Balanced XLR from pre-amp to Unbalanced RCA on Sub

[snike3]

Ok, let me start by saying I've been doing a lot of searches and talking with some friends that build guitar amps for some time now and haven't come up with a solid answer.

 

I'm getting a pre-amp (probably the Marantz AV8802A) that has balanced XLR and unbalanced RCA output for the subwoofers. One of my subs will be located fairly far from the pre-amp (I've measured and it will be about 50-60 feet in cable), and after testing my new amps with my Klipsch speakers I realized I have a fairly electrically noisy environment. Given the noisy environment and fairly long run to the subwoofer, I would like to use the balanced XLR connection.

 

Now to the real problem...

My subs are Klipsch R-115SW which only have RCA input. To connect these beasts to the XLR output of the pre-amp I'm not sure of the direction to go. To connect a pro amp to the LFE output of my normal AVR I needed an active DI box to adjust the gain levels. However, I'm not sure that's needed in this situation so using just a passive DI box might be sufficient. It seems like using an XLR to RCA cable would eliminate the benefits of using the balanced connection.

So does anyone know, from specs or experience, if the output from a pre-amp (Marantz AV7703/AV8802A, Yamaha CX-A5100, or something similar) will work with a passive DI box or will I need an active one to adjust gain levels? If a DI box isn't needed and just a converter cable will work (obviously using it from the wall to the sub, and not the pre-amp to the wall), feel free to correct me.

 

If I don't need ground lift or gain adjust something like the Neutrik NA2M-D2B-TX seems like a easy and inexpensive adapter...

http://www.fullcompass.com/prod/284423-Neutrik-NA2M-D2B-TX

 

I do realize I'm probably being a bit paranoid about getting noise in the signal using unbalanced RCA, especially if I'm using quality quad-shielded RG-6 in the wall, but with the cost of all this stuff I want to eliminate as many sources of problems as I can.

 

Thanks in advance for any advice or experience info!

[carlthess40]

I’ve done the same thing with my last marantz 7702mkII and my cable run for my sub was 68’ and I had no problems at all. I did use a solid copper core and copper shield rg6 cable and it had great sound with my svs and with two different Klipsch subs. Just give it a try with a test cable and see if you hear anything bad in the sound

[DizRotus]

Does the Klipsch sub have balanced/differential inputs?  I don’t think so.  The balanced/differential outputs are intended to go to amps with balanced/differential inputs, such as iNukes.  

 

The use of “XLR” as synonymous with balanced and “RCA” as synonymous with unbalanced is common, but incorrect.  Each type of connection can be part of balanced or unbalanced circuits.

[snike3]

3 hours ago, carlthess40 said:

I’ve done the same thing with my last marantz 7702mkII and my cable run for my sub was 68’ and I had no problems at all. I did use a solid copper core and copper shield rg6 cable and it had great sound with my svs and with two different Klipsch subs. Just give it a try with a test cable and see if you hear anything bad in the sound

 

Most RG-6 I'm finding is copper clad steel core. Do you mind sharing what kind/where you got copper core RG-6? I have some really long runs of RG-6 quad that I'm running standard RCA signals through with good results, so I'm not ruling this method out just yet.  I actually am in the process of finishing this room so I will be running the cable in the walls within the month.

 

3 hours ago, DizRotus said:

Does the Klipsch sub have balanced/differential inputs?  I don’t think so.  The balanced/differential outputs are intended to go to amps with balanced/differential inputs, such as iNukes.  

 

The use of “XLR” as synonymous with balanced and “RCA” as synonymous with unbalanced is common, but incorrect.  Each type of connection can be part of balanced or unbalanced circuits.

 

You are correct, the balanced/differential inputs are meant to go to amps with balanced/differential XLR inputs. I have an iNuke that I used to use to power a sub in my old system, so fully aware of those. There are powered subs with XLR inputs by the way (HSU ULS and Monoprice Monolith both have them). The balanced/differential output can be used avoid signal degradation due to noise over long cable runs (commonly used for extremely low power instrument signal), which is my point in using them here. If you need further justification about using the balanced output to send signal to the unbalanced R-115SW you may want to check out the articles at the end of this post...

 

I did not refer to XLR and RCA as balanced and unbalanced, respectively. In my post I actually used the term "balanced XLR" and "unbalanced RCA". XLR can be an unbalanced connection depending on how the source and destination actually process the signal and how the cable is made. That aside, if you look at the manual for the pre/pro you'll see that it has a balanced/unbalanced selector switch, and the instructions indicate that selecting 'balanced' will enable the XLR and 'unbalanced' with enable the RCA.

 

One other thing to note, I've seen one other person say that there's such thing as a "balanced RCA" but I've yet to see anyone confirm it or back it up with a reference or diagram. I won't go into details about why I don't believe that works given only two conductors, but if you have a reference on how balanced RCA works or some equipment that can do it, I'd gladly give it a look. Always nice to learn something new...

 

 

 

I finally found the appropriate search term in Google to actually find the information I was looking for. Here's a link to an articles that talks about connecting up separates, and it specifically talks about balanced to unbalanced connections.

https://www.soundandvision.com/content/how-set-power-amp

That article then points here for info on balanced to unbalanced connections: http://www.rane.com/note110.html

 

 

Edited May 11, 2018 by snike3

[snike3]

1 hour ago, snike3 said:

 

Most RG-6 I'm finding is copper clad steel core. Do you mind sharing what kind/where you got copper core RG-6? I have some really long runs of RG-6 quad that I'm running standard RCA signals through with good results, so I'm not ruling this method out just yet.  I actually am in the process of finishing this room so I will be running the cable in the walls within the month.

