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JL Audio RD400/4

4-channel car amplifier — 75 watts RMS x 4

Item # 13698621

JL Audio's RD400/4 4-channel amplifier will power each your car's front and rear speakers with 75 watts RMS, so...

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JL Audio's RD400/4 4-channel amplifier will power each your car's front and rear speakers with 75 watts RMS, so...

Item # 13698621

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About the JL Audio RD400/4

Buck Pomerantz

Power your music

JL Audio's RD400/4 4-channel amplifier will power each your car's front and rear speakers with 75 watts RMS, so you'll hear and feel your music clearly over the whistle of the wind and rumble of the road. For driving bass, you can switch the RD400/4 to 2-channel mode and run a couple of subwoofer with up to 200 watts RMS each.

Power your music

JL Audio's RD400/4 4-channel amplifier will power each your car's front and rear speakers with 75 watts RMS, so you'll hear and feel your music clearly over the whistle of the wind and rumble of the road. For driving bass, you can switch the RD400/4 to 2-channel mode and run a couple of subwoofer with up to 200 watts RMS each.

Low current demand, low distortion, and integrates well

The RD400/4 plays your music with true fidelity and impressive impact, without straining your car's electrical system, thanks to JL Audio's proprietary NexD™ amplifier technology. "Differential-balanced" inputs keep the incoming signals clear of outside noise, and allow you to connect this amp to just about any radio. The speaker-level inputs feature two auto turn-on modes so you can readily incorporate this amp into a factory system — no need for RCA connections or a remote turn-on lead for this setup. 

LED clip lights and other cool features

High- and low-pass filters help you dial in the perfect tone for your speakers or subs. Onboard LED clip indicators let you set the amp gain quickly and accurately with the use of a test tone. A removable cover hides the amp's controls, resulting in a handsome appearance and a tamper-resistant installation. Cool blue LED accent lighting adds some charm to this amp's rugged good looks.

JL Audio RD400/4

JL Audio RD400/4 controls and accent lighting


Product highlights:

  • Featured in our article: Best 4-channel car amplifiers for 2023
  • 4-channel car amplifier
  • 75 watts RMS x 4 at 4 ohms (100 watts RMS x 4 at 2 ohms)
  • 200 watts RMS x 2 bridged at 4 ohms (4-ohm stable in bridged mode)
  • Class D amplifier design with NextD™ switching amp technology
  • differential-balanced inputs help eliminate noise
  • variable low- and high-pass filters (50-500 Hz, 12 dB/octave)
  • LED clip lights help set gain
Other Features:
  • blue LED accent lighting
  • preamp inputs and outputs
  • speaker-level inputs (speaker wire to RCA adapter required)
  • 4-gauge power and ground leads and a 40-amp fuse recommended — wiring and hardware not included with amplifier
  • no onboard fuses — when installing multiple amps in a system, an additional 40-amp in-line fuse between the distribution block and the amplifier is recommended
  • 9-13/16"W x 2-1/8"H x 7"D
  • warranty: 2 years
  • MFR # 98621

What's in the box:

  • 4-Channel amplifier
  • 2.5mm Allen wrench
  • Self-adhesive JL Audio badge
  • Owner's manual

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Customer reviews for JL Audio RD400/4

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Product Research



RD Series: The JL Audio RD series amplifiers feature big audio performance from a small footprint. With exclusive NexD switching technologies, the RD amps run cooler than most other amplifiers. Because the RD amps employ smaller cast alloy heat sinks, they can be installed in smaller spaces. RD amplifiers feature a black powder-coat finish with a brushed aluminum cover for the controls.

Four-channel Class-D Amplifier: The RD400/4 is a Class-D 4-channel amplifier, designed specifically for full-range operation, but can be used to power subwoofers. The amplifier is stable down to 2 ohms in stereo, or bridged to a 4 ohm load.

Amplifier Rating: The RD400/4 has the following power output:

  • 75 Watts RMS x 4 channels at 4 ohms
  • 100 Watts RMS x 4 channels at 2 ohms
  • 200 Watts RMS x 2 channels at 4 ohms bridged

Differential-Balanced Inputs: The RD400/4 features differential-balanced input architecture to effectively control noise and ensure compatibility with a wide variety of source units, including many OEM systems. Without resorting to line output converters, you can directly connect most OEM source units to the amplifier without creating any noise or compatibility issues. Unlike conventional single-ended inputs, the differential-balanced design does not connect one side of the audio signal to chassis ground. This eliminates any voltage sensitivity between one input terminal and ground and greatly reduces the incidence of alternator whine in a typical car audio installation.

Note: You will need a Speaker Wire to RCA adapter (sold separately) for each pair of channels to connect this amplifier to your head unit via speaker level.

