Unbalanced line to balanced XLR line?

[DMahalko]

I am confused about what I need to do to go from an unbalanced line connector to a balanced XLR line connector.

I am not looking for ultra-pro audio performance, just the basics. I am trying to output 5.1 / 7.1 sound over XLR from a $32 StarTech USB converter box that only has 1/8" stereo output connectors.

This is for movie-like surround sound in a gymnasium using powered XLR speakers.

A direct wire is probably good enough if I keep the unbalanced part of the cable as short as possible, but it seems that an autotransformer is needed to properly center-tap the ground wire for the twisted pair XLR cable.

Is just a straight 1:1 ratio center-tap audio autotransformer / balun needed to properly balance the XLR output?

I am trying to find a part that I can look up on DigiKey / Newark, or hopefully a preassembled cable with the autotransformer already included.

 

[JonSnell Electronic]

Use an isolating transformer to reduce earth loop hum.
Something like; https://uk.rs-online.com/web/p/audio-transformers/8513686/
as an example. Cheap but good quality.

 

[DMahalko]

Thanks, and yes I see from the spec sheet, it's a straight 1:1 600 ohm center-tap isolation transformer.

https://docs.rs-online.com/bd0a/A700000007069994.pdf
http://www.farnell.com/datasheets/1872256.pdf

 

[justanalogue]

Isn't 2mW a way too small to deal with 0dB or above levels?

Willem.

 

[MAAC0]

but it seems that an autotransformer is needed to properly center-tap the ground wire for the twisted pair XLR cable

Try first with the middle XLR pin (inverting input) connected to ground.
That should work fine with short cables.

If You hear buzzing when HDD spinning or mouse moving try a laptop with an ungrounded PSU.

 

[Mark Tillotson]

Isn't 2mW a way too small to deal with 0dB or above levels?

Willem.

1.1Vrms into 600 ohms is 2mW, or 3dBu (by definition as 0dBu is 1mW into 600 ohms.)

 

[justanalogue]

Thanks, but I like lot's of headroom, at least +9dB.
Don't like saturation that turns into distortion.

Willem.

 

[DMahalko]

Part of the difficulty of solving a technical problem like this is knowing the proper search terms.

This discussion has helped to point me in the direction of a preassembled solution, which is really what I am trying to find, but also doesn't break the bank for what I'm trying to do.

SONIFEX RB-UL4 1U Quad Stereo Unbalanced to Balanced Converter
New US $600, Used about $150

For the used device, that is about $18.75 per channel, for 7.1:
Front stereo, Mid stereo, Rear stereo, Subwoofer + Center

 

[justanalogue]

SONIFEX makes good gear!

Willem.

 

[stratus46]

Isn't 2mW a way too small to deal with 0dB or above levels?

Willem.

0 dBm would be 1 mW. It's a little closer to the edge than I'd like but it's probably OK. Personally, I
avoid transformers UNLESS you have serious offsets between the input and output.

G²

 

[PRR]

Get a 1/8" plug, an XLR, and some clip-leads. About 9 out of 10 times there is a simple direct connection which works flawlessly. Because there are more than 3 possibilities, try-and-see is quicker than thinking.