An unbalanced RCA sub out to a balanced XLR input

[RazB]

I would like to run two cables through a conduit for the back subwoofers.
The conduit was a bit of an after thought, but fortunately I was still able to place it in. It had to take the path around the room from the front to the back wall. The length this path is approximately 10-15 meters / 35-50ft (probably a bit less), depending on which corner it reaches.
The AVR has only unbalanced RCA subwoofer outputs, the subwoofers have both RCA and XLR inputs.

The idea is the following:
AVR RCA sub out -> RCA to XLR adapter cable -> front wall XLR plate
Back wall XLR plate -> (short flexible) XLR cable -> subwoofer XLR input

Why XLR if I could simply go RCA to RCA (with a coaxial cable)?
Future proofing. Whenever I will replace the AVR with a processor or a higher tier AVR which offers XLR sub outs, I will be ready to go. No need to replace cables nor wall plates.
I already have a couple of short RCA to XLR cables (Canare L-4E6S Star Quad) wired in the "pseudo-balanced" way, as shown in this diagram:

1. What do you think about my approach? Is there anything else I should be aware of?
2. The cable I want to run in the conduit (~10-15m/35-50ft) is the Belden 8451. Do you agree with my choice or should I consider a different type?
3. What type of XLR cable should I use for the short run from the wall plate to the sub? L-4E6S as the one in the RCA to XLR? For some flexibility in sub positioning, I would like the cable to be about 2m/6.5ft long.

I might consider using an active converter to get a true balanced signal, but if there aren't any noise problems I would prefer sticking with the simple (passive) adapter cable.

 

[Wayne A. Pflughaupt]

No reason why your proposal won’t work. The 8451 cable is fine for this application – seems I’ve heard that foil wrap shielding approaches 100% effectiveness. The only thing is that the 8451 probably does not have the twisting conductors that the 4E6S does.

If you end up with hum, it'll probably be from the back subs being on a different circuit or phase from the rest of the gear.

Regards,
Wayne A. Pflughaupt

 

[RazB]

No reason why your proposal won’t work. The 8451 cable is fine for this application – seems I’ve heard that foil wrap shielding approaches 100% effectiveness. The only thing is that the 8451 probably does not have the twisting conductors that the 4E6S does.

If you end up with hum, it'll probably be from the back subs being on a different circuit or phase from the rest of the gear.

Regards,
Wayne A. Pflughaupt

Thank you for your response Wayne!
Glad to hear that my plan should be fine.
The 8451 is a twisted pair cable while the L-4E6S is a star quad cable, with four twisted conductors. Not sure how and if it can affect the signal in any way. From my understanding it shouldn't, the differences are about interference protection, but I don't have any experience with mixing cable types.
All the gear related to the HT is on the same circuit and breaker, so I should be safe from hum issues, but maybe the long cable run can still pick up some interference (hopefully not!).

 

[somebodyelse]

Mixing twisted pair and starquad shouldn't be an issue so long as they're wired correctly. I would expect twisted pair to have enough interference rejection with a line level signal, but as it's pseudo-balanced it will have less rejection than normal. If you know the output impedance of the source you can add that to the cold leg at the RCA end to improve the impedance match and hence the interference rejection. Or just move anything with a big alternating magnetic field a little further from the cable. If that fails you have mentioned a good fallback, the active converter to make it balanced. Overall it seems a decent plan.

 

[RazB]

Mixing twisted pair and starquad shouldn't be an issue so long as they're wired correctly. I would expect twisted pair to have enough interference rejection with a line level signal, but as it's pseudo-balanced it will have less rejection than normal. If you know the output impedance of the source you can add that to the cold leg at the RCA end to improve the impedance match and hence the interference rejection. Or just move anything with a big alternating magnetic field a little further from the cable. If that fails you have mentioned a good fallback, the active converter to make it balanced. Overall it seems a decent plan.

Thank you, that's a great input!
I will go as I planned and in case there is an issue I will try to solve it.

I don't know what's the impedance of the sub out on the AVR, this kind of information doesn't seem to be available. Same goes for any information in regards to the balanced input circuitry on the subwoofer.
What I do know is that someone was able to use an adapter such as this one:

(which I wouldn't even consider) and had no problems with the same subwoofer.
Had one of these laying around so I checked it with a multimeter and it seems that pin 1 and 3 are both connected to the RCA sleeve and therefore shorted together.

 

[Speedskater]

Countless recording and broadcast studios have been wired with Belden 8450, 8451 & 9451 (all very similar) balanced cables.
And have been doing so for at least 4 decades.
But they would be a very poor choice for a RCA cable.

 

[RazB]

Countless recording and broadcast studios have been wired with Belden 8450, 8451 & 9451 (all very similar) balanced cables.
And have been doing so for at least 4 decades.
But they would be a very poor choice for a RCA cable.

Sure, that's why I thought it is a good choice for a permanent installation. Its low capacitance is also a good feature, though for subwoofer signal it might not be so important.
What worried me is the unbalanced aspect, never ran more than a couple of meters going from RCA to XLR. Wanted to hear from people who probably have more experience than me before I go with it.

 

[DVDdoug]

The general rule is that's it's OK to go from unbalanced to balanced, and you still get most of the noise reduction benefits of a balanced connection. And It's OK to short one of the balanced input connections to ground.

BUT you have be careful going the other way around because depending on the equipment it's not always OK to short the balanced output connections.

I don't know what's the impedance of the sub out on the AVR, this kind of information doesn't seem to be available. Same goes for any information in regards to the balanced input circuitry on the subwoofer.

The impedance will be fine. Line level inputs are typically 10K-100K and that's what line-outputs are intended to drive. We don't "match" impedance and the actual output impedance of whatever's driving it will be much lower.

Sometimes the signal level is an issue when mixing consumer and pro equipment. They have different "standards", line Levels aren't tightly controlled, and some program material is louder than others, and sometimes there is a volume control, etc.

What I do know is that someone was able to use an adapter such as this one...

...it seems that pin 1 and 3 are both connected to the RCA sleeve and therefore shorted together.

The same as what your schematic shows.

I'm not sure how I like the "mechanics" of that thing hanging-off the back of a subwoofer with an RCA cable plug attached.

 

[Wayne A. Pflughaupt]

What worried me is the unbalanced aspect, never ran more than a couple of meters going from RCA to XLR. Wanted to hear from people who probably have more experience than me before I go with it.

I’ve run unbalanced lines on the order of 50-75 ft. with no issues. The key, IMO, is cable with top-notch shielding. Which the 8451 has.

Regards,
Wayne A. Pflughaupt

 

[Speedskater]

Depending on the situation and condition of the associated AC power circuits, Yes RCA unbalanced interconnects cane be run 50 to 75 feet.
But for best results, use a coax with a heavy braided shield.
In this setup, do not use Belden 8450, 8451, 9451 or any other cable with just a foil shield.
Foil shields are great for noise and interference above 50 megahertz.
But braided shields are great for noise and interference below 10 megahertz.

 

[RazB]

The same as what your schematic shows.

I'm not sure how I like the "mechanics" of that thing hanging-off the back of a subwoofer with an RCA cable plug attached.

Correct
It really does exactly the same as my adapter RCA to XLR cables, but no way I'm going to have it sticking out! Will be using the cable.

In this setup, do not use Belden 8450, 8451, 9451 or any other cable with just a foil shield.

Alright, I knew there are differences regarding interference rejection, but now I understand it's about frequencies.
Do you have any specific recommendation for braided shield cables which are meant and rated for in-wall permanent installations (instead of the 8451)?

 

[Julf]

Recommended reading: The G Word, or How to Get Your Audio off the Ground