Pin 1: Multiple XLRs between two amplifiers - diyAudio
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Old 17th November 2014, 06:06 PM   #1
JFace is offline JFace  Canada
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Default Pin 1: Multiple XLRs between two amplifiers

I am designing a headphone amplifier for use of live sound in ear monitoring. A mixing board will send two balanced signals for stereo operation, and the headphone amplifier will receive these two XLR connectors. How should I connect the pin 1 of the two receptacles on the headphone amplifier?

I am working with the assumption that all shields are connected to the mixer chassis. I am also assuming that the mixer earth may not be the same as the earth on stage.
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Old 17th November 2014, 06:10 PM   #2
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Pin 1 of every XLR should connect to the chassis by the most direct route possible. Google "Pin 1 problem" for details.
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Old 17th November 2014, 06:13 PM   #3
JFace is offline JFace  Canada
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Quote:
Originally Posted by scopeboy View Post
Pin 1 of every XLR should connect to the chassis by the most direct route possible. Google "Pin 1 problem" for details.
I've read articles and posts on the matter. The simplification of this information seems to exclude multiple conductor scenarios.

This would create a chassis ground loop. Is this a problem?
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Old 17th November 2014, 06:31 PM   #4
JFace is offline JFace  Canada
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Looking at Rane's posted headphone amplifiers, they do connect pin 1 to chassis of all the XLR inputs. I suppose loops in shields don't cause any issues?
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Old 17th November 2014, 07:31 PM   #5
dmills is offline dmills  United Kingdom
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Nope, no issue at all.

The current flows in the chassis and not in the (internal) single ended reference (Unless you screw that up by connecting it to chassis at more then one point).

Now there is a proviso to all this, google "shield current induced noise" for the residual issue, which is seldom a problem in practise.

Also see Tony Walderons stuff on "designing interference free audio systems", worth the time.

Regards, Dan.
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Old 17th November 2014, 07:57 PM   #6
AndrewT is offline AndrewT  Scotland
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From my reading of all the pin 1 problem papers/reports:
Pin1 always goes to chassis at the socket entry.
Preferably OUTSIDE the Chassis, rather than a pigtail looped inside the Chassis.

Pin1 is NOT part of the Signal.

The two signal pins 2 & 3 are taken to the signal processing In/Out of the circuit.
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Old 17th November 2014, 08:20 PM   #7
dmills is offline dmills  United Kingdom
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And the internal single eneded reference for the electronics is connected to chassis at ONE point (to define the common mode level), it never leaves the box.

Regards, Dan.
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Old 17th November 2014, 08:59 PM   #8
JFace is offline JFace  Canada
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Great, makes sense. Typically would you make this one connection between audio and chassis grounds near the input, or is it mostly a trial and error type thing?

I use Oshpark to prototype my designs. They are great, but they take about a month to get the pcbs in my hands. I'd like to iron out as many details as possible before the testing stage.
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Old 17th November 2014, 09:00 PM   #9
forr is offline forr  France
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Quote:
Originally Posted by JFace View Post
I suppose loops in shields don't cause any issues?
Think of induced voltages in the signal path instead of worrying about all apparent ground loops which are often innocuous.
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Old 17th November 2014, 09:09 PM   #10
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Balanced line audio has no reference to ground. The input floats. Pin1 and the shield are there purely for safety.
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