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Line level signal to Mixing Desk through XLR Multicore Mixers (Digital)
Old 1 week ago
  #1
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Line level signal to Mixing Desk through XLR Multicore

Hello, guys. My friends will be gigging next week and I am helping them with technical stuff. They need to connect their audio interface (PreSonus AudioBox 44VSL) to club's Allen & Heath Qu-32 digital mixing console. The audio interface has 2 balanced 1/4 TRS line outputs. The console has 32 balanced line inputs.

Here is the problem. There is a very long distance from stage to console. In the club they use Klotz 32/8 multicore with XLR connectors. Yes, this multicore has only XLR connectors, no TRS. I can make TRS to XLR cables, but I am worried about problems like (1) different signal levels, (2) phantom power.

How do you solve problems like this?
Old 1 week ago
  #2
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Richard Crowley's Avatar
 

Quote:
Originally Posted by Ez Presso View Post
I am worried about problems like (1) different signal levels, (2) phantom power.
(1) If running mic-level through that snake cable doesn't affect signal levels significantly, it surely won't affect line-level.

(2) There will be no phantom power on line-level inputs.
Old 1 week ago
  #3
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Quote:
Originally Posted by Ez Presso View Post
... club's Allen & Heath Qu-32 digital mixing console. The audio interface has 2 balanced 1/4 TRS line outputs. The console has 32 balanced line inputs.... this multicore has only XLR connectors, no TRS. I can make TRS to XLR cables, but I am worried about problems like (1) different signal levels, (2) phantom power. ...
Signal level management is easy with a little cooperation between the stage and the FoH sound person. Protecting your equipment from phantom power requires either transformer isolation via a passive DI box, or an active DI (some of which are phantom-powered) box between your stuff and the FoH mixer.

I always bring an ART T8 transformer isolator with me to remote setups. It's completely passive, and has all the combos for in/out ... RCA, TRS, XLR. Similar solutions will be beneficial to you.
Old 1 week ago
  #4
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Quote:
Originally Posted by Richard Crowley View Post
(2) There will be no phantom power on line-level inputs.
Richard, in the club they use XLR multicore between our equipment (on stage) and their mixing console. We have no chance but to use their XLR multicore. The audio interface has TRS line outputs, so we have to make a balanced TRS-XLR cable. It means there is a chance our audio interface may be damaged by the phantom power if it is switched on.
Old 1 week ago
  #5
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Quote:
Originally Posted by MediaGary View Post
I always bring an ART T8 transformer isolator with me to remote setups. It's completely passive, and has all the combos for in/out ... RCA, TRS, XLR.
Will it affect sound quality on low frequencies (below 10Hz)? I heard transformers introduce some phase and frequency distortions to audio signal.
Old 1 week ago
  #6
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Quote:
Originally Posted by Ez Presso View Post
Will it affect sound quality on low frequencies (below 10Hz)? I heard transformers introduce some phase and frequency distortions to audio signal.
Very likely yes it would attenuate low frequencies way down there. If I'm the FoH sound person, the last thing I want it to pass <30Hz signals through my board to the subwoofers, so I generally have "safety/protection" high pass someplace in the chain.

I also have a T8 in my studio and the fidelity has been fine. I use it between the unbalanced outputs of my Yamaha ES8 and Roland RD-600 going to the XLR inputs of my Midas M32. All is well.

The spec sheet for the T8 lists:
- Frequency response -0.5dB down points are 10Hz-50kHz @ +4dBu
- THD 0.01% typical @ 1kHz @ +18dBu (around -80dB)
- THD <0.1% @ 100Hz @ +24dBu (about -60dB)
- Phase error < 5% 20Hz to 20kHz

Last edited by MediaGary; 1 week ago at 11:04 PM.. Reason: typos
Old 1 week ago
  #7
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Quote:
Originally Posted by Ez Presso View Post

How do you solve problems like this?
Since it's only two channels, a pair of quality passive DIs would be how I would handle it.

Yes, transformers are not completely transparent, but something like the Radial JDI (or even the less expensive models) will be closer to transparent than anything else in the system besides the snake itself. You will lose some level (all passive DIs lose level), but if it's really coming out of the interface at line level, the QU-32 should have no trouble boosting it back up to where it needs to be without adding any significant noise. Just don't use a cheap $30 DI. They will have low end distortion problems

My experience with the QU-16 is that the XLRs can accommodate line level (nominal +4 dBu) without clipping, but your concern about accidental phantom is valid, so putting a transformer somewhere in between devices is good practice.

In fact, after checking the flow diagram for the QU series, the mic XLR and line TRS are essentially connected in parallel, except the TRS has some extra resistance and is after the phantom blocking caps, so some simple XLR>TRS adapters at the console end might do the trick, with no danger of phantom being fed back to your interface. What you won't get (that the DI gives you) is the ability to lift the ground if you run into a pin 1 problem. I've used the QU-16 with simple splits from another console (no ground lifts), and not had a problem, but you don't want to show up at the club and discover you have a problem. That's when the finger pointing will start.

Geoff
Old 1 week ago
  #8
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Richard Crowley's Avatar
 

@Ez Presso apparently I read your question wrong. I thought you were asking...
Quote:
I have a TRS line-level source which I need to feed into a venue FOH mixer which has 32 line-level inputs.
The venue has an XLR analog snake from the stage to the FOH mixer which has TRS balanced line inputs.
The venue XLR snake is normally connected to the XLR mic-level inputs.
I want to send my balanced, line-level audio to the FOH mixer.
So, the solution there would be a couple of adapters for each end of the venue snake:
1) At the stage end, a TRS to male XLR adapter cable between your TRS balanced line-level source and the venue snake input (assumed to be female XLR)
2) At the mixer end, a female XLR to TRS adapter cable to feed your signal into the TRS line-level inputs.

