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Old 24th March 2012, 09:31 AM   #1
Nisbeth is offline Nisbeth  Denmark
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Default Grounding in balanced passive pre?

This may be a stupid question, sorry. I need a small balanced passive volume control for test purposes, but what is the proper way to connect the XLR "grounds" in this setup, i.e. a quad Alps pot and XLR in/out?

Any help appreciated!

/U.
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Old 24th March 2012, 09:52 AM   #2
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All the pots probably have one terminal connected to the ground?
So you would need to use the XLR ground - that is not really what is ment to be. Actually that is just a shield.
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Old 24th March 2012, 10:15 AM   #3
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No, pin1 of the XLR is really a ground - to maintain some degree of restraint over the common mode voltage. The shell is the shield.

With a balanced signal and with the two pots on the one channel also balanced there will be no net current returning to pin1. But of course no pot is actually that good in terms of matching so pin1 comes to the rescue to remedy the slight imbalance in practice.
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Old 24th March 2012, 10:23 AM   #4
Nisbeth is offline Nisbeth  Denmark
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It is one pot (quad Alps) for both channels actually. Does that make a difference?

/U.
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Old 24th March 2012, 10:39 AM   #5
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Quote:
Originally Posted by abraxalito View Post
No, pin1 of the XLR is really a ground - to maintain some degree of restraint over the common mode voltage. The shell is the shield.
Really what I ment was that the ground is there just as an electric potential link (connect equipments grounds, shields, cases, power supply capacitors protection). Is not ment to carry/return SIGNAL. It probably can, but is not supposed to.
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Originally Posted by Nisbeth View Post
It is one pot (quad Alps) for both channels actually. Does that make a difference?
It is a quad. Therfore four potentiometers. Each has a connection to the ground that is not supposed to be for signal return.
That's the problem with the ballanced equipment volume adjustment.

Last edited by SoNic_real_one; 24th March 2012 at 10:45 AM.
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Old 24th March 2012, 11:44 AM   #6
sek is offline sek  Germany
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There is more than one way to employ a quad potentiometer for balanced volume control. I think it would be easier to recommend an approach as soon as the applied schematic was clear.

I've drawn one for you. I assume that your question is regarding the wire with a question mark?

Two things are to be determined:
wether the connection is to be made at all,
wether it is only a ground or also a shield connection.

One thing is for sure, though: using pin 1 for one function while using the shield connector for another (and not somehow connecting those two) is not a viable option.

Cheers,
Sebastian.
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Old 24th March 2012, 02:16 PM   #7
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Quote:
Originally Posted by SoNic_real_one View Post
Really what I ment was that the ground is there just as an electric potential link (connect equipments grounds, shields, cases, power supply capacitors protection). Is not ment to carry/return SIGNAL. It probably can, but is not supposed to.
Right, I got ya and I agree. If the pot is truly balanced (and the source is a true balanced signal) then it won't (to a first order).
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Old 24th March 2012, 02:33 PM   #8
AndrewT is offline AndrewT  Scotland
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Can pots achieve sufficiently accurate balance of impedances to maintain the rejection ratios of the balanced impedance connection?

Or, must we move to precision switched attenuators to maintain those required rejection ratios?
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Old 24th March 2012, 02:43 PM   #9
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I think if we want to be concerned about unbalancing the ground then we either need to
  • remove the ground connection totally and accept output unbalancing, or
    use the pot just as a variable resistor with series pads (and accept a lower than 0dB minimum attenuation), or
    go active, or finally
    use stepped attenuators with precision resistors as you suggest Andrew
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Old 24th March 2012, 04:29 PM   #10
Nisbeth is offline Nisbeth  Denmark
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Quote:
Originally Posted by sek View Post
There is more than one way to employ a quad potentiometer for balanced volume control. I think it would be easier to recommend an approach as soon as the applied schematic was clear.

I've drawn one for you. I assume that your question is regarding the wire with a question mark?

Two things are to be determined:
wether the connection is to be made at all,
wether it is only a ground or also a shield connection.

One thing is for sure, though: using pin 1 for one function while using the shield connector for another (and not somehow connecting those two) is not a viable option.

Cheers,
Sebastian.
Thanks Sebastian, that's exactly what I mean. The question is mainly regarding the screen connection (pin 1 on the XLR) although of course the "chassis" connection of the XLRs is also important.

/U.
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