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Installing a car stereo in a vehicle with a factory amp

Should you bypass the stock amplifier or use it?

Factory amplifier — bypass it or keep it? That's the question we'll explore in this article. Hint: if you want the best sound from your new stereo or speakers, the extra work it takes to bypass the factory amp is totally worthwhile.

The factory stereo systems in many of today's vehicles include an external amplifier. This factory amp is designed to make cheap factory speakers sound better. But it doesn't do you any favors when you replace the stereo or those stock speakers.

  • Retaining the factory amp usually makes it easier to install a new stereo, and in some vehicles that's your only option. But your new stereo's sound is limited to what that amp can do.
  • Bypassing the factory amp takes a little more work because you have to run the bypass harness to the factory amp, which is often located in some other part of the car. It's usually worth the extra effort because you'll get better sound from your new stereo, even if you're keeping those factory speakers.

While planning an article about a car audio installation, we came across an unexpected opportunity to showcase a question that our Advisors answer regularly here at Crutchfield:

Should I bypass the factory amp or should I keep it?

In many cases, we'll have wiring solutions for a vehicle that either work with the factory amp or let you bypass the amp. Which way is better? We decided to compare both options and hear for ourselves.

JVC KW-R925BTS CD receiver in car

We chose the JVC KW-R925BTS CD receiver for the features Dave wanted, including preamp outputs that worked with the amp integration harness.

A willing car audio test subject

Dave, one of Crutchfield's editors, wanted to replace the factory sound system in his family's 2002 Toyota Highlander. A couple of us on the car writing team got together, researched Dave's vehicle, and recommended the JVC KW-R925BTS CD receiver and JVC speakers to meet his requirements for the new setup.

We noticed that the stock system in Dave's Highlander included a factory amplifier, and Crutchfield offers ways to either integrate or bypass the factory amp in this Toyota. We decided to use this opportunity to test our theory that it's always better to bypass the factory amp. We connected Dave's new stereo both ways to see what really sounded the best.

Setting the stage with new speakers

Dave changed out the speakers before installing the new stereo so they'd be ready for the test. The right pry tools and comprehensive Crutchfield MasterSheet™ instructions led him through the procedure, and once he had the first door speaker done, the others went smoothly.

Dave replaced the front speakers with JVC CS-DR1700C 6-3/4" component speakers up front, using a universal backstrap to make a bracket for each tweeter. JVC CD-SR1720 6-3/4" 2-way speakers mounted directly into the factory brackets in the rear doors.

Wiring on back of receiver

To use the amp integration adapter, Dave plugged the adapter's RCA plugs into the JVC receiver's preamp outputs.

Getting the new radio ready for the test

To install the JVC radio, Dave began by securing the Toyota-specific kit that Crutchfield offers to the radio chassis. He also ran his Bluetooth® microphone cable from the cavity to the dashboard. Once the radio was secured with a few screws, it was time to move on to the wiring.

Dave thought that wiring to the existing factory amp in his Highlander would be fine, since it still worked. The installation would've been a little easier, since he wouldn't have to run any cables to the amp location. Retaining the factory amp meant he could connect the new stereo to the plug in the dash cavity, where the old radio had been plugged in.

Here's where we chimed in, explaining that Dave could expect better sound by bypassing that factory amp and directly feeding the JVC receiver's power to his new speakers. Keeping the factory amp in play would negate many sound attributes his new stereo offers.

plugging in the amp

Easy access to the factory amp's plug enabled us to test both methods.

Switching between the bypass harness and the integration adapter

We had easy access to the wiring harness that connected to the factory amp in Dave's Highlander. So we helped him wire the radio in a way that safely let us switch between the amp integration and bypass setups to compare the sound.

How to bypass the factory amp

To bypass factory amp, Dave simply unplugged the cable from the amp and plugged it into the bypass harness's plug.

How did it sound?

To put things in perspective, Dave works primarily on the home audio side of our business, so he's been exposed to some of the best sounding home audio systems out there. Here are his thoughts on the two methods of connecting his new car stereo:

"At first, I thought I wanted to use the factory amp for more power and better sound. It also looked like an easier installation, since I didn't have to get to the amp. With the amp integration harness, the sound was better than what I was used to. It had a lot more detail than the factory radio and speakers did."

So, he heard some improvement in the sound when he kept the factory amplifier in the system. Even going with the easier installation, his music would benefit. But then we moved on to the second part of the experiment:

"There's that missing bass... Bypass it is!"

"However, when the bypass was connected, I exclaimed 'Ah, there's the missing bass!' Bypassing the factory amp returned the low end that had been missing on the first go-round. The middle and high frequencies were also much more natural without losing the detail. It was a huge difference. Bypass it is!"