 

Ok, apparently I didn't search very hard....  I found some copper core with copper shielding made by Belden. Unfortunately, I don't want 1000 ft!  Oh well, the search continues.

 

BTW, that Neutrik transformer I'd mentioned in my first post was the wrong one...  It should have been the NA2F-D2B-TX for the female XLR.

[DrWho]

I don't recommend using a transformer for subwoofer duty.....they tend to roll-off the low-end (unless you get something really large and expensive).

 

Honestly, I would start with the unbalanced connection. If the subwoofer was designed well, then you shouldn't have noise coupled through the connection. A good unbalanced solution will outperform a lousy balanced solution all day long.

 

If you're pulling cables through the wall, then spend the extra hundred bucks to run conduit so you can pull new cable.

 

There are two mechanisms for noise injection into a cable:

1) Currents along the cable shield.

2) Radiated noise coupling into the "loop antenna" (the structure of the signal and signal reference).

 

In an XLR cable, your signal reference and signal exist on a twisted pair, which is then shielded by a third ground shield.

In an RCA cable, your signal reference is the ground shield for the signal.

 

Since the ground shield has some impedance, currents flowing along the cable shield create a common mode voltage. With a balanced connection, the common-mode noise gets cancelled out (the key is that the currents see the same impedance so that the voltages are the same on both legs). With an unbalanced connection, this current gets converted directly into the signal voltage. The only way to reduce the voltage is to lower the impedance (or improve the power supplies so that they leak less current down the cable). These noises tend to be very low because the cable impedance is low (<1 ohm ideally) and the leakage currents are low (~1uA)...roughly -120dBV. Most low noise inputs are roughly -90dBV white noise, maybe -110dBV on a really low noise circuit.

 

If your noise is radiated (#2), then you can solve this with a shielded twisted pair for the RCA. The shield only connects to the source (the subwoofer output of your receiver), and then the twisted pair connects to the signal and ground on the receiving end (your subwoofer input). This routes the noise currents into the low output impedance of the source, which keeps the noise off the twisted pair. And by using a twisted pair, any noise that gets through the shield is common to both the signal and the ground cable, which helps provide some common-mode rejection. Some refer to this as a directional cable, or maybe a "balanced rca" cable. The ~10ohm output impedance of your source attenuates the field currents a lot better than the 10k input impedance on the sub.

 

 

 

Summary: Use a shielded twisted pair cable for the subwoofer cable, and start with the shield connected on both ends - if you're still getting buzz, then try lifting the shield at the subwoofer connector. If it gets worse, then the problem is conducted - and your best solution would be to find the three-prong power cable that is causing the problems (and lift it, although lifting it is unsafe for chassis ground faults). The usual culprits are the Cable TV lines - and the transformer isolation for those is a lot cheaper (and better) than a transformer for a low frequency audio signal.

 

Sorry for the brain dump. Don't forget to run conduit...1" PVC should be sufficient (and leave the pull cable in the pipe when you're done so you don't have to snake it twice).

[carlthess40]

 
Ok, apparently I didn't search very hard....  I found some copper core with copper shielding made by Belden. Unfortunately, I don't want 1000 ft!  Oh well, the search continues.
 
BTW, that Neutrik transformer I'd mentioned in my first post was the wrong one...  It should have been the NA2F-D2B-TX for the female XLR.

How many feet do you need? I may be able to make you up a cable. Just pay me for the shipping and the rca plugs and just a very small fee for the cable

[DizRotus]

7 hours ago, snike3 said:

One other thing to note, I've seen one other person say that there's such thing as a "balanced RCA" but I've yet to see anyone confirm it or back it up with a reference or diagram. I won't go into details about why I don't believe that works given only two conductors, but if you have a reference on how balanced RCA works or some equipment that can do it, I'd gladly give it a look. Always nice to learn something new...

 

I use a Pono player every day to send balanced output to the balanced input of my class D amps using RCA connections.  The RCA connections could be replaced by XLR plugs, if wired correctly.

 

The attached diagram was drawn for me by Gary Marsh, the designer/builder of the “Wiener” TPA3116 Class D board.  I acknowledge balanced connections, especially using RCA connections, are somewhat counterintuitive. It takes two RCA plugs per channel.  The hot from one pin, the cold from the other pin, the collars are tied to the cables’ shields and to ground.  

 

The attached photos photos show a Pono player’s balanced/differential outputs connected to the balanced/differential inputs of a Wiener following Gary Marsh’s diagram.  It takes 4 RCA plugs to preserve the balanced circuit.

 

 

 

[babadono]

My understanding of the Marantz 8802 balanced outputs is both pins 2 and 3 have signals on them inverted in phase by 180 degrees. I personally would not take either of those signals to ground on my single ended unbalanced input subwoofer with any cable or simple adapter. If you want to keep the benefits of a balanced output all the way to your sub get a high quality balanced to unbalanced transformer based converter device like an ISO MAX box from Jensen Transformer. This will also have the added benefit of breaking any potential ground loop or Common Impedance Coupling.

[DizRotus]

The photo with the Meanwell power supply shows a single ended/unbalanced connection to a TI TPA3255EVM.  Only two RCA plugs are required.  The shields/collars of each channel share a common ground.

 

The other photo shows a balanced/differential connection to the same 3255.  Jumpers are repositioned to isolate the RCA collars from ground.  Four RCA plugs are required.

 

The 3255 is in the process of being put in a case.  The input connectors will be XLR, one for each channel.  Two pins for the inverted signals and one pin to ground the shield.