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SE Out to Balanced In: Hybrid Grounding?

P48

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#1
Hello everyone,

is anyone familiar with the "hybrid grounding" method where a disc capacitor is used to filter out RF noise from the common ground path on XLR connections? There is also a suggestion to solder a low resistance (say 100 Ohm) resistor in parallel for enhanced noise reduction.

commongroundfilter.jpg


Source: https://cdn.shopify.com/s/files/1/0247/3799/files/preventing_hum_and_rfi.pdf
See also: https://www.ranecommercial.com/kb_article.php?article=2107 (search for "hybrid shield termination")

So far I have only seen this schematic applied to the cable shield of fully balanced circuits where the twisted pair is used for the transmission of the hot and cold signal. Now, let's say we have an unbalanced source like the RCA out of a HiFi DAC and want to feed it into a balanced XLR circuit ...

unbal_out_to_bal_in.jpg


Source: https://www.ranecommercial.com/kb_article.php?article=2107

Here you can see the unbalanced out (RCA) to balanced in (XLR) termination recommended by RANE. Note that the shield is connected only at the receiver, leading to pin 1 of the XLR connector. The common ground from the transmitter is sent through one of the twisted pair conductors to pin 3.

If we now apply the "hybrid" schematic illustrated above to this configuration the RF interfering with the cable shield is getting filtered by the capacitor and attenuated by the resistor. So far so good. But what about pin 3 receiving the common ground of the transmitter ... ?

With the termination recommended by RANE (configuration No. 17) the differential input can maintain a certain amount of CMR by substracting the ground signal (pin 3) from the useful signal (pin 2). In case we manipulate the impedance at pin 3 by adding a (say 100 Ohm) resistor the "impedance balance" of the input will break, hence corrupt the CMR. So, no use here. But what if we would manipulate the capacitance of the signal path by adding a 100 pF disc cap (or even 10 nF or 100 nF as suggested by RANE)? Could this actually improve RF rejection without adding further noise? And what if we would add this to pin 2, as well?

Do you see any other simple way to enhance EMI and lower the effects of ground loops in this configuration without adding components like a transformer?

I found the NC3MXX-EMC by Neutrik which utilizes a "ferrite bead with 24 Ohm at 1 MHz between pin 1 and the cable screen". But I doubt that this connecter will be compatible with unbalanced sources as illustrated above.

Any thoughts and suggestions on this topic?
Is the method desribed above really worth it?

I would appreciate your contribution.

Regards
P48
 
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Speedskater

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#2
For a RCA output to an XLR input the above RANE (configuration No. 17) is great just the way it is.
The hybrid connection is only for long XLR to XLR balanced interconnect cables.
 
OP
P48

P48

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Thread Starter #3
Thanks for your evaluation! :)

Will the twisted pair do actually any good in this semi-balanced setup with an unbalanced source?
I'm just asking because I have some different types of cables at hand.
 

Speedskater

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#4
Heck yes! The major problem (by far) with RCA to RCA interconnect systems (and this is more important with long cables) is common impedance coupling noise. The RANE (configuration No. 17) eliminates this problem. There are no chassis to chassis leakage currents with this system.
 
OP
P48

P48

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Thread Starter #5
Sorry, I should have defined my question more precisely.
I wanted to know if the twisting of the conductors itself is of substantial relevance for the noise rejection in this "semi-balanced" configuration above. I was asking because I thought I had a piece of cable with straightly routed (non-twisted) pairs. But I just checked that, it is actually twisted. The winding does not appear to be too strong, though. I will compare that to my other mic cables later.
 

Speedskater

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#6
While all multi-conductor cables should be twisted (interconnect, data, speaker & power), it's more of a good engineering practice thing. Only rarely will there be interference problems with cables, but then it's too late.
 

somebodyelse

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#7
I have hazy memories of having to do the maths to calculate the difference between a parallel pair and a twisted pair, but I can't find a ready reference. I can't remember if we had to do starquad too. All I can find now is general descriptions, not specifics.
 

Speedskater

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#8
The reason for twisting is to reduce interference pickup. Not much to calculate. Twisting might hold the conductors closer together which increase the capacitance and reduce the inductance. But it will also make the conductors a little longer so you will have to recalculate.
 
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