DYI hack to control headphone switch

[aagaag]

I often need to switch my Sony STR-DN1060 receiver from headphones to speakers and vice versa. The receiver is hosted in a cabinet, and it would be practical to remote control the switch via a relay.

My understanding is that the insertion of any plug into the headphone jack causes the receiver to switch, suggesting that a mechanical switch is incorporated in the jack. I am considering a hack to redirect the switch to a pair of wires which I would then control through a potential-free relay.

I have found a circuit schematics that might be the right one. I deduce that shorting CL1185 with CL1187 will trick the system into believing that no headphones are attached, whereas interrupting them will switch the headphones on. Am I right? Or will I fry the receiver, the headphones, and possibly also my ears and anything between them?

 

[betwixt]

You have to bear in mind two factors:

1. headphones (at least normal dynamic ones) need only a small voltage to drive them compared to loudspeakers.
2. many amplifiers get unhappy if you run them without any load (no loudspeakers).

So some designs use headphone sockets that carry the signal to the loudspeaker through them and a mechanical contact isolates the speaker wire as a plug is inserted, generally the sound is then diverted to the headphone contact through a resistor to limit the volume, this is scenario 1 above.

Some, including yours if that is the correct schematic, will croak if the loudspeaker is disconnected (as you can't hear anything the first sign of trouble is smoke!). In that scenario, when the headphone is plugged in, the main amplifier has to be shut down to prevent damage. The other factor here is that some smaller amplifier has to take over just to drive the headphones.

Shorting those contacts will probably work but test it before turning the power on. Measure the resistance across CL1185 and CL1187 with and without headphone plugged in and make sure it really does work as the schematic suggests. Be sure the resistance goes from high to almost zero Ohms as the switch operates, there is a danger of measuring the headphone resistance which is usually only a few Ohms but still higher than a complete short. If the switch does indeed swap the output from speakers to headphones it should be safe to wire an external switch to do the same thing.

The alternative method is to take the signal from the speaker connections all the time and use a changeover switch to divert the signal externally from the unit. You need to use some power rated resistors to fool the amplifier into thinking the speakers are still connected and some dropper resistors to limit the headphone volume. I would not recommend this method if the amplifier regularly runs at high power (> 25W) as the switch contacts and resistor ratings will make it impractical to build.

Brian.

 

[aagaag]

Dear Brian
thank you for your very fast, detailed and helpful response, which is highly appreciated. When I insert a jack adapter, the loudspeakers disconnect even if no actual headphone is inserted, thereby indicating that the switch is indeed mechanical. I would imagine that this would be the best scenario to measure the resistance without running the risk of measuring the headphone's resistance.
Many thanks again!