Protection Circuitry With Status Indicator Lights: The amp features a bi-color LED light strip on the top panel of the amplifier to indicate the amplifier's operating status:

  • Temporary Red: When the amplifier is powering up, the light strip turns red for 3 seconds and audio output is muted.
  • Constant Blue: The LEDs light blue to indicate that the amplifier is turned on and operating normally.
  • Constant Red: The LEDs light constant red to indicate that the amplifier has exceeded its safe operating temperature, putting the amplifier into a self-protection mode, which mutes the power output of the amplifier. The red LEDs will shut off and the amplifier will return to normal, full-power operating mode when the temperature drops back to a safe level.
  • Momentary Red: The light strip will light red for 1/2 second and the output will be muted to indicate that an over-current condition has occurred. Over-current condition can be caused by a speaker impedance lower than the optimum load impedance range for the amplifier or a short-circuit in the speaker wiring.
  • LEDs Off: The only condition that will shut down an undamaged RD400/4 completely is if battery voltage or remote turn-on voltage drops below 10 Volts. The amplifier will turn back on when voltage climbs back above 11 Volts.

Cooling System: The outer shell of the JL Audio amplifier is designed to remove heat from the amplifier circuitry. For optimum cooling performance, this outer shell should be exposed to as large a volume of air as possible.  Enclosing the amplifier in a small, poorly ventilated chamber can lead to excessive heat build-up and degraded performance.


Control Panel Cover: The RD400/4 features a brushed aluminum Control Panel Security Cover which protects and hides the amplifier's controls. When installed, the cover ensures that the amplifier's settings are not accidentally changed while creating a clean aesthetic look for the amplifier and your installation. The control panel security cover is pre-installed at the factory and must be temporarily removed to access the amplifier's controls. The security cover screws can be removed with the supplied 2.5mm Allen wrench.

LED Clipping Indicators: The amplifier is equipped with onboard LED clipping indicators which let you quickly and accurately set the amp's input sensitivity without special equipment. The simple, 7-step procedure using a Test CD and a screwdriver properly sets the input sensitivity to its optimum level.

Flexible Input Section and Active Crossover Network: The RD400/4 amplifier provides a flexible input section and a comprehensive crossover network to fine tune your system effectively and accurately. This 4-channel amplifier is divided into two separate pairs of channels, each with their own crossover:

  • Filter: The front and rear channels are equipped with a 12 dB/octave Butterworth filter which can be configured as a low-pass (LP) or high-pass (HP) filter, or turned off. You can set the Filter Mode via a three-way switch.
  • Filter Frequency: The filter frequency can be selected using the rotary control. There are markings at 50Hz, 60Hz, 80Hz, 120Hz, 200Hz, and 500Hz. You'll need a small screwdriver (not included) to set the frequency.

Turn-on Options: The Turn-on Mode Switch lets you choose the method for turning the amplifier on and off:

  • +12V Remote Turn-on Lead: The preferred method for turning the amplifier on and off is by running a turn-on lead from the source unit and connecting it to the Remote terminal. When +12V is present at the terminal, the amplifier will turn on. When the +12V is turned off, the amplifier will shut off. The set screw connector on the Remote terminal will accept 18 AWG to 12 AWG wire.
  • DC Offset-Sensing: The amplifier will turn on and off when the presence of a very small DC signal (or offset) is detected on the amp's RCA left channel input. The amplifier will turn off when this DC Offset is no longer present. This method is designed for high-level (speaker-level) input signals, not for low-level (line-level) signals.
  • Signal-Sensing: The amplifier will turn on and off by detecting the presence of a full-range audio signal at the left-channel input. The amp will turn off approximately 30 seconds after the audio signal is no longer detected. This method is designed for use with high-level (speaker-level) input signals and not for low-level (preamp level) signals.
Note: When the DC Offset or Signal Sensing turn-on methods are being used, the amp's Remote terminal can be used as a remote turn-on output for other amplifiers or accessories in the system.

Input Controls: You can select the Input Voltage and Input Mode via two way switches:

  • Input Voltage: The amp will accept a wide range of signal input voltages, from 200mV to 8V. The range is split into two sub-ranges: "Low" for preamp level signals from 200mV to 2V, and "High" for high-output preamp level signals and speaker level signals from 800mV to 8V.
  • Input Mode: The Input Mode switch lets you send four discrete channels into the amplifier, or use only two channels to feed all four amplifier channels. You can set the Input Mode to 4 CH or 2 CH.


Power/Ground/Turn-On Terminals: This JL Audio RD amplifier features set screw clamp terminals for the power, ground, and remote turn-on connections. A 2.5mm hex-wrench is supplied for tightening or loosening the set screws.

