USB-C Headphone Jack Wiring: Identifying M+, M–, R, L Positive and Negative Connections
User question
I have a type c USB that I removed from female headphone jack which two wires are positive and negative M+ R L M-
Artificial Intelligence Response
Direct answer to the question
- In the set of four conductors you listed
• M+ = Microphone signal / “positive”
• M– = Microphone return / common / “negative” (usually tied to ground)
• R = Right-channel audio signal (AC, referenced to ground)
• L = Left-channel audio signal (AC, referenced to ground)
- Therefore, the only true positive / negative pair is M+ (positive) and M– (negative) for the microphone circuit.
- R and L are neither permanently positive nor negative; they are alternating (audio) signals that swing above and below the common reference.
Detailed problem analysis
- Origin of the four wires
• You removed a short pigtail or PCB from a USB-C-to-3.5 mm (TTRRS) headset adapter.
• Such adapters rely on “Audio Accessory Mode” in USB-C. Inside the plug is either:
– A passive wiring harness that routes analog signals straight out of the phone/tablet that supports analog mode, or
– A tiny DAC/codec IC plus a microphone bias circuit if the host only provides digital audio.
- Mapping to TRRS tip-ring-ring-sleeve
The overwhelming majority of modern headsets follow the CTIA (Apple) standard:
Tip = Left, Ring1 = Right, Ring2 = Ground, Sleeve = Mic.
Internally that becomes:
L → Tip, R → Ring1, Common (GND) → Ring2 (often shared with M–), M+ → Sleeve.
Some low-cost adapters split the mic into two wires (M+, M–) to allow differential input on the codec, which improves noise immunity. If so, M– is still the reference conductor.
- “Positive” vs. “Negative” in audio lines
• Audio R and L are AC signals centred about ground; polarity matters only for left/right phase coherence, not in a DC “+ / –” sense.
• The microphone, however, is usually referenced to ground and is powered by a bias network (2 V – 3 V). Hence M+ (biased signal) and M– (return) can correctly be described as + and –.
Current information and trends
- USB-C Audio Accessory Mode is becoming less common on flagship phones; many now output only digital USB audio, forcing adapters to integrate a DAC and mic bias circuitry. In those adapters you will still find the same four analogue wires on the jack side.
- Newer codecs increasingly offer true differential microphone inputs (hence M+, M–) for better common-mode noise rejection.
Supporting explanations and details
- Why no 5 V? The 5 V power present on VBUS (A4/B4) is isolated from the audio path by the adapter’s circuitry; you cannot safely draw power from R/L/M+/M–.
- Colour coding (if any) is not standard—always verify with a multimeter:
Continuity from the USB-C plug to:
• A3/B3 → L
• A4/B4 → R
• SBU1 → M+
• Shield/A1/B1 → GND (often tied to M–)
Ethical and legal aspects
- DIY rewiring of USB-C cables can short VBUS to audio lines if done incorrectly, risking device damage.
- For commercial products, USB-IF compliance requires adherence to USB-C pin-mapping rules; altering them voids certification.
Practical guidelines
- Identify ground first: probe the shell or braided shield to find M– / common.
- Verify L and R with a signal generator or audio source; swap only if stereo image is reversed.
- When reconnecting a microphone element, respect polarity (M+ to mic capsule hot, M– to mic cold/shield).
- Insulate each splice with heat-shrink; finish with overall tubing for strain relief.
Possible disclaimers or additional notes
- Some legacy headsets follow the older OMTP layout where Mic and Ground are swapped; your cable labels (M+, M–) already tell you which is which, but be aware when mating with other hardware.
- Without the original adapter’s codec, plugging these four wires directly into a USB-C port will not give you working audio; the host must support analog pass-through.
Suggestions for further research
- USB-IF “USB Type-C™ Cable and Connector Specification, Rev 2.2” (particularly §4.9 Audio Accessory Mode).
- Qualcomm/Realtek codec datasheets for examples of differential microphone connection.
- Application note: “Designing Differential Electret Microphone Circuits” (Texas Instruments).
Brief summary
Only the microphone conductors form a DC-referenced pair: M+ is the mic’s hot (positive) line, M– is its return/ground (negative). R and L are AC audio signals referenced to the same ground; they are not strictly “positive” or “negative.” Always confirm with a continuity test before wiring, and remember that these wires carry analog audio—not 5 V power.
Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.
