Switchcraft Mono 12A jack

[Phend]

I have one and it will work as a simple mono jack.
But what are these NC 3 prong jacks commonly used for ?

[GGBB]

Often used for amp input jacks with the shunt connected to sleeve/ground so that when no plug is inserted, the input is grounded to eliminate hum/noise.

[PRR]

> what are these NC 3 prong jacks commonly used for ?

To connect something when a plug is NOT inserted.
Ground, another input, a mix network, an attenuator, etc.



Jackery and circuit stolen from Fender, just 'power' tube changed.

[Kevin Mitchell]

When you get more into modular systems these switching jacks become far more prevalent.
In a stompbox they're commonly used to ground the input when nothing is connected.
Similarly but different, stereo jacks are used to connect battery ground to circuit ground by shorting out the sleeve & ring via mono cable - keeping the battery from draining when no input jack is plugged in.

In a modular system (think FX in and outs, expression pedals and of course synths) they are used all over.
For example, my latest synth project uses a crap ton of these switching jacks. When nothing is connected they are routed to their default sources. Once I insert a jack that connection is broken and the cable's tip takes over.

The image you've provided is an example of a NC (normally closed) switching jack and is the popular choice. However I have had to use a NO jack for at least one pedal project but I can't recall which one it was. It certainly threw me for a loop at the time. I think that was also for some sort of power saving gymastics as well - with supporting circuitry.

[mozz]

And............used on the outputs of tube amps, so when there is no speaker hooked up, it shorts the output to ground, which is better than than leaving a open connection which can lead to blown output transformer or arcing tubes.

[andy-h-h]

In a modular system (think FX in and outs, expression pedals and of course synths) they are used all over.
For example, my latest synth project uses a crap ton of these switching jacks. When nothing is connected they are routed to their default sources. Once I insert a jack that connection is broken and the cable's tip takes over.

This is called Normalling on patch bays.  There's a few options to play with - if you google patch bay normalling, you'll see what I mean

[Kevin Mitchell]

This is called Normalling on patch bays.  There's a few options to play with - if you google patch bay normalling, you'll see what I mean

Yup. Upon the handful of definitions of normalization in audio electronics, this is one of them. When a signal is applied to the NC tip sw lug this is considered "normalized".

[andy-h-h]

This is called Normalling on patch bays.  There's a few options to play with - if you google patch bay normalling, you'll see what I mean

Yup. Upon the handful of definitions of normalization in audio electronics, this is one of them. When a signal is applied to the NC tip sw lug this is considered "normalized".

I thought you would be pretty across this if you've built synths - I'm yet to dabble in that rabbit hole

[amptramp]

In the early days of transistor radios, this type of jack was used to disconnect the speaker when the headphone was plugged in.  But even earlier, there were battery tube portable radios and this one from SDR shows an intelligent use of a shorting jack:



In this application, the outer shell of the jack carries the signal from the output of the first audio stage and the inner terminal carries the ground.  The shorting bar goes to the output tube filament.  When the crystal headphone is plugged in, the ground line to the output tube filament is disconnected, shutting the output tube off.  This allows headphone use to reduce the A and B battery power drain, since the output tube doesn't draw current from either supply with a headphone plugged in.  Only a crystal headphone works here since the 1S5 tube cannot supply much current.