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Issues with aux output from Allen + Heath QU series - any thoughts?

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Issues with aux output from Allen + Heath QU series - any thoughts?

Postby Ben » Wed Oct 30, 2019 2:26 pm

Hi SOS forum

I shoot video and I'm getting some unexpected behaviour from venues that use this series of mixers.

I would expect the aux outputs to be at line level, but I can't get a signal at my camera when set to 'line'. When I switch to 'mic' level at the input stage, the signal is distorted (later, I can see the waveform is heavily limited/clipped), but the aux output levels on the mixer and input levels on the camera are all safe and show no clipping.

I can't find much info from Canon about the audio inputs on my camera that might explain what's going on, but I haven't noticed this issue with other mixers. Can anyone suggest possible causes?

Many thanks
Ben
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Re: Issues with aux output from Allen + Heath QU series - any thoughts?

Postby Hugh Robjohns » Wed Oct 30, 2019 2:58 pm

There are no dedicated Aux outputs on the Qu mixers, of course, just assignable line outputs, but they are all at nominal line level (+4dBu) with peaks to +22dBu, and from actively-driven symmetrical balanced outputs.

There's really nothing odd about them, although the Qu manual stresses the need to link the cold side of the output (pin 3) to ground (pin 1) if feeding an unbalanced destination... so I wonder if that is what's causing your problem? Are you feeding an unbalanced input on the camera?

The difference between the Qu and the dedicated aux outputs of most budget and mid-range mixers is that most of the latter provide 'impedance-balanced' which put all the signal on the hot side with just the ground reference on the cold. Thus the interface is fully compatible with unbalanced destinations by default.

So I think what we need to know is what kind of cable are you using, and how are you connecting into the camera?

If you're near a Qu mixer, then as an experiment, open the XLR connector and use a piece of wire to short between the middle pin (3) and the one the cable screen is connected to (1)... and see if that makes any difference!
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Re: Issues with aux output from Allen + Heath QU series - any thoughts?

Postby MarkPAman » Wed Oct 30, 2019 3:29 pm

What range of input level do the meters on your camera display? Analogue outputs from the desk (with 0dB on the meters) will be +4dBu.

When you say Aux outputs, which outputs are you referring to?

The "2 Track" output, which you may be using has 2 x TRS which outputs a copy of L&R , has no independent volume control, and if I'm not pushing the PA hard it tends to be rather low in level , so I tend not to use it.

I'd recommend the "Alt Out"*, also on TRS, which IIRC can be set up to output pre or post fade out of most (if not all?) of the desk's busses. This has its own dedicated hardware level control on the desk too which is useful. If a spare mix is available, I'll usually set that to be post fade and route that through the Alt Out for recording, to allow a different balance for the recording to be set up.

Qu series desks also have their Mix outs on XLR and the larger ones have Matrix outputs on XLR too.

* Not available Qu-SB though.
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Re: Issues with aux output from Allen + Heath QU series - any thoughts?

Postby MOF » Wed Oct 30, 2019 4:44 pm

Are you getting NO signal when fed a line level signal?
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Re: Issues with aux output from Allen + Heath QU series - any thoughts?

Postby pk.roberts » Wed Oct 30, 2019 5:41 pm

Can you tell us the model of the camera? Does it have XLR or mini-jack audio connectors for the audio inputs. Are you manually controlling the input levels on the camera or using 'auto' level control?
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Re: Issues with aux output from Allen + Heath QU series - any thoughts?

Postby Ben » Wed Oct 30, 2019 6:00 pm

Thanks, all, for the thoughtful responses.

I'm using an XLR cable to the camera. Yes, it's one of the general outputs from the mixer being used as a pre-fade send of the relevant channels.

The input gain is all fine - manual adjustments and peaking well below what I'd normally want. Reducing the input gain gives me a quieter distorted signal. I don't know the mixer inside-out (and neither do most of the venue techs, who are sometimes agency), but the meters for the output layer I'm using are all fine and there is no issue with the output to the house.

I don't have the mixer in front of me, but perhaps the 'line' input setting on my camera was giving me a tiny signal, rather than no signal. Not workable, even with maximum gain applied. During the most recent shoot, my assistant was in the next room with the same camera model and a little analogue Soundcraft mixer instead - everything was as expected, line level input at unity, adjust aux send, go.

Perhaps I'll explore Hugh's suggestions and the Alt-output idea next time I encounter this mixer. I'm usually thrown an XLR cable with minutes to spare before shooting, so it's always a bit nervy. I guess I should monitor the output layer directly with some headphones and, perhaps, confirm an issue at the camera end.
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Re: Issues with aux output from Allen + Heath QU series - any thoughts?

Postby Hugh Robjohns » Wed Oct 30, 2019 6:24 pm

The most likely reasons for low level and distorted sound are either that:

1. The connection is 'one-legged' -- meaning that it's not receiving or handling one side of the balanced signal -- causing distortion and low level.

2. The camera's input impedance is ridiculously low which is stressing the output drivers of the sending device, causing distortion and low level.

3. There is phantom (or even plugin-power) on the connection which is causing biasing issues with the sending device's output driver, resulting in distortion and low level.

4. Very unlikely, but it is just possible that the Qu console is putting out digital 'shash' on the line outputs which is upsetting the camera's electronics.

Beware that not all devices with XLRs actually have properly balanced inputs...

This is all just guessing, though, without more information about the camera and its inputs.

Some experimental connections via a line isolating transformer box -- like the ART DTI -- would eliminate and/or identify all four possibilities...

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Re: Issues with aux output from Allen + Heath QU series - any thoughts?

Postby Ben » Wed Oct 30, 2019 7:46 pm

Thanks Hugh, I'll explore.
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Re: Issues with aux output from Allen + Heath QU series - any thoughts?

Postby James Perrett » Wed Oct 30, 2019 9:05 pm

Remember that, as a backup, you can record to a disk drive or USB stick on the QU itself and then match up the audio in post. They're a little fussy about which drives they like but, in my limited experience, are fine if you get yourself an approved drive.
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Re: Issues with aux output from Allen + Heath QU series - any thoughts?

Postby Dave B » Thu Oct 31, 2019 12:43 pm

Ben, if you want some other mixers to try out to debug this, I'm only down in Maidenhead and have a couple of similar digital mixers. You're welcome to bring the Cannon down and see if we can recreate (then hopefully, fix) your issue.
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Re: Issues with aux output from Allen + Heath QU series - any thoughts?

Postby Ben » Sat Nov 02, 2019 12:54 am

Very kind, thanks Dave.
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Re: Issues with aux output from Allen + Heath QU series - any thoughts?

Postby CS70 » Sat Nov 02, 2019 12:18 pm

My bet is on the cable pin connections. You may also check on the camera which kind of line level is expecting (the lower -10 dBV "consumer" rms voltage or the higher