Using input jack as on/off switch

[Seven64]

I am building a MXR headphone amp for my pedalboard, and wanted to incorporate something i see in the boss multi effects.  I am running a splitter from the last pedal in my effects chain into both the output of the pedalboard and into this headphone amp.  I am going to have a jack on the surface of my board, and wanted the headphone amp to turn on, and it to cut off the output to the amp when headphones are plugged in.  an led indicator would also be mounted next to the headphone jack to let me know when the headphone amp is on.

is this possible, and if so how?  i have no idea where to even start looking for something like this.

[Gurner]

Yes it is possible....you need a tip/ring/sleeve socket (TRS)....and use the extra lug (the ring) to 'complete' your circuit's ground rail ..... because I'm lazy, I'll use someone else's diagram....

http://....org/viewtopic.php?f=19&t=2007&start=140  (about 3 or 4 posts down - I did post the gif here, but then saw a 'permission not given' statement on it!)


(ignore the bit about jack....unless you intend using a dc connector too ....also where it says 'switch' - that's your signal 'hot')

[dthurstan]

If your using headphones you'll want a 3 pole jack not a 2 pole one.

[Seven64]

it is going to be for headphones with a 1/4" jack.  in switching jacks, doesnt it seperate the contact points upon putting the plug in?  how would that switch it on?

[Gurner]

I assumed a mono headphone amp (ie with the guitar being mono).....you'd need to use a mono jack plug.

The left/right headphone wires are soldered together & then are both soldered to the jack 'tip'

The jack 'ring' lug goes to your circuit ground

The jack sleeve goes to your battery -ve (& your signal ground)

Now when the cable jack is plugged in to the socket, the sleeve is effectively shorted to the ring    which connects your battery -ve to your circuit ground.

[petemoore]

  Mono jack = long sleeve that connects the sleeve/ring lugs of a stereo jack [if it's the right kind of stereo jack].
   ..generally, this causes a pop, however on a small amplifier it will probably just be a small-ush pop that isn't too terrible or cause turn-on-pulse damage to the headphone motors.

[earthtonesaudio]

Switching stereo 1/4" jacks like this are not that uncommon.

From there you can do whatever switching you want. 
Personally I would keep the headphone amp always-on for simplicity and only switch the line output.

The jack I linked to says it contains an isolated DPDT, which is pretty cool.  That would be more than enough to break the line output connection and light an LED. 

[GGBB]

I assumed a mono headphone amp (ie with the guitar being mono).....you'd need to use a mono jack plug.

If you use a mono jack with a stereo headphone plug, you'll only get sound in one ear regardless of whether the tip has both channels or not.  You need to use a stereo jack with the tip and ring connected to each other in order to deliver one channel of sound to both channels of the headphones.  And the usual pedal technique for powering up a pedal when the input is inserted won't work here because that relies on a mono plug being inserted into a stereo jack which grounds the ring.  So you need something fancy like what earthtonesaudio suggested.

[Gurner]

If you use a mono jack with a stereo headphone plug, you'll only get sound in one ear regardless of whether the tip has both channels or not.

you obviously missed my next line....

The left/right headphone wires are soldered together & then are both soldered to the jack 'tip'

you can readily use a mono jack plug with stereo headphones, just do as I outlined above (the driving amp will see half the load, but it's seldom a problem in practise)

[GGBB]

If you use a mono jack with a stereo headphone plug, you'll only get sound in one ear regardless of whether the tip has both channels or not.

you obviously missed my next line....

The left/right headphone wires are soldered together & then are both soldered to the jack 'tip'

you can readily use a mono jack plug with stereo headphones, just do as I outlined above (the driving amp will see half the load, but it's seldom a problem in practise)

Didn't miss anything - what are you talking about?  A jack or a plug?  I'm not aware of anything called a "jack plug" - mono or stereo.  There are jacks and there are plugs. The OP is asking about what king of jack he/she needs.  A mono plug simply won't do without modifying the headphone plug or using a stereo jack to mono plug adapter - neither of which is at all necessary or practical and probably not desirable.

[Gurner]

Didn't miss anything - what are you talking about?  A jack or a plug?  I'm not aware of anything called a "jack plug" - mono or stereo.  There are jacks and there are plugs.

type "jack plug" into google & your very first hit is a wikipedia for a TRS plug! If you've never heard a jack plug being referred to as an erhm a jack plug, well.....  [heads off to look for an icon that shows shrugging shoulders, fails but settles for this   ,to be fair the article suggests it's a Brit thing - news to me -  so I consider the usage as going some way to settling the score for those overseas calling a water tap a faucet.]

Re Seven64's request .... if I'm reading it right, he wants a circuit to be 'switched on' when a headphone connector is pushed into the corresponding socket. Being a guitar rig, I assumed mono...in which case it's quite easy to use a mono headphone plug, to 'make' the ground connection between supply 0V & circuit 0v...the socket needs to be a TRS though (see how quickly I can adapt!) ....the supply ground connects to the TRS 'sleeve' lug ,the follow on circuit ground connects to the 'ring' lug  .... a mono headphone plug gets plugged in  ...the circuit 'supply' is made.....but you'd need to mod the headphone jack plug accordingly, which brings me onto this....

A mono plug simply won't do without modifying the headphone plug or using a stereo jack to mono plug adapter

Modding, changing, customizing,  hey on occasion even using adapters ...it's the lifeblood of what we all do here!!

[GGBB]

Sorry - my bad.  Now that I understand that in the UK "jack plug" means plug (we just call them plugs here in Canada and we call "jack sockets" just jacks or sockets - I've honestly never heard the term "jack plug" before), I realize that you have correctly instructed the OP how to rewire his headphones so that he can plug them into a stereo jack socket and have that turn on his headphone amp.  But you left out the part about disconnecting his amp output.  Instead, I would recommend he leave his headphones alone and buy the kind of jack mentioned earlier which will do both the headphone amp turn on and amp output disconnection, and also work with any pair of headphones.