Key Takeaways
- A line output converter (LOC) turns the factory speaker signal into an RCA preamp signal, so you can add an amp and sub without replacing the head unit.
- Wiring order: factory speaker wires into the LOC speaker inputs, RCA out to the amp, remote turn-on lead to the amp's remote terminal. The diagram below shows it.
- Most LOCs self-trigger by sensing audio or DC offset on the speaker wires, so you usually do not need a separate switched wire for the amp.
- Use an active LOC for high-power factory amplifiers; a passive unit is fine for low to moderate stock power.
- Set the LOC output just below clipping, then set the amp gain. Too much LOC output sends clipped signal the amp will pass to your speakers.
A line output converter, or LOC, takes the high-level speaker signal from a factory radio and converts it to a low-level RCA preamp signal an aftermarket amplifier can use. It is the standard way to add an amp and subwoofer to a factory stereo that has no RCA outputs, while keeping the stock head unit. You tap the speaker wires, run them into the LOC, take RCA out to the amp, and connect a remote turn-on lead so the amp powers up with the radio.
This guide covers the wiring, where to tap the signal, passive versus active units, and how to set the gain so you do not feed your amp a clipped signal. Once the amp is connected, our guide to bridging a 2-channel amp covers wiring it to a sub, and our best car amplifier guide helps you pick the amp itself.
What does a line output converter do?
It converts a speaker-level signal into a preamp-level signal. A factory radio without RCA jacks sends its audio out as a high-voltage speaker signal meant to drive speakers directly. An aftermarket amplifier wants a low-voltage RCA signal at its inputs. The LOC sits between them, stepping the speaker signal down to the roughly 0.5 to 5 volt range an amp's RCA inputs expect, so you can add an amp and subwoofer without touching the head unit.
The wiring is a straight signal chain: factory speaker output, into the LOC, RCA out to the amplifier, amplifier out to the sub. The only extra wire is the remote turn-on lead that tells the amp to switch on with the radio. The diagram below shows the full path.
How do you wire a line output converter?
Disconnect the battery ground first, then work the signal chain from the factory side out. The table below maps every connection. The only step that varies by vehicle is where you tap the speaker signal, which the next section covers.
| Connection | From | To |
|---|---|---|
| Speaker level in | Factory L/R speaker wires (+ and −) | LOC speaker-level inputs |
| Signal out | LOC RCA outputs | Amplifier RCA inputs |
| Remote turn-on | LOC remote lead (auto-sense) | Amplifier remote terminal |
| Amp power | Battery positive (fused) | Amplifier +12V terminal |
| Amp ground | Amplifier ground terminal | Bare chassis metal within 18 in |
| Output | Amplifier speaker output | Subwoofer or speakers |
Where do you tap the speaker signal?
Tap a full-range speaker output, usually a front door speaker or the wires behind the head unit, matching left and right and keeping polarity correct. Front channels are the safest source because they are least likely to carry factory bass roll-off or rear-fader processing. Avoid tapping a tweeter wire, which is high-pass filtered and gives the LOC no bass to work with.
Some factory amplified systems complicate this. A few platforms, several Chrysler and GM systems among them, put out a signal the LOC will not see correctly until the amp sees a speaker load, and some run factory equalization on every channel. On those vehicles a quality active LOC or a vehicle-specific interface harness is the clean answer, because it reads the signal the way the factory amp expects. If your factory radio is amplified and high-powered, plan on an active unit from the start.
Passive or active LOC: which do you need?
Match the LOC to your factory power. A passive converter uses a transformer or resistor network, needs no power or ground, and handles low to moderate factory output for the price of a cheap part. An active converter has its own power and ground and a buffer stage, which keeps the signal clean from a high-power factory amp and usually adds adjustable output and reliable auto turn-on. If your stock system is amplified and loud, the active unit is worth it.
| Factor | Passive LOC | Active LOC |
|---|---|---|
| Power needed | None | +12V and ground |
| Best for | Low to moderate factory power | High-power factory amps |
| Auto turn-on | Sometimes | Usually, more reliable |
| Output adjust | Often fixed | Adjustable level |
| Noise immunity | Good | Better, buffered |
How do you set the gain without clipping?
Set the factory volume to about three-quarters of maximum, the level where a stock radio usually starts to distort on its own. Turn the LOC output up until you just hear it break up, then back it off slightly. That sets the LOC to hand the amp the cleanest signal the factory deck can produce. Only then set the amplifier gain to match the amp's rated output.
The order matters because a LOC turned up too high passes clipped signal straight through. The amp cannot tell clean from clipped; it amplifies whatever it receives and sends it to your sub, which is how voice coils cook on factory-integrated systems. Keep the LOC conservative and let the amp gain do the level matching. For a full walkthrough of setting amp gains by ear and with a meter, see our amplifier guide.
Common line output converter mistakes
Three errors cause most LOC install problems.
Running RCA next to the power wire
A low-level RCA signal next to a power cable picks up alternator whine. Run the RCA and remote leads down one side of the vehicle and the power wire down the other. This single habit prevents most LOC noise complaints.
Tapping a filtered or processed channel
Pulling signal from a tweeter, a rear channel with fade processing, or a factory-EQ'd output gives the amp an incomplete signal. Tap a front full-range speaker, and on a processed factory system use an active LOC or an integration harness built for that vehicle.
Cranking the LOC output to get more volume
More LOC output is not more clean power; past a point it is just clipping. Set it below distortion and make up level at the amp gain. If you still want more output after that, the fix is a bigger amp, not a hotter LOC.
Frequently Asked Questions
What does a line output converter do?
Where do you connect a line output converter?
Do I need a remote wire with a line output converter?
What is the difference between a passive and active LOC?
How do you set the gain on a line output converter?
Can I add a subwoofer to a factory stereo without replacing the radio?
Where to go next
With the LOC feeding signal, wire the amp to your sub with our guide to bridging a 2-channel amp, pick the amp itself in the best car amplifier guide, and if you want to fix factory EQ rather than just tap around it, the car audio DSP guide covers the cleaner integration path.
If your factory system fights you, and some amplified platforms do, we can spec the right converter or integration harness for your vehicle. Send your year, make, and model through our contact us page and we will point you to the clean way in.
About the Author
Scott Welch is a Multi Time IASCA National and MECA World Sound Quality Champion, an active SQ judge since 2019, and the owner of Audio Intensity in Tullahoma, Tennessee. He cuts every Proline X enclosure on the shop's CNCs and tunes every customer system before it leaves. Audio Intensity is the original US importer for Goldhorn DSP and an authorized dealer for Prodigy, Crescendo, Image Dynamics, Wavtech, Tru Technology, and more.
