XLR to RCA Subwoofer new idea?

[JimA84]

I have been going back and forth with particularly subs with manageable weight and low extension.

It seems like you can never get the perfect set of features. Some have XLR and some are more affordable but only have RCA.

Assuming a XLR preamp or processor this is a dilemma. Spend thousands more to get XLR throughout or compromise with some potentially gimpy XLR->RCA converter.

I was just thinking with my EE hat.

Instead of grounding or leaving pin 3 floating, what if we produce a "stereo" XLR to RCA cable to connect to a subwoofer with L+R "stereo" inputs.

So pin 2 is connected to the typically left or LFE and pin 3 is connected to the "R" stereo input.

Assuming the input channels are indeoendent, this should maintain a fully differential input with nothing more than a simple adapter cable with a single XLR input and two RCA outputs.

Am I wrong or crazy, or why hasn't anyone done this?

 

[DVDdoug]

Assuming you don't have a noise problem, unbalanced RCA connections are fine for "home use" and short connections. In pro audio it's nice to use XLR cables for everything (except passive speakers).

Instead of grounding or leaving pin 3 floating, what if we produce a "stereo" XLR to RCA cable to connect to a subwoofer with L+R "stereo" inputs.

As an EE, you know that the signal has to be relative to something. The input needs a ground or it needs to be differential.

You can ALWAYS go from an unbalanced output to a balanced/differential input because with one input grounded there is still a "difference". And you get some of the noise immunity of a balanced connection.

But you can't always go the other way around. Sometimes you can't safely ground (short-out) one of the signals, and occasionally the ground isn't a proper signal ground. (Most of the time you CAN just leave one of the signals unused.)

So pin 2 is connected to the typically left or LFE and pin 3 is connected to the "R" stereo input.

You can wire an XLR connector that way but there is no advantage. 1/4-inch TRS connectors are sometimes used for differential/balanced connections and sometimes for stereo (like with headphones). But you rarely see one XLR connection used for stereo.

Without a ground connection to the subwoofer or a proper balanced connection, you can end-up with a differential connection and L-R. i.e. If the ground connection on headphones breaks, that's what you get... A center-channel "vocal remover" which also removes the bass since it's usually common to left & right (and you get mono with both earpieces connected in series).

 

[JimA84]

I think you're not understanding this idea at all.
With a "stereo" RCA input you typically have two independent unbalanced channels that are summed internally.

So if you passively connect them to the two balanced channels of a XLR cable you have effectively created a balanced input from two unbalanced ones.

Your notions of "grounds" are irrelevant because the ground in this case is effectively the center tap of the XLR and the common ground of the RCA.

If you look at the circuitry of a balanced XLR input compared to a stereo unbalanced RCA input summed to mono in a subwoofer it's the same.

 

[JimA84]

I have been going back and forth with particularly subs with manageable weight and low extension.

It seems like you can never get the perfect set of features. Some have XLR and some are more affordable but only have RCA.

Assuming a XLR preamp or processor this is a dilemma. Spend thousands more to get XLR throughout or compromise with some potentially gimpy XLR->RCA converter.

I was just thinking with my EE hat.

Instead of grounding or leaving pin 3 floating, what if we produce a "stereo" XLR to RCA cable to connect to a subwoofer with L+R "stereo" inputs.

So pin 2 is connected to the typically left or LFE and pin 3 is connected to the "R" stereo input.

Assuming the input channels are indeoendent, this should maintain a fully differential input with nothing more than a simple adapter cable with a single XLR input and two RCA outputs.

Am I wrong or crazy, or why hasn't anyone done this?

Obviously you should make the adapter cable very short at the subwoofer (output) end.

You can also use a Jensen transformer but again you want to limit the RCA cable to no more than 2 feet of quality RG6.

The point is that a Jensen transformer costs hundreds of dollars just to compensate for a manufacturer who chooses to save a few dollars by not offering a XLR input.

This option which should work equally well costs a few dollars if you make it yourself.

The issue here serms to be people who can't imagine turning a stereo input summed to mono into a differential mono input.

Electrically it is completely equivalent.

 

[Speedskater]

Do you have:

Left XLR output
Right XLR output
* * * * * * *
Left RCA sub input
Right RCA sub input
note that the sub has a circuit to sum L & R

 

[JimA84]

No the point is to sum the differential mono XLR output using two unbalanced RCA inputs, which are the equivalent of one balanced XLR input.

 

[mcdn]

This simply won’t work, because the L and R unbalanced signals are in phase so will have almost no difference between them. You’d need to invert one of them first.

 

[JimA84]

They are already inverted.

