Two RCA phono jacks to balanced XLR at DVinfo.net

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Old March 13th, 2010, 08:54 PM   #1
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Two RCA phono jacks to balanced XLR

My 'field mixer' outputs a analog signal to two RCA phono jacks (L and R). I want to route that output via XLR to a camcorder. Can anyone point me towards a resource that might explain how to do that?

Thank you.
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Old March 13th, 2010, 09:01 PM   #2
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Clarification?

When you say "XLR to a camcorder", are you asking about ONE XLR connector (i.e. monaural, mixing the L & R signals from the mixer RCA outputs), or TWO XLR connectors (stereo, Left RCA to left XLR, and Right RCA to right XLR).

What is the rated output level from the (unidentified) mixer? If it is RCA, presumably it is "consumer line-level" or around -10dBu. Assuming that the XLR input(s) on your (unidentified) camcorder can be set for either mic level or line level. Or do you need an attenuator also to knock the mixer (presumed) line level down to mic level for the camcorder?

Other issues include how far you are sending the signal, and whether there are any shielding, grounding, ground-loop potential problems to deal with. Those may require a more expensive solution like isolation transformer(s) vs. a simple adapter cable.
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Old March 13th, 2010, 11:10 PM   #3
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Fair enough,

The 'field mixer' is a Sound Devices USBPre connected to a Asus EeePC Netbook via USB which is running Sony Sound Forge Audio Studio for primary recording. The EeePC is small enough to fit in the audio bag if needed. The camcorder hook up would be the secondary audio recorder where BOTH the L and R channels would be combined into a single XLR cable.

While I do not know, the output signal is almost certainly a line level signal.

This is a compromise solution for budget reasons. The maximum cable run would not exceed 75'.
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Old March 14th, 2010, 07:50 AM   #4
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As Richard said, when you say "line level" you have to be very clear WHICH line level you are referring to. "Consumer" line level is -10dBv while "Professional" line level is +4dBu. Because dBv and dBu have different zero points, the difference between the the two line levels is about 12dB. According to the manual, the USBPre's RCA computer audio output is unbalanced consumer level, a nominal -10dBv. So you need to look up the sensitivity of the XLR line level input to the camcorder and make sure they match - you may need additional amplification or conversely, a pad to lower the feed's level down to mic level, depending.

Why would you put both left and right signals onto a single XLR connector? At the camcorder end the XLR input is certain to be mono. Using a standard 3-conductor XLR for left and right stereo is incredibly rare, if it is ever used at all. I know I've never seen it, except at the microphone end of a Y on some stereo mics. A single XLR connector is normally a mono, balanced connection and carries one channel only. Are you actually combining the left and right signals into a single mono feed? While you can easily make up a Y cable that would parallel the two RCAs and send the result to a single mono XLR input, note that the result will be UNBALANCED cable run and connection and the noise immunity of a normal, balanced, XLR cable would be lost. A 75 foot cable run is going to be very iffy.

If you really want to send the left and right outputs combined into a single mono channel, terminating in an XLR at the other end, you need to arrange a Y cable that connects both of the two RCA's centre pins to XLR pin 2 (signal hot) and the RCA shell's to XLR pin 3. Use a 2 conductor plus shield cable as one would for a normal balanced XLR cable and use the two centre conductors for the above connections. Connect the shield to XLR pin 1 and either tie it to the RCA shells or leave it unconnected at the RCA end. A combination of the following adapters and cable would do it, though I hate the idea of stacking adapters like this.
Hosa YRA 105 Audio Y-Cable, RCA Female to Dual RCA Male, 6" | Full Compass
Neutrik NA2MPMM Male XLR to Male RCA Connector, Wired | Full Compass
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Old March 14th, 2010, 08:18 AM   #5
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You could get a balanced XLR output with the left and right
signals combined to mono by using two RCA to TS cables (or a Y
cable) hooked
into the looping inputs of a direct box. Your signal would need to
be mono-compatible to begin with and some devices don't like
having their outputs Y'ed together, but at least the XLR run
would be balanced.
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Old March 14th, 2010, 10:38 AM   #6
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Thanks one and all. Huge help!
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Old March 14th, 2010, 11:08 AM   #7
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The RCA outputs on the USBpre only output sound from the connected PC, they do not contain the input audio unless your PC/recording software puts it there....latency is usually an issue here.

If you turn the mix knob all the way counterclockwise (to pre) you will get the input audio on the two headphone outputs, the headphone gain obviously affects this level.
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Old March 16th, 2010, 01:01 PM   #8
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You should also be aware that summing to mono with a Y-cable is not really a great idea. It may work for a while, but it can also be hard on the output amplifier stage of the device driving the load. You really need a couple low-value resistors in-line with the outputs to help keep the line output amps from "driving" each other--or that's the best I remember the explanation as to why this is a bad idea.

Check out the Rane article on this. Why Not Wye?

From the article: "A wye-connector used to split a signal into two lines is being used properly; a wye-connector used to mix two signals into one is being abused and may even damage the equipment involved."

Andrew
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Old March 16th, 2010, 01:05 PM   #9
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More and more I think I'll skip the USBPre and live with a cheap field mixer.
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Old March 16th, 2010, 01:23 PM   #10
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Quote:
Originally Posted by Andrew Hughes View Post
You should also be aware that summing to mono with a Y-cable is not really a great idea. It may work for a while, but it can also be hard on the output amplifier stage of the device driving the load. You really need a couple low-value resistors in-line with the outputs to help keep the line output amps from "driving" each other--or that's the best I remember the explanation as to why this is a bad idea.

Check out the Rane article on this. Why Not Wye?

From the article: "A wye-connector used to split a signal into two lines is being used properly; a wye-connector used to mix two signals into one is being abused and may even damage the equipment involved."

Andrew
Good catch! Only suggested the Y as a budget option, it's definitely not the right way to do it, as you rightly point out.
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