Balanced to RCA

@sunnybaker

As Bob and Mike indicated earlier the best approach to connecting a balanced output to an unbalanced input is widely considered to be with a Jensen Iso-Max transformer.

A suitable transformer, given that you are using a two-channel amplifier (as opposed to monoblocks) would be the Jensen model PI2-XR. It is available here for about $270 in a configuration having XLR inputs and RCA outputs. (That page shows a PI2-XX, having XLR connectors for both inputs and outputs, but you can specify RCA output connectors in the options menu near the bottom of the page). Further details can be found at the Jensen site.

A transformer such as that would eliminate the possibility of ground loop issues, and it would provide essentially the same noise rejection as a balanced interface would provide. Or perhaps even better noise rejection, because its bandwidth limitation of about 55 kHz at -3db will tend to filter out RFI that may be present. It would also eliminate whatever audible significance the "micro-capacitive flutter" effect that Teo referred to may have. (Although I would not expect that effect to be significant if an XLR-female to RCA-female adapter leaving pin 3 open is placed at the output connector of the dbx unit, and is used in conjunction with an RCA-to-RCA cable. I suppose it might be significant if an XLR cable is used in conjunction with an XLR-to-RCA adapter at the input of the destination component, due to capacitive coupling that may occur within the cable. And there certainly may be an issue if pin 3 is left open on an XLR **input**; when adapting an RCA output to an XLR input pin 3 should be grounded).

Regarding the need for additional cables with a Jensen transformer, as you can see in the manual for the PI2-XR the cables on its output side must be very short (no more than 3 feet long, and preferably less, depending on the capacitance of the particular cable). So given the minimal length they shouldn’t cost a great deal, and whatever sonic effects they may introduce should be less than if they were longer.

Regarding the use of adapters or adapter cables at the outputs of your dbx unit, to expand on my previous comment a bit the only situation in which it is necessary to ground pin 3 when adapting an XLR output to an RCA input is when the signals on pins 2 and 3 are "floating" relative to circuit ground (i.e., the voltages between those signals and circuit ground are not controlled or defined, or putting it another way the signals are essentially "isolated" from circuit ground). That is not the case in most designs, and I suspect it is not the case with your dbx. So I think it likely that connecting pin 3 to pin 1, either directly or via the 10 ohm resistor you referred to, won’t do any good and conceivably could do harm, eventually if not sooner. But without a schematic or other detailed knowledge of the specific design I can’t say that for sure.

Regards,
-- Al

NEVER short pin3 to pin1 on a balanced XLR output.

This would have been a perfect topic to post in the "pro audio" section.

First let's explain the XLR pins :
1 : ground
2 : hot (+) normal signal
3 : cold (-) inverted signal

A XLR output is always a male XLR and an XLR input is always a female XLR. An easy trick to remember this is just to think of how a microphone looks like : it has a pins -> male XLR and it is a audio source -> output

Although it is not recommended you can make adapter cables from RCA to XLR and vice versa.

The rules are simple :
- RCA ground to XLR ground (pin 1)
- RCA center (non-inverted signal) to XLR hot (pin 2)
- on the MALE XLR cable part ONLY!!! Short cold (pin 3) to ground (pin 1)
The short is to avoid unwanted signals into pin 3. If you leave it floating it can cause unpredictable input (noise). 

Pin 3 on the female XLR of a XLR to RCA cable is left floating otherwise you would short the output and risk damage (you will "overload" the output). Balanced output stages are mostly opamps (some use tranformers) and nearly all of them are protected against shorts, but protections can fail and overloading of the cold(-) signal can negatively influence the hot(+) output signal.

Leaving an output floating is not problem and if you think it is I have one question : do you shorten all the unused RCA outputs on your devices? I guess not.

PS : normally you don't connect XLR chassis to GND (pin 1) but in case you have any hum (ground loop) try connecting those. Some professional devices (especially amplifiers) have a switch for that purpose (ground lift, which factory defaults on "lift").

btw, wasn't it DBX that had an example of those cables in their manual?

@danip,
Though I am not an EE, I think the OP should be aware that not all equipment with balanced interconnects abides by the AEC standard.
Therefore, in an abundance of caution, I suggested using the Jensen Transformers. 
Almarg suggested contacting the manufacturer for a schematic of the wiring just to make sure it abides that standard. If it should, your advice would be sound.
Bob 

NEVER short pin3 to pin1 on a balanced XLR output.

@danip

It’s never a good idea to say "never" :-)

As my previous posts should make clear, I agree with your post when it comes to most designs. But there are exceptions.

You referred to balanced output stages utilizing op amps, and a notable exception to the general guideline of not shorting pin 3 to pin 1 when adapting a balanced output to an unbalanced input applies to op amp-based output stages that are "cross-coupled," which thereby provide outputs that are essentially floating with respect to circuit ground. See pages 11 and 12 of the following reference, in which such circuits are referred to as "active floating sources." And note the statement on page 13 that "Grounding one output line at the driver, which is REQUIRED [emphasis added] to guarantee stability of most "active floating" circuits, degenerates the interface to a completely unbalanced one having no ground noise rejection at all."

https://www.jhbrandt.net/wp-content/uploads/2014/11/Design_of_High-Performance_Balanced_Audio_Interfaces.pdf

That is further confirmed in the Rane document that was referenced earlier in the thread by Rauda1. See diagram 6 on page 7 of that document.

Also, while I’m not specifically familiar with the designs of the output stages of Atma-Sphere’s balanced tube-based preamps, Ralph (Atmasphere) has explicitly stated in past threads here that connecting the balanced outputs of his preamps to unbalanced inputs without grounding pin 3 will result in a huge hum. Which can be expected, given that the balanced outputs of his preamps, although not transformer coupled, are nevertheless floating with respect to circuit ground. And consequently if pin 3 is not grounded the signal on pin 2 will be received by the destination component with an uncontrolled and undefinable voltage relative to pin 1.

There are undoubtedly other such examples.

Regards,
-- Al