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I'm planning to use an amplifier that has both (distinct) speakers and headphones outputs and an input to select between them.

Such input switch the amplifier output on headphones if logic level is high and to speakers if logic level is low. It's not specified if this input has an internal pull-up or pull-down.

I would like to trigger such changeover through headphones jack insertion, but I'm a little bit confused by solution examples found (of course because I don't know how such kind of amplifiers are working).

This is one of the solution schematics I found.

Solution schematic

Image source: EE Times - Using comparators to detect accessories in portable audio applications by Arpit Mehta

In my case I need to reverse the detection logic, but my amplifier does not describe either the headphone output impedance, so I'm guessing where the current will flow or if headphones output level will be somehow affected by the Vcc connection.

Assuming the headphone output stage is always in high-impedance state and that Vcc is not affecting the output level, should this schematic work?

My solution

The yellow rectangle portion in the upper left corner is the amplifier I'm planning to use.

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While the circuit will work, it's really a workaround for a socket that doesn't have a switched contact on the sleeve terminal. It may use a whole additional IC if you haven't got a spare inverter handy in the design, it relies on the headphones having a a certain DC resistance and it applies a DC voltage to one side of the headphones, which might not always be a good thing.

A better option is to use a socket that does have a switched contact on the sleeve, like this one from https://uk.rs-online.com/web/p/jack-plugs-sockets/9131018:

RS PRO Jack Connector 3.5 mm PCB Mount Stereo Socket, 6Pole 5A

The circuit is then much simpler and only needs a pull-up resistor, which can probably be increased to 10K or more depending on your amplifier spec:

enter image description here

Depending on the actual amplifier you use, the output capacitors may not be needed - check the datasheet or application notes.

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  • \$\begingroup\$ Ok ok now I got this... just a clarification because on this schema the sleeve contact seems a NO but I think is NC ... right? From the datasheet it seems to be NC anyway... \$\endgroup\$
    – weirdgyn
    Oct 13, 2023 at 7:00
  • \$\begingroup\$ the problem is to have some more info about the amplifier (I asked the manufacturer and I'm waiting for answer)... I'm pretty sure output capacitors are needed because they are present in the typical application schematics on the datasheet. \$\endgroup\$
    – weirdgyn
    Oct 13, 2023 at 7:10
  • \$\begingroup\$ The contacts are NC - they open when you insert a jack. That's pretty normal for jack sockets. \$\endgroup\$
    – Finbarr
    Oct 13, 2023 at 9:33
  • \$\begingroup\$ yes I see altough there are not so much manufacturer with sleeve switch contacts... \$\endgroup\$
    – weirdgyn
    Oct 13, 2023 at 12:11

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