I've researched this for a while now and can't find any answers for the life of me.. How can I add a headphone jack to an audio amp with speakers that disables the speakers when headphones are plugged in? There are only two possible explanations I can think of: there would have to be some sort of physical switch that cuts the speaker lines when headphones are plugged in, or something would have to be coded in a chip to disable the speakers. Can someone tell me how it's done?
Electrical – Switching Headphone Jack
amplifierheadphonesspeakers
Related Solutions
A single FET won't do the job properly because the signal is AC, and the FET has a body diode that conducts when voltage on the Drain is negative with respect to the Source. To fix this you need two FETs, connected back-to-back.
Another problem is that to keep the FET fully turned on the Gate-Source voltage must be higher than its threshold voltage. Since the amplifier output swings (almost) from rail to rail, the Gate drive voltage must be a few volts higher than Vcc. If your circuit is powered from a higher voltage that is regulated down to 3.3~5V then you may be able to tap into the unregulated supply, otherwise you might need even more parts to generate the required Gate voltage.
Assuming you have a sufficiently high voltage available, the circuit could look something like this:-
The LM4863 is a dual amp with headphone functionality. Its datasheet shows exactly how to connect speakers and a headphone jack to implement the application you describe.
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Best Answer
On the mechanical option for disconnecting speakers...
This is as old as the hills...probably 1960's. (An interweb search for '3.5mm socket changeover buy' leads you to some.)
1 is GND 2 is AmpL and normally connected to 4 SpkrL 3 is AmpR and normally connected to 5 SpkrR
When you plug the headphones in, 2/3 are sprung away from 4/5 to disconnect the speakers and are now connected to the headphones. Unfortunately, all of the speaker power is now being thumped into the headphones. If the headphones have a different/higher impedance to normal speakers, as they certainly do these days, it sounds trebley.
On the electronic option...
Here's an example circuit, showing one way of doing it. When the headphones are plugged in, the resistors pull the detect voltage to high and constant. This signals to the amplifier circuit to deliver a signal suitable for headphones.