Line Output Converter Diagram: How to Wire a Speaker-Level to RCA Converter
This is a free printable metra line output converter diagram: download the diagram as SVG or open it and print to paper or PDF.
A line output converter (LOC) takes speaker-level audio output from a factory head unit and converts it to a low-level RCA signal compatible with aftermarket amplifiers, without replacing the factory radio.
A line output converter (LOC) is a passive or active electronic device that accepts high-level speaker output (typically 2–40 V AC, depending on amplifier power and impedance) and attenuates it to a standard pre-amplifier line level (typically 150 mV to 2 V RMS) suitable for feeding an aftermarket external amplifier's RCA inputs.
Factory-fitted head units increasingly use integrated amplification, digital signal processing, or error-correction systems that make it impractical or undesirable to replace the OEM radio. A line output converter solves this by connecting across the existing speaker wires and deriving a usable line-level signal without interrupting the existing speaker circuit.
In the wiring diagram, the LOC accepts two pairs of inputs for stereo operation: left positive (+) and left negative (−), and right positive (+) and right negative (−). These connect directly across the speaker terminals — or across the existing speaker wires feeding the left and right speakers — with the OEM speakers remaining connected in parallel. The LOC presents a high input impedance, so it draws negligible additional current from the head unit amplifier.
On the output side, the LOC provides one or more pairs of RCA phono connectors (left and right channel, sometimes with separate front/rear outputs for four-channel amplifiers). These connect to the RCA inputs on the aftermarket amplifier via standard shielded RCA cables.
Many LOCs include a gain adjustment trimmer (typically a small potentiometer accessible via the device body) to set the output level relative to the head unit's maximum volume before distortion. Setting this correctly is critical: gain set too high causes audible clipping at the amplifier; too low results in insufficient signal level and a noisy floor.
Some higher-specification LOCs include a remote turn-on output that detects audio signal and generates a 12 V DC trigger to automatically switch on the amplifier — useful when the factory head unit does not provide a conventional amplifier remote wire.
All wiring is illustrative reference. Consult the specific LOC and amplifier documentation for your installation.
How to wire metra line output converter diagram
- Identify the speaker wires to tap at the head unit or speaker location Using the vehicle wiring diagram, identify the front left and right speaker wire pairs at either the head unit connector or at the speaker itself. Note polarity (positive and negative) for each channel. In most OEM systems, the negative speaker wire is not connected to chassis ground — it is a floating output from the head unit amplifier. Confirm this with a multimeter before connecting.
- Connect the LOC input wires to the speaker wires Connect the LOC's left input positive wire to the left speaker positive wire, and the left input negative wire to the left speaker negative wire. Repeat for the right channel. Use posi-tap connectors or solder-and-heat-shrink splices — avoid insulation displacement (scotchlok) connectors for permanent installations as they are prone to poor contact and corrosion. Leave the existing speakers connected.
- Route the RCA output cables to the amplifier Connect shielded RCA cables to the LOC output connectors. Route the RCA cables away from any DC power cables to minimise hum induction. RCA signal cables should ideally run along the opposite side of the vehicle interior to the main power cable. Secure cables with appropriate clips or tie-wraps.
- Connect the remote turn-on output (if available) If the LOC includes a signal-sensing remote output wire, connect it to the amplifier remote input terminal. Connect the LOC's 12 V supply wire (if active LOC) to an ignition-switched 12 V source, and the LOC ground wire to a clean chassis ground point. Verify the ignition-switched feed switches off with the ignition to prevent battery drain.
- Set the LOC gain trimmer Before completing the installation, set the LOC gain trimmer to its minimum position. Play test audio through the vehicle, turn the head unit volume to 75–80% of maximum, then gradually increase the LOC gain until the amplifier output reaches the desired level without audible distortion or clipping on the amplifier's clip indicator LED (if fitted).
- Verify system operation and check for noise With the engine running and the audio playing, listen for alternator whine (a pitch that increases with engine RPM) which indicates a ground loop. If present, check that the LOC ground, amplifier ground, and head unit ground are all connected to the same chassis ground point, and that RCA cables are fully routed away from power cables.
Specifications
| Typical LOC input voltage range | 2–40 V AC (speaker level) — verify against specific device datasheet |
|---|---|
| Standard RCA line-level output voltage | 150 mV to 2 V RMS (adjustable via gain trimmer on most units) |
| LOC input impedance (typical) | ≥ 1 kΩ (high impedance; presents negligible load to OEM amplifier) |
| OEM speaker output impedance (typical) | 4 Ω or 8 Ω nominal speaker load |
| Remote turn-on output (signal-sensing type) | 12 V DC switched output; typically 200–500 mA capacity (sufficient for amplifier remote input) |
| Frequency response (typical quality LOC) | 20 Hz – 20 kHz ± 1 dB |
Safety warnings
- Disconnect the vehicle battery negative terminal before beginning any wiring work to prevent short circuits and damage to vehicle electronics.
