I'm building an electronic musical instrument which I would like it to have a 3.5mm audio output socket, so it would work with any "regular" headphones, like a cellphone does.

For a long time, I've tried to googling the electrical standard for such a socket (having TRRS + CTIA connection) and couldn't find one.

  • How should I design my signals for the connection?
  • What are the voltage ranges?
  • What is the output impedance my circuit should show?
  • Is the analog signal being the signal being played? or the socket output signal contains some sort of modulation?

If you could please clarify how should I accessing the socket subject,

Thank you.


1 Answer 1


If the goal is to have phones only (not a headset with microphone), it only needs 3-pin 3.5mm TRS socket, so TRRS is not necessary. Sleeve is ground, Tip is left speaker and Ring is right speaker. Output impedance should be fairly low, as typical headphones have 16 to 32 ohms impedance. A speaker would be 8 typically, and there are higher impedance headphones up to 600 ohms. Voltage range is not much, 2 to 3 volts peak-to-peak should be unbearably loud already. There is no modulation, headphones are really just like speakers, the driver elements are directly connected to the connector.

  • \$\begingroup\$ USB 2.0 has a specification describing the data transfer protocol, electrical requirements, etc.. Is there such a document describing the standard/specification of TRS socket? \$\endgroup\$
    – Taru
    Aug 21, 2019 at 21:34
  • 1
    \$\begingroup\$ Not really, as the connector has been used for so many things, and even as audio output it can line level output or headphone output. And there are wildly different headphones that just have different sound levels when driven with same voltages. There are industry and safety norms that describe what is the acoustic power limit for the whole system, and since users can swap headphones easily, the voltage with unknown phones is limited to 150mV. \$\endgroup\$
    – Justme
    Aug 22, 2019 at 5:51
  • \$\begingroup\$ So when designing for a general headphones (like a smartphone designers do) should I aim for max voltage of 150mV and output impedance of 16/32 ohms? \$\endgroup\$
    – Taru
    Aug 22, 2019 at 7:17
  • \$\begingroup\$ Preferred output impedance is zero of course. It's the load that would be 16 ohms minimum. \$\endgroup\$
    – Justme
    Aug 22, 2019 at 7:53

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