Adding a Wired Headphone Socket to a Sofa
This project is part of my larger build thread here. This thread was created to make it easier to find information regarding this project and is one of several upcoming dedicated project threads to include extra information and bonus photos that are not included in the main build thread.
Like many people I sometimes enjoy wearing headphones instead of using my main speakers, especially when listening to music. As an audiophile I don't much care for wireless headphones. I want to feel like I'm getting the best quality audio that I can and there is no better way than a wired connection. It avoids using batteries or relying on wireless technology that could be hindering the audio quality.
I have a pair of Audio-Technica ATH-M50 studio monitor headphones. These are absolutely amazing sounding headphones, especially for music. I decided I want a wired headphone socket built into my new sofa. This will allow me to store the headphones in the console of the sofa and switch to them without the need to get-up and run the cable across the room to the AV receiver.
Sounds simple enough right? The problem is like most devices the AVR automatically switches to headphones when they're plugged into the headphone port. This is a mechanical switching operation by the headphone jack itself. Some newer devices will detect the impedance of the headphones before switching-over but my AVR (SONY STR-DG820) does not. This means a headphone extension cable would switch the AVR to headphone mode even without headphones on the end.
I took apart my AVR to figure-out exactly how the headphone jack operates. I determined it uses a switched headphone jack with integral "normalling" contacts. The type used in my case is 2x switching stereo (4th diagram below).
This was linked in a normally-closed arrangement to a relay on the AVRs main PCB. This switch is opened when the headphones are inserted into the jack. Excellent! This means it's a simple modification to add an external headphone socket and retain the automatic operation by connecting them in the same manor.
I did not want to render the front headphone socket on the AVR unusable, so I decided to add a switch onto the back of the AVR to swap between front & extension headphone mode. Both ports are connected simultaneously in parallel, but only one of them can actuate the automatic switching when a plug is inserted. This also means I can manually swap between headphones and main speakers without the need to actually unplug them from the sofa; if I so desire.
It is possible to connect multiple switching headphone jacks without a switch, however due to the normally closed nature of the switching the circuit must remain intact for it to work correctly. I want to be able to unplug the switched headphone extension cable from the AVR. Since this would break the circuit a switch was needed to bypass it.
I used 5-pin MIDI style plugs & sockets to form the new extension cable to the sofa headphone socket. Strictly speaking we only need 4-pin: left, right and ground for the stereo audio plus 1 extra pin for the normalling switch. 5-pin MIDI is just the cheapest option so that's what I used.
This AVR doesn't really have a whole lot of connections on the back so there's plenty of space to add new ones. I created a dish shape with masking tape to catch the metal swarf and prevent it from falling into the AVR, drilled-out for both the 5-pin MIDI socket and a rotary-action switch I had left-over from my university bench power supply project. I chose a rotary switch because it seems like the least-likely to be accidentally switched when pushed back into the rack-cabinet.
The schematic sketch below shows the existing switched headphone jack (left), the connection for the normalling cable back to the AVR main PCB (bottom middle), the new headphone jack (right) and the switch in-between.
I cut the normalling cable from the main PCB and soldered it to the new switch instead. Both the existing and new headphone sockets normalling connections are connected to this switch. The audio connections are simply connected together in a Y-splitter configuration.
With the rotary switch in the first position (below); the front headphone socket works as normal. When the switch is moved to the other position; the extension headphone socket takes-over switching. In the event that no headphones are plugged into the extension socket, or if the extension cable is not plugged into the AVR, the main speakers will be used instead.
Since I need an extra wire for the normalling switch, I had to order some multi-channel audio cable to create the extension cable between the AVR and the sofa.
The new headphone socket to be installed into the sofa also needs to have normalling contacts. The T-638JS is a 9-pin 6.35mm panel mount socket, with two separate changeover switching contacts. We only need one normally-closed switch but these were priced the same as the others so I figured why not; I may want the other contacts for something in the future. The T-637S would also work for this project.
I used an off-cut of 40 x 3mm mild-steel flat bar to make a mounting bracket for the new headphone socket to be installed into the sofa, plus another to mount the MIDI socket.
Before I could put-together the connectors for the sofa I had to figure-out where to put them. With the sofa turned over I discovered the base for the console storage compartment is removable with just 4 screws, so that makes things easier.
The MIDI socket can go anywhere really. It doesn't matter if it's upside down because I'm using locking MIDI plugs so they can't fall out or come loose.
The headphone socket needs to be mounted at a right angle to avoid crap falling into it, so this means reaching down into the console and drilling through into the console storage compartment.
The storage compartment sides are made out of 1mm plywood of some type, far too thin to mount the headphone jack to directly. This is where the steel bracket comes in. My impact driver with a Dewalt right-angle drill attachment was able to just fit into the tight space to mount the bracket. Although I did have to put it on one side, not that it matters really.
With the bracket mounted I was able to use it as a guide to drill through into the console storage compartment. The ply was so thin I managed to drill it by just turning the drill bit with my hand.