 

[antcollinet]

No the point is to sum the differential mono XLR output using two unbalanced RCA inputs, which are the equivalent of one balanced XLR input.

What do you get if you do the sum +x + -x?

 

[JimA84]

This simply won’t work, because the L and R unbalanced signals are in phase so will have almost no difference between them. You’d need to invert one of them first.

The whole point of the XLR interface is that they are out of phase. Thus you are effectively creating the same thing by summing two RCA inputs fed by the out of phase XLR signals.

 

[Chrispy]

Can't imagine basing a subwoofer choice based on rca vs xlr input. Or worrying about it.

 

[antcollinet]

The whole point of the XLR interface is that they are out of phase. Thus you are effectively creating the same thing by summing two RCA inputs fed by the out of phase XLR signals.

OK - lets try again.

XLR Cold = (- XLR hot) (yes? that is what out of phase means)

So when you add them together by putting them into the sub L and R,

The result going into the sub amp is (XLR hot) + (XLR cold)
= (XLR hot) + (-XLR hot)
= 0

 

[Blumlein 88]

Should work. I don't see a big benefit vs just shorting one of the pins other than a level increase. I think people are missing that you are tapping each XLR so the two channels are inverted which actually puts them back in phase when the sub sums the two channels.

 

[mcdn]

The whole point of the XLR interface is that they are out of phase. Thus you are effectively creating the same thing by summing two RCA inputs fed by the out of phase XLR signals.

My bad, typing before coffee rarely goes well!

 

[antcollinet]

Should work. I don't see a big benefit vs just shorting one of the pins other than a level increase. I think people are missing that you are tapping each XLR so the two channels are inverted which actually puts them back in phase when the sub sums the two channels.

Bear in mind with a normal balanced input, the input is differential. IE you subtract one from the other creating 2x the single ended signal.

If you add them together (as a sub does to its LR inputs) then you end up with 0. You also double any common mode noise (rather than cancelling it out as you do with a differential input). So you end up with 0 signal, and 2x noise. The exact opposite of what you want.

 

[JimA84]

What do you get if you do the sum +x + -x?

Zero in effect of noise which is the whole point.

Look at the circuit of a differential interface like the XLR.

The (-) simply extends the signal in the opposite direction from reference ground.

In other words you need to drive the (-) channel below reference ground.

I guess I might have overthougt this because you would have to invert the phase on the (-) channel to achieve summation given a common reference ground. Once again the stupid "ground" is a problem.

Of course reintroducing the always plaguing ground issues. Damn, I guess I may have been a bit over enthusiastic.

I guess I should just buy the B&W subs with the XLR inputs.

I just want to avoid the hum and noise problems with RCA cables and the stupid grounding problems that go along with them in trying to interface with unbalanced interfaces

I would rather pay several thousand dollars more for B&W subwoofers than to deal with gimpy attempts to connect unbalanced circuits.

I hate RCA unbalanced cables and would avoid them at all costs if possible.

There is no reason for them to exist.

It's like twin lead 300 ohm TV antenna cables.

I wonder how much it really would have cost for Kef to put a XLR input on the KF92

Especially given the elaborate crossover and high pass filter output features they included.

Why can't we have equipment that is designed for end to end sound quality instead of either crap consumer features or simply cost cutting by MBAs?

I give up. I am so frustrated by these issues, stupid marketing and poor quality and customer service due to MBA ideology.

I think I will just put a Rolls MB15b ProMatch box on my TV and plug a couple Adam T8V boxes with possibly their T10S and just become a spectator.

 

[Cbdb2]

Your presuming you will have a problem without even trying an XLR to rca cable? Your probably wasting your time unless its a long run. Most people connect there subs with rca and theres no problem.

 

[a2Thompson]

I use the matchbox HD systems recently reviewed here to convert balanced out from my Topping DM7 (which has a non-zero floating ground or whatever it is called so that there is hum if you don’t have an active conversion to rca) to unbalanced in on my sub. It works beautifully. I’m balanced for the 30’ to the matchbox and then unbalanced only for the last 12” to the sub. Got the matchbox HD for $50 on eBay.

 

[Kal Rubinson]

I wonder how much it really would have cost for Kef to put a XLR input on the KF92

Because then all the owners of their wireless speakers would be demanding XLR outputs.

 

[JimA84]

Your presuming you will have a problem without even trying an XLR to rca cable? Your probably wasting your time unless its a long run. Most people connect there subs with rca and theres no problem.

The last time I used a RCA cable (an "adapter" cable is unbalanced) with active monitors I got a nifty AM radio receiver. I generally try to avoid unbalanced for anything beyond about 6 feet.