- Do not connect the LOC input to any circuit carrying mains (AC) voltage — the LOC is designed for low-voltage DC automotive speaker signals only.
- Ensure the LOC's 12 V supply (if active type) is protected by an inline fuse as close as possible to the power source.
- Route RCA signal cables away from the main power cable to prevent induction of electrical noise (alternator whine, ignition interference) into the audio signal path.
- Do not tap into safety-critical vehicle circuits (airbag, ABS, engine management) when identifying speaker wires — always identify audio system wiring specifically.
Tools needed
- Digital multimeter (for identifying speaker wire polarity and verifying ignition-switched supplies)
- Wire strippers sized for OEM speaker wire gauge
- Soldering iron and solder (for solder-splice connections, preferred over insulation displacement connectors)
- Heat gun (for heat-shrink tubing on splices)
- Trim removal tools (to access factory speaker wiring without damaging panels)
- RCA cable routing rods or cable fish tape (for running signal cables under carpet)
Common mistakes
- Connecting the LOC input wires to the wrong pair of speaker wires — for example, tapping a tweeter output that has a high-pass crossover applied in the OEM amplifier, resulting in a signal missing bass frequencies at the LOC output.
- Using unshielded wire or domestic audio cable instead of shielded RCA interconnect, which picks up alternator noise and ignition interference.
- Running RCA signal cables in the same loom as the main amplifier power cable — the switching current in the power cable induces noise into the signal cable.
- Setting the LOC gain too high so that the head unit's full volume range clips the LOC output before the head unit's own amplifier clips — resulting in distortion at moderate volume levels.
- Grounding the LOC to a different chassis point than the amplifier, creating a ground potential difference that appears as a hum loop in the audio signal.
Troubleshooting
- Alternator whine (pitch rises with engine RPM) audible through speakers
- Cause: Ground loop — potential difference between LOC ground, amplifier ground, and head unit ground, causing alternator ripple current to flow through the signal path Fix: Ensure all audio component grounds (LOC, amplifier, head unit chassis) connect to the same chassis ground point. Try lifting the shield at one end of the RCA cable (connect shield only at the amplifier end) to break the ground loop. Keep RCA cables as far from power cables as practical.
- Audio signal is present but very low level — amplifier barely drives the speakers
- Cause: LOC gain trimmer is set too low, or the LOC is connected to a speaker circuit that the OEM system has already attenuated (volume-dependent fader, parking sensor duck, etc.) Fix: Increase the LOC gain trimmer incrementally. If maximum gain is insufficient, the LOC input signal level is too low — verify you are tapping a speaker circuit at full OEM amplifier output. Check whether an OEM DSP is reducing the signal for reasons unrelated to volume setting.
- Only one audio channel works correctly — other channel is silent or distorted
- Cause: Incorrect polarity connection on one channel's LOC input, or a faulty LOC channel Fix: Verify the positive and negative connections for the non-working channel at the LOC input. Swap the left and right RCA outputs temporarily to confirm whether the fault follows the LOC output or the amplifier input.
Frequently asked questions
What is the difference between a passive and active line output converter?
A passive LOC uses only resistive or transformer-based attenuation to reduce the speaker-level signal to line level — it requires no power and introduces no gain. An active LOC includes a powered circuit that can buffer, filter, or even provide some signal processing. Active units generally offer lower noise and better control over output level, but require a 12 V supply connection.
Do I disconnect the existing speakers when wiring a line output converter?
No — in a standard LOC installation, the existing speakers remain connected and continue to work normally. The LOC connects in parallel across the speaker wires, presenting a high impedance that does not significantly load the head unit amplifier. Disconnecting the speakers is only necessary if you are also replacing them or if the amplifier drives the speakers directly.
Which speaker pair should I connect to the line output converter?
Choose a speaker pair that is always active when audio is playing and that does not have a crossover applied in the OEM system that would affect the full-range signal. Front door speakers are often the best choice. Avoid subwoofer outputs if you want a full-range signal. In some vehicles, the OEM system applies digital processing or time alignment per channel — check for audible phase or tonal differences between channels before committing.
How do I set the gain on a line output converter?
Turn the gain trimmer to minimum. Connect the amplifier and speakers. Play programme material at about 75–80% of the head unit's maximum volume (the level before audible distortion). Slowly increase the LOC gain trimmer until the amplifier output reaches the desired listening level or until you hear distortion — then back off slightly. This sets the LOC to operate in its linear range without clipping.
My aftermarket amplifier has a remote turn-on wire — do I need a LOC with a signal-sensing remote output?
If your factory head unit does not provide a switched 12 V remote output (most factory radios do not), you need either a signal-sensing LOC with a built-in remote output, or a separate signal-sensing relay module. The signal-sensing circuit detects audio signal presence and switches 12 V to the amplifier remote terminal, turning the amplifier on when audio plays and off when audio stops.
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