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grounding inputs with switching jacks
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monkeyxx
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7th September 2011
Old 7th September 2011
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grounding inputs with switching jacks

Hi,

I've noticed a lot of designs and schematics call for a switching jack to short the tip (of a 1/4" unbalanced signal) to ground when no plug is inserted. What is accomplished by this?

I have two practical concerns/applications, an API-style preamp DI with a 1/4" input which I currently have un-switched

and an old Fender solid state guitar amplifier that seems to give me a lot of trouble with loud pops when plugging and unplugging things... It is supposed to be switched to ground, but I installed a non-switching plain jane jack that looks a lot more sturdy than the old shorting one, which was broken

I am trying to advance my electronics know-how and DIY and design skills...thanks
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7th September 2011
Old 7th September 2011
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The shorting jacks terminate the unused inputs which can help minimize interaction between multiple inputs, reduce noise, perhaps switch in components/circuits in more advanced applications.

Regarding the API type pre it really depends on the application. The switching jack in some pres might enable/disable the mic pre section. If your pre works fine the most likely benefit to the switching jack would be lower noise when using the mic inputs, but again depends on the actual circuit and implementation. You could build a 1/4" shorting jack and see if it makes an audible difference.

Regarding the amp a shorting jack may help, but if it only has one input just turn down the volume when plugging and unplugging things.
monkeyxx
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7th September 2011
Old 7th September 2011
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hmm I'm wondering why it's suggested in a schematic that already has a MIC/DI switch that disconnects the DI when in mic mode..

here's an example
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7th September 2011
Old 7th September 2011
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In general it is good engineering practice to terminate (short) any unused inputs, helps prevent unwanted funny business. A lot of designers will just do this out of habit, it's a good habit to get into.

With this particular circuit most likely will make no difference. Though there is the off chance if you don't the DI could act as an antenna and leak RF onto your circuit ground. So if you are building it from scratch might as well include it as a preventive measure, since you already built it would just say leave as is, or maybe replace if/when convenient, but don't lose sleep over it. As I mentioned you could always try inserting a shorted 1/4" and see if you can hear or measure any difference.

One curious thing in the schematic is why xlr pin 1 is tied to circuit ground and not chassis ground?
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7th September 2011
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thanks for the tips, appreciate it

that's an interesting observation. I don't know much about ground issues yet. what kind of negative effect would you get from grounding an xlr input to the circuit ground?
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7th September 2011
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Essentially you want the XLR pin 1 to connect directly to the chassis with the shortest possible signal path. This way the cable shield and the chassis enclosure form one big conductive box that will keep the RF "crud" out.

Here's some good reading Grounding and Shielding Audio Devices

If you google pin 1 problem you will find a lot of good papers on the subject.
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8th September 2011
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Quote:
Originally Posted by johnnyc View Post
In general it is good engineering practice to terminate (short) any unused inputs, helps prevent unwanted funny business. A lot of designers will just do this out of habit, it's a good habit to get into.
Agreed.

I terminate 1/4" inputs all my designs unless there's a good reason not to do it. Most are terminated to gnd... in some, the high & low inputs are shorted.
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monkeyxx
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8th September 2011
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Quote:
Originally Posted by johnnyc View Post
With this particular circuit most likely will make no difference. Though there is the off chance if you don't the DI could act as an antenna and leak RF onto your circuit ground.
what would the difference be if all three contacts were shorted to ground instead of just the ring and sleeve?

actually wouldn't connecting the tip and the following circuitry to the ground make the antenna scenario more likely? vs just leaving it floating
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9th September 2011
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Quote:
Originally Posted by monkeyxx View Post
what would the difference be if all three contacts were shorted to ground instead of just the ring and sleeve?

actually wouldn't connecting the tip and the following circuitry to the ground make the antenna scenario more likely? vs just leaving it floating
Not quite sure what you mean with three contacts ring and sleeve. Are we still talking about the 1/4" DI?
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9th September 2011
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Quote:
Originally Posted by johnnyc View Post
Not quite sure what you mean with three contacts ring and sleeve. Are we still talking about the 1/4" DI?
sorry, I was getting confused with a different schematic with a TRS Jack on a similar circuit. Yes I am talking about the 1/4" DI, I'll rephrase it

what would the difference be if both 1/4" jack contacts were shorted to ground instead of just the ring?

actually wouldn't connecting the tip and the following circuitry to the ground make the antenna scenario more likely? vs just leaving it floating
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9th September 2011
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Quote:
Originally Posted by monkeyxx View Post
what would the difference be if both 1/4" jack contacts were shorted to ground instead of just the ring?

actually wouldn't connecting the tip and the following circuitry to the ground make the antenna scenario more likely? vs just leaving it floating
No wouldn't make it more likely. The whole point of grounding the tip is to minimize any undesireables. In practical terms for this particular circuit probably will make little to no difference though.
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