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Old 24th February 2008, 08:39 AM   #1
schodge is offline schodge  United States
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Question XLR Shorting Plugs

Most of what I've found on the web regarding creating XLR shorting plugs (to try and reduce noise picked up through unused inputs - worth a shot) shorts only hot to ground, and not negative as well. Is there a reason for this?

With my RCA shorting plugs, I "shorted" using a 1k resistor - partly because I was worried they might get unintentionally stuck into an output and fry an output stage, and partly for a reason that I thought was quite good but I can't currently remember. Given that the first is impossible for XLRs, is there a good reason to use resistors instead of straight shorts?

Thanks,

Shayne Hodge
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Old 24th February 2008, 12:00 PM   #2
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XLR shorting plugs are usually used when a RCA is wired is parallel with one side of the XLR input. The shorting plug would be inserted into the unused side of the XLR when the RCA is in use. An XLR iput on it's own should inject very little noise into the system as XLR uses a balanced system with an inherently very high CMRR. Most any noise that is picked up should be common to both the +ve and -ve line in the XLR and so cancelled before it can really get too far into the system and cause problems.
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Old 25th February 2008, 08:13 AM   #3
schodge is offline schodge  United States
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Thanks, that was so obvious in retrospect I have not a clue as to what I was thinking...

Shayne Hodge
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Old 22nd May 2016, 08:28 AM   #4
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Originally Posted by AudioFreak View Post
XLR shorting plugs are usually used when a RCA is wired is parallel with one side of the XLR input. The shorting plug would be inserted into the unused side of the XLR when the RCA is in use.
An XLR iput on it's own should inject very little noise into the system as XLR uses a balanced system with an inherently very high CMRR. Most any noise that is picked up should be common to both the +ve and -ve line in the XLR and so cancelled before it can really get too far into the system and cause problems
Hi and sorry to take up again this old discussion.
But i am doing some noise measurement on usb interfaces and i would like to be sure to measure only noise generated by the interfaces.
So the idea of shorting the mic inputs.
From the thread above i understand that is not necessary ?

Moreover, in the case the shorting is recommendable, i read somewhere that is better to short the 1 and 3 pin to ground using small value resistors (like 100-200 ohm)
I did not quite understand why these resistors are needed.
I am doing now the tests with the mic ins open.
But i would like to have it shorted to be sure that they not capture disturbs around.
This noise measurements are very important to me because i am looking for a very silent interface.
I have noticed already that many interfaces have quite noisy mic preamps when gain goes up.
I am looking for a really silent interface.
If all this is silly tell me without problem.
Any advice would be very welcome and appreciated.
Thanks a lot indeed.
Kind regards, gino

Last edited by ginetto61; 22nd May 2016 at 08:42 AM.
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Old 22nd May 2016, 10:09 AM   #5
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The idea of a balanced input is exactly that; it is balanced. If each pin is the same size and length, which they are, the noise (if any) will be present on both +ve and -ve inputs, (pins 2 & 3) and as the inputs are balanced the noise will cancel.
If however you find that by connecting pin 2 to pin 1 stops any noise, the input is not balanced.
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Old 22nd May 2016, 10:20 AM   #6
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Originally Posted by JonSnell Electronic View Post
The idea of a balanced input is exactly that; it is balanced. If each pin is the same size and length, which they are, the noise (if any) will be present on both +ve and -ve inputs, (pins 2 & 3) and as the inputs are balanced the noise will cancel.
If however you find that by connecting pin 2 to pin 1 stops any noise, the input is not balanced
Hi and thanks a lot indeed for the very kind and valuable advice.
Then it could be of some value to use them in ordet to check if the inputs are really balanced of what. Very very interesting. I think i will do it. I am curious. I am in the process of evaluating some usb interfaces and noise is indeed a very important aspect. I like very low noise.
Issue closed.
Thanks a lot again, gino
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Old 22nd May 2016, 10:30 PM   #7
Enzo is offline Enzo  United States
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The main noise (and I am agreeing with the others above about balanced) source would be on the balanced line somehow. if you are testing, measure noise with unterminated input, then short pins 2 and 3 together - that shorts the balanced line. If you see a reduction in noise, then you might do something about it. But I am suspecting it won't matter.
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Old 23rd May 2016, 06:48 AM   #8
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The main noise (and I am agreeing with the others above about balanced) source would be on the balanced line somehow.
if you are testing, measure noise with unterminated input, then short pins 2 and 3 together - that shorts the balanced line.
Hi and thanks a lot for the valuable reply.
You mean connect 2 with 3 and leave the ground input open ?

Quote:
If you see a reduction in noise, then you might do something about it. But I am suspecting it won't matter.
Thanks a lot i will do that.
I think that a lot can be learned from these noise measurements.
Actually with THD measurements should provide a good overall measure of the quality of an interface.
For instance i have started measuring a usb powered interface.
Replacing the power from the usb bus with a 2A Samsung charger (i have a special cable to do this) has no impact on the noise of the unit.
So i ordered a new 10A usb charger ... just to play a little
Thanks a lot again, gino
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Old 23rd May 2016, 09:40 AM   #9
DF96 is offline DF96  England
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Originally Posted by ginetto61
You mean connect 2 with 3 and leave the ground input open ?
Ground is not part of the input for a balanced system. It is already connected to ground. That is why it is called ground.
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Old 23rd May 2016, 10:11 AM   #10
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Ground is not part of the input for a balanced system. It is already connected to ground.
That is why it is called ground
Hi and thanks for the very helpful reply.
In my mind, maybe wrongly, shorting an input meant to connect it to the ground. So shorting a XLR meant to connect both + and - to the ground.
But if connecting - and + together is a more telling test i would do that for sure. I am open to good options.
Thanks a lot again, gino
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