Adding a stereo to a vehicle with a factory amp
Should you bypass the stock amplifier or use it?
In a Nutshell
Many of today's vehicles have an external amplifier connected to the factory stereo system. This amplifier 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 DIY car audio system, we came across an unexpected opportunity to showcase a question that our Advisors answer regularly here at Crutchfield:
"Should I keep the factory amp or should I bypass it?"
Many vehicles include an external amplifier as part of a factory audio system. In many cases, we'll have a wiring solution for a vehicle that either works with the factory amp or bypasses it. So, which way is better? We decided to compare both options and hear for ourselves.
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.
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 onto 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 didn'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.
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.
To use the bypass harness, Dave simply unplugged the cable from the factory amp and plugged it into the 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 EFX 9-wire Ultra Flex speaker wire bundle, to reach the output section of your factory amp.
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.
Dave ran the bypass harness from the dash cavity to the factory amp location, using the vehicle's trim to conceal the wiring bundle.
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 via phone, chat, or email to help.
Dave's new sound system puts a smile on his face.