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"ULTRA" Balanced signals/systems... (1 Viewer)

Clay Autery

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The following ideas came to me when I discovered the presence of a 4th contact on the Neutrik XLR connectors I used in the cables between my receiver to BFD to sub amp...
I created this as a separate thread from my BFD cable thread because... well, this is a totaly separate project and will be VERY involved. I need the eyes and brains from you guys with electronics (passives especially) to make sure I have this headed in the right direction...
I'm SERIOUSLY considering testing out approach #2 below:
The tech from Neutrik confirmed that the 4th contact in the X-series XLR connectors is indeed a ground pin, but did not/would not elaborate on its use. I however, have come up with TWO approaches to using it that I think MIGHT be something to look at for "ULTRA" balanced signals circuits/systems... Here they are:
BACKGROUND:
in the MX3 series... typically, you would have a cable with a 2/4 conductor twisted pair and a braided shield overall.
Generally, the wiring is as follows:
Pin 1 = shield/ground
Pin 2 = signal (non-inverted)
Pin 3 = signal (inverted)
Now, I've looked through my various amp and other docs, and the ACTUAL wiring depends on whether the circuit that the cable is completing is 1) balanced/unbalanced, and 2) whether the input and output devices have 3-pin mains or a floating chassis... Lets confine the discussion to balanced circuits ONLY.
For a balanced circuit, if one device is floating, then the shield is connected at both ends. If both, devices are grounded, then the shield is lifted at one end (to avoid possible ground loop) of the cable (it appears that in this case, it is a bit of a philosophical decision as to whether to use source output or destination input grounding).
It was my understanding that the shield/ground (Pin 1)was used for RFI/EMI rejection in a balanced circuit...
But if that 4th contact is ALSO used as a ground/shield, I was curious as to its specific applications....
Here are my thoughts on the possible uses for it:
APPROACH 1: deleted
APPROACH 2:
Alternatively, and I think a safer (and better) approach would be to (both devices still on same circuit):
a) Pins 1, 2, and 3 as normal using 3 conductor twisted pair (using Star-Quad would be better I think... using solid red for Pin 2, clear red, for Pin 3, and using BOTH white conductors on Pin 1, which would place a ground on each side of both signal conductors).
b) The AC mains connections at the IEC would need to be checked on both devices and modified if necessary. You would need to insure that the IEC ground pin was isolated from the chassis. Whatever connection it has to it from the PCBs in the device would be maintained, but then isolated from the chassis with a phenolic spacer or similar means.
c) You would need to examine the Pin 1 circuit from the chassis XLR connector to see where the ground is taken. If it goes directly to the PCB, fine. If it has a connection to the chassis, that connection must be broken. If it is connected to the chassis via the IEC ground pin to chassis connection, you would maintain the direct connection to the IEC ground pin, but the chassis connection would be broken via the isolation of the IEC ground pin.
d) You would need to examine the device PCBs et al. to break ANY and ALL connections to the chassis by using phenolic or other non-conductive isolators (e.g. PCB mounting posts, et al.) Thus, the device electronics get their ground ONLY by direct connection to the IEC ground pin, and the chassis is floating with respect to the AC mains.
e) FINALLY, and this I think is the trick... The overall shield is connected to the XLR connector housings at both ends of the cable via the housing ground pins. The chassis XLR connector housings are provided a direct connection (probably via mounting to the back panel) to the chassis. Then, a grounding lug would be mounted in the back panel of each device's chassis (I would put it adjacent to each IEC connector). Then, you would run dedicated ground wires from each of the chassis ground lugs to a dedicated EARTH ground as close to the electronics installation as possible. (I would install a heavy, copper, gounding bus bar on a panel in the nearest outside wall; run a very large solid copper conductor from the bus bar to a grounding rod driven min. 8ft into the ground; minimum 8" under the surface, and minimum 8ft from the foundation of the house. The goal here is to provide the best earth ground possible.... preferably less than 1 Ohm between the ground lug on the chassis and the ground rod... preferably MUCH less than 1 Ohm.
This alternative approach, while maintaining the mains configuration to get all voltage references from the AC system (preferably via an AC conditioner/regulator), provides a SYSTEM shield... not just in the external signal paths, but over the entire device assembly.... via the chassis'.
NOTE: Personally, I would also add conductors inside each device to physically connect each separate chassis panel in the device to insure that all panels have low resistance connections to the shield.... preventing paint and the like from insulating one or more panels (or increasing the resistance to ground).
OK.... I know this was a lot of stuff, but am I "off in left-field" with this approach?
It seems to me that on a cost-no-object system (or a DIYer with a lot of time, patience, and attention to detail), this could add that last little bit of signal protection to get best-case sound reproduction.
What do y'all think?
 

Clay Autery

Stunt Coordinator
Joined
Oct 23, 2000
Messages
158
OK, I did a quick sketch in pencil. I hope it helps make it a bit clearer...
Sketch of "ULTRA Balanced" system
The following pic shows a "normally" wired male XLR core and shows the 4th contact. In my approach, the reds would be split between Pins 2 and 3, the whites would go to Pin 1 (where the shield is in the pic), and the shield would go to the connector housing contact (at the top of the pic).
Pic showing XLR housing contact
 

Dan Pawlowski

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Joined
Aug 12, 2002
Messages
100
Clay
Nice doodling.
A lot confusing
Do you live in a Cell site??? What sources of noise are you trying to rid?
I suggest you gain a better understanding of balanced circuits and how and why they are used. Check out: Balance Line Driver :)
 

Clay Autery

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Joined
Oct 23, 2000
Messages
158
Kick a$$ web site... I'll have to go read it... BUT, you really must understand that I do NOT want to design balanced circuits... I was just postulating on the possible benefits of making the Pin 1 ground/shield a ground only and adding in a separate shield system...
Again... just brain storming... My system has a long way to go before EMI/RFI, etc. become a consideration again. (I do not by the way have any audible ground loop hum in my system... I fixed that already with a new common earth grounding system for the AC mains and all the various antennae, leads, etc. :)
 

Wayne A. Pflughaupt

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It seems to me that on a cost-no-object system (or a DIYer with a lot of time, patience, and attention to detail), this could add that last little bit of signal protection to get best-case sound reproduction.
Certainly a worthwhile objective, Clay, but you already get that for “free” when you use a balanced signal path.

Regards,
Wayne A. Pflughaupt
 

Clay Autery

Stunt Coordinator
Joined
Oct 23, 2000
Messages
158
Wayne,
Thanks for the response! :)
Perhaps you are correct, and I am wasting my time... I don't know... yet. I am spending a lot of time studying the projects and articles over at ESP now trying to get a deeper understanding of balanced inputs/outputs and several other possibly relevent concepts...
I just can't help but think that even WITH balanced circuits (which BTW, not all balanced circuit designs are equal... some don't do much... some actually hurt the signal...) an additional ISOLATED shield system over the entire system would offer additioanl benefits... If nothing else, it would serve as a filter to reduce the noise et al that the balancing circuitry, et al has to deal with...
I've got a lot of time to think about this though, as I have a lot of construction-type projects in line ahead of this idea. ;)
 

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