value!

  • 2-, 3-, 4-channel, or Tri-Way output
  • 120 watts x 2 in bridged mode
  • 60 watts x 4 at 2 ohms
  • MOSFET power
  • preamp outputs
  • speaker-level inputs with Auto Turn-on — automatically turns on amplifier when it senses voltage from the radio (no remote lead needed)
  • adjustable bass boost (0-18 dB) centered at 40 Hz
  • defeatable 18 dB/octave crossover at 85 Hz, selectable for high-pass, low-pass, full-range (rear channels) or high-pass, full-range (front channels)
  • dimensions: 12-5/8"W x 2-1/8"H x 9-1/16"D
  • 1-year warranty

My system has plenty of power. Who needs an amp?

You might think you don't need an amp because you have a 400-watt system in your car, or because your new stereo puts out 50 watts a channel. But those are peak wattage ratings, which may boil down to 15 or 20 watts per channel, using the more realistic RMS rating. An amp sends your speakers the power they need to deliver your music with clarity and impact, whether you're using factory speakers or high-performance components. And low-frequency reproduction requires lots of power, so an amp is an absolute must if you're adding a sub to your system.

MTX Road Thunder RT4240 Reviews

Hands-on research

What's in the box?

Download owner's manual

  • Black MTX Road Thunder amplifier
  • Two 25-amp blade fuses in chassis
  • 2 Speaker-level input wiring harnesses (each 8" long)
  • Owner's Manual (English/French/Spanish/German)
  • Warranty Registration and Customer Survey card
  • Certified Performance Certificate
  • MTX self-adhesive sticker
  • Bridge Mode Application Impedance Requirement warning sticker
  • NOTE: Mounting hardware is not included!

Product Research

Inputs and Outputs


Color: The amplifier is black with an MTX logo on top. 

Power supply: The amplifier uses a patented PWM MOSFET switching power supply with class A 100% Discrete Driver Circuit Topology (Preamp input is class A).

RCA output: The gold plated Sum RCA output provides a line level summed output of the right front/right rear and left front/left rear input signals. It is typically used to drive an external subwoofer amp. This is a full range output and is not controlled by the unit's built in crossovers.

Bass boost: Continuously adjustable and defeatable bass enhancement circuitry (centered at 40Hz, rotary control for 0-18dB adjustment).

SmartEngage: If you use the speaker level input on this unit and tie the front L+ and L- speaker leads from a source unit with BTL outputs (most high powered radios) the amplifier will turn on whenever there radio is turned on, allowing for easy factory head unit integration.

The amplifier has noiseless turn on-turn off..


Gain controls: There are variable rotary front and rear gain controls.

Bass boost: The bass boost is centered at 40Hz and may be adjusted from 0-18dB.

Input select switch: Allows you to select 4 channel (when a head unit with four line level outputs is being used) or 2 channel (when a head unit with two line level outputs is being used).

Crossover frequency: The internal crossovers are set at 85 Hz.

Crossover select switches: The front channel switch allows you to select whether the front internal crossover functions as a full-range (no filtering) or high-pass crossover. The rear channel switch allows you to select whether the rear internal crossover functions as a full-range, low-pass, or high-pass crossover. Both crossovers have a slope of 18 dB.

Power LED: When the power is on the LED lights red.

Inputs and Outputs

There are gold plated left and right, front and rear RCA preamp inputs.

There are gold plated left and right sum RCA preamp outputs. This output is full range.

There are gold plated +12V, Remote and Ground screw terminals.

There are gold plated Left and right, positive and negative screw terminal speaker outputs . To bridge the amplifier you use the R(-) and L(+) for the rear and front channels.

The speaker level (high level) inputs are Molex connections used with the supplied wiring harnesses.


RMS Power measured at 12.5 Volts DC:

45 watts x 4 into a 4 Ohm load with less than 0.5% THD

60 watts x 4 into a 2 Ohm load with less than 1% THD

120 watts x 2 into a 4 Ohm load with less than 1% THD

Dynamic Power (IHF-202 Standard) measured at 14.4 Volts DC

55 watts x 4 into a 4 Ohm load

100 watts x 4 into a 2 Ohm load

200 watts x 2 into a 4 Ohm load

Note: RMS power is given ONLY for 12.5 volts, so that is what is listed in the Crutchfield database.

Mounting hardware is NOT INCLUDED. You will need four self-tapping screws to mount the unit, wire for power, ground and remote turn on.

The amplifier uses two 25 Amp ATC type fuses (blade)

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.