Because this is a completely line-level, balanced connection from end to end, there should be no issue with signal levels.
And there will be no phantom power coming out of the mixer TRS line-level input connectors.
The only problem with this solution is that it requires the venue people to interrupt their normal mic-level XLR connections into the FOH mixer and substitute your adapter cable. (and then restore "normal operation" when done.) This solution is the cleanest with respect to the audio signal, but it requires "special handling" by the venue which you may want to avoid.

But apparently you seem to equate "Yes, this multicore has only XLR connectors" to mean "there are only mic-level inputs available at the stage end of the snake." Which confuses me because you mention the line-level inputs available on the FOH mixer, but you seem unwilling (unable?) to use an adapter at the mixer end to turn a couple of snake pair into line-level inputs.

So, perhaps you are asking:
Quote:
I have a TRS line-level source which I need to feed into a venue FOH mixer.
In order to avoid fooling around with anything special at the mixer end, I would like to simply feed mic-level signals into the venue snake and keep it simple.
There may or may not be phantom power on these XLR mic inputs at the stage, so I need to protect my TRS line-level balanced source.
In that case, a "direct-box" solution would be the most straightforward and least fiddly when dealing with different venues with varying technical understanding.

My favorite solution for line-level to balanced mic-level is the Rolls DB-25 "Matchbox".
It handles a wide variety of input levels from almost mic/instrument-level up to speaker-level with a continuously-variable attenuator "volume control".
It has a completely transformer-isolated mic-level XLR output which will totally protect you if phantom power is present in the venue mic input.
And best of all, they are $24 each. I keep four of them in my mobile kit. Recommended as a perfect solution for your scenario.
This will take any line-level source, balanced or unbalanced, consumer level (-10dBv) or pro-level (+4dBv) and convert into what appears to be an ordinary mic signal as the venue would see from any ordinary microphone. So they would have to do nothing special at their end for connection or operation.

Rolls Corporation - Real Sound - Products DB25b

Old 1 week ago
  #9
Richard,

Rolls is not known for really top end equipment. Have you checked out this box for frequency response and distortion? Just wondering. I trust your judgement.
Old 1 week ago
  #10
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Richard Crowley's Avatar
 

Thanks, Thomas, that is a fair question.

i have disassembled my Rolls DB-25a (the previous version without continuous attenuator). I disassemble most of my gear to see inside to judge the components and build quality, etc. The components (steel case, connectors, switches, PC board, etc) are equivalent quality to well respected audio gear.

The most important component, the isolation transformer, is quite substantial and larger than you might imagine. It indicates to me that it won't skimp on low-frequency response and distortion. Especially compared to the size of the transformer in many even high-end gear like microphones. And some high-end isolation products I have disassembled use significantly smaller transformers. Of course, it is also a passive circuit, so there is no issue with the effects from active components.

I am generally NOT a believer in the theory that certain brands are inferior just by name. I have been stung by brands with "high-brow" reputations. I judge things individually on their own merits, not on their populist reputations.

If I were in a situation where very accurate performance was required, (like a formal commercial recording) I would certainly try different gear to see what works best. But I admit that in cases of casual performance on the road, my perception is that the quality of the iso transformer doesn't make the top-10 list of issues to worry about. A ham-handed, inexperienced (possibly impaired) FOH operator can trivially defeat even the most expensive gear. Not a week ago I was involved in a live performance that had every possibility of being quite glorious, but turned out terrible because the FOH kid was asleep at the switch.

If I were to be surprised by an AudioPrecision analyzer under my Christmas tree, I would love to make some comparative measurements between prosaic gear like the Rolls DB-25 and the famous high-end equivalent gear. If someone has already published such a comparison, I would like to see it.

If spending more money on a brand with "better reputation" actually makes a significant improvement in quality (or even makes the user feel better) I wouldn't discourage that. But I am only testifying on my first-hand experience with gear that I use myself.
Old 4 days ago
  #11
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Richard, Thanks for your very detailed and thorough reply!

We ended up building a pair of TRS to male XLR cables and a pair of female XLR to TRS cables as you suggested. Also we will bring a pair of direct-boxes, just in case. No one knows if those venue guys are going to be cooperative or not.
Old 4 days ago
  #12
Quote:
Originally Posted by Ez Presso View Post
Will it affect sound quality on low frequencies (below 10Hz)? I heard transformers introduce some phase and frequency distortions to audio signal.
Yes, Fletcher is the main proponent of this approach. Had pre’s go below 10hz (a freq. one might hold useless) -and this is a totally distinct topic than ´bold low end’ character of some vintage, class A pre’s.

Getting more bottom doesn’t imply boosting this freq. range.
Old 4 days ago
  #13
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Richard Crowley's Avatar
 

Where are you getting "low frequencies (below 10Hz)"? Do you need to record an organ with 128 foot pipes (or electronic stops)? Are you recording volcanoes erupting?

If you are recording phenomenon other than acoustic audio, then laboratory data recorders are more suitable than the kind of audio gear we discuss here. Lab data recorders typically go all the way down to 0Hz (DC).
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