Factory amps can also include factory frequency settings

It's not just about the power. The factory amp can also include a filter or crossover network for specific frequencies that may not sound optimal with a new system. By bypassing the factory amplifier and feeding the JVC receiver's power directly to the speakers, Dave also bypassed any filters in the factory amp, sending a pure, full-range signal to the speakers.

Occasionally, we may not have a specific bypass harness available for a vehicle. But if you don't mind splicing some wires, you can use a speaker-wire bundle, like the Crutchfield CSW9W-20 9-wire speaker cable, to reach the output section of your factory amp.

Running wire

There it is! The Toyota's factory JBL amp is mounted under the rear seats. It doesn't have to be removed to be bypassed, which saves some time.

Installing the bypass harness in the vehicle

Once Dave decided to stick with the improved sound from the bypass adapter, he had to route the adapter's wiring to the factory amp, which is located under the rear seat in the Highlander. He found this wasn't as hard as he initially thought.

He fished this wiring bundle from the radio cavity, under the glovebox, and behind the kick panel trim on the passenger's side of his vehicle. Dave removed some of the door threshold trim, which revealed very handy channels that accommodated his wiring nicely. He neatly hid all the wires under the trim and plugged this bundle into the connector that plugged into the factory amp.

running wire

Dave ran the bypass harness from the dash cavity to the factory amp location, using the vehicle's trim to conceal the wiring bundle.

Final thoughts

Dave did a final sound check, making sure everything was working properly before replacing all the trim pieces. The new stereo system makes his family driving duties much more fun. By bypassing the factory amplifier, the music sounds more detailed and clear than it did when he used the amplifier integration harness. Whenever there's one available, we recommend bypassing the factory amp to get better sound.

If you're considering a new stereo for your vehicle, keep this option in mind. Our OutfitMyCar™ tool will show you the connection options for your vehicle. And if you have any questions, we're available to help via phone or chat.

Dave in his car

Dave's new sound system puts a smile on his face.

  • Hector

    Posted on 12/1/2023

    What if you don't change the radio, can you still bypass the factory amp and add your own amp,s and replacement speakers

  • Mac

    Posted on 11/27/2023

    In my 1993 Toyota T100 SR5 which had there version of a premium sound system LOL. When I acquired the truck almost the first thing I did was replace the 4 stock speakers well, that was a joke. So, I got a kenwood head unit with blu-tooth from crutch, retained the factory wiring did some soldering splicing etc much better but still no real bass. Since it is not an extended cab, behind the seat was gonna be tight so I got 2 dual voice coil 10" subs and a cab wide box (not ported) from the local yack shop, then picked up a Crunch amp from the crutch with remote bass ran it mono so I simply picked up a wiring kit from crutch used line out hid all the wires etc. Fired that puppy up, tuned it and OMG. that's alot of SPL for a small cabin even the windshield bounces. It's funny cause it is a work truck and has dents dings scratches etc and I pull up next to a yahoo who thinks he has it going on, looks over at me and smiles then I crank it max out the bass the looks I get are funny.

  • kalpesh from houston

    Posted on 11/13/2023

    i have toyota sienna 2021 and i want to add on auxiliary on jbl fabric stereo how i can do i have my amp and dvd player but need to connect car speakers also with my amp so can u tell me how i can add aux o rca on sienna 2021

  • John from PHOENIX

    Posted on 10/29/2023

    I am looking for more bass in a BMW 428i. 2014 convertable. My car has an ok stereo, not the harmon kardon upgrade though. I got a set of the Harmon Carden woofers,tweeters and door speakers, and center dashspeaker still sound same, center speaker ,and tweeters are only thing that sound better. I want to add a sub amp and run 2 new 8's under the seat,or maybe a small box with 2 8's. Do you suggest a intergral adPtor of some sort to connect to my cars amp,or do i run a line level, RCA off of say the underseat woofers? Do they make an afaptor for that?? Please help John.

  • Jack from Charlotte

    Posted on 7/18/2023

    Always bypass the factory amp. It's going to give you a fuller and much better sound..

  • D from Fort Pierce

    Posted on 6/2/2023

    Bypass, new stereo has own Amp, factory radio 5w aftermarket is 200w unless your hooking up more speakers with better quality no need to add aftermarket amp, will just blow factory speakers. Hobby 4 fun.

  • Bryan from Florence

    Posted on 3/29/2023

    What part # is the amp bypass harness for 2002 Chevy suburban

  • Chuck Mann from Shelbyville

    Posted on 2/1/2023

    Do you remove the factory amp or do you have to have both?

  • F. Gainey from Browns Summit NC

    Posted on 1/16/2023

    I am glad that this article was posted. I have not purchased a replacement radio because I had concerns to replace the amp or not in my Nissan Z.

  • John from Marietta

    Posted on 11/19/2022

    But Dave changed the speakers. A Factory amp makes low impedance speakers sound good. So with higher impedance speakers installed of course it's gonna sound better using the head end power due to impedance matching.

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