  • Power and Ground: The amplifier's set screw clamp Power and Ground connections are designed to accept up to 4 AWG power wire, as recommended by the manufacturer.
  • Remote Turn-On: The amplifier's set screw clamp turn-on lead will accept 18 to 12 AWG wire.
Note: The Amplifier is not equipped with a fuse. An external 40-amp AFS, AGU, or MAXI fuse must be installed on the power wire. If a wiring kit is used which includes a fuse with a value other than 40 amps, the original should be replaced with a 40-amp fuse.

Speaker Output Terminals: The JL Audio RD400/4 features eight set screw clamp speaker terminals for easy connection. Markings showing the proper way to bridge the amplifier are provided on the speaker terminals. The terminals will accept 16 to 8 AWG speaker wire.

Preamp Inputs: The amplifier features two sets of differential-balanced RCA jack inputs, each with an Input Voltage switch and rotary Input Sensitivity control. The amplifier's input voltage range is 200mV-2V (LOW) and 800mV-8V (HIGH).

Preamp Outputs: The RD400/4 incorporates a buffered pass-through preamp output section, so that additional amplifiers can be easily added to the system. The preamp output signal is not affected by the amp's crossover networks. The pass-through preamp output is a summed stereo signal (Channels 1&3 / Channels 2&4) to another amplifier.

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Customer Q&A

10 questions already asked

Does the Amp come with the test CD to set up the speaker level inputs?
mike r  May 01, 2021
3 answers
Yes, mine did. I love this amp.
scott  May 14, 2021
No it does not come with the cd but you can google it and play it through your phone for tuning.
david  May 14, 2021
No CD is included.
james  May 13, 2021
Do I need rca cords to head unit? My previous amp got stolen, I have just speaker wire and power wire at my amp location.
edwin  Dec 29, 2020
1 answer
Yes you will. You can wire the speakers to the amp but than you need the RCA cables to go from the amp to the head unit.
raymond  Dec 30, 2020
Will one channel handle 2 sets of 4 ohm 3 way components?
mike  Feb 25, 2019
5 answers
Don't know
mark  Feb 28, 2019
Sorry Mike, I haven't had a chance to install my amp been very busy and we have had a lot of snow and rainy weather,you can check with JL tech department and they should be able to answer your questions.
kirkwood a  Feb 28, 2019
Yes; you will need to wire them in series, but the RD400/4 is 2-ohm stable.
chris  Feb 28, 2019
Most definitely
james  Feb 28, 2019
Probably not. It will degrade the sound. You'd be better off with a second amp
anne  Feb 26, 2019
I bought this amplifier and connected the inputs via rca out from the head unit.The speakers output channels are connected by bare wires that run to the speakers. i do not need any rca adapter in my application, or do i need any rca adapter. i have my input voltage set to low. Have I set it up correctly?
ornob  Feb 14, 2019
3 answers
This amp is designed to except wire input so it should be fine
anthony  Feb 15, 2019
If your radio is stock . Yoiu may need it. Aftermarket radio yoi do not need.
rafael  Feb 15, 2019
Did you hook up the blue remote lead?
torger  Feb 14, 2019
Sub would have to be a 4 ohm load to bridge off of channels 3 and 4.
tyler  Mar 12, 2017
I wouldn't suggest running channels 3&4 at a 2ohm load, I believe the manual says it's only capable of 4ohm .
jonathan l  Mar 11, 2017
What speakers would you suggest with this Amp for good Bass without a Sub ?
prakasam  Jan 26, 2017
1 answer
Thanks for your question. For information on selecting the best speakers to match the amp I would recommend reaching out to an Advisor by phone (1-800-324-9695) or online chat. They will be able to help determine the differences and help you make the best choice for your system.
kenneth  Jan 27, 2017 (Staff)
Does this Amp JL Audio RD400 come with fuse in built ?
prakasam  Jan 24, 2017
3 answers
No it does not. I used a distribution block to fuse the sub amp and this amp separately. This one needs a 40 amp fuse
erik  Jan 26, 2017
It does not have an onboard fuse, but requires at least a 40 amp fuse within 18" of the battery, I ran a 0 gauge to a fuse block and fused the amp there as well, it might be a bit overkill, but better safe than sorry. I use this amp to run my front stage which is the focal 165 Krx3 Series and it runs them great. This is a new line of amps from JL and I actually like them better than the hd series. Hope that helps you out. Keep it Loud!
jonathan l  Jan 26, 2017
No it does not. You will have to fuse your power lead at the battery or install a proper rated fuse at your distribution block if you have one.
tyler  Jan 26, 2017
How big is the amp?
d  Nov 03, 2016
2 answers
easily fits under 2018 toyota tacoma front seat with room to spare.
scott  Aug 19, 2020
This JL RD400/4 amp is unbelievably small, sleek, and incognito. Definitely smaller than the competition with the same and or similar specs which is why I chose this amp....
damon  Nov 04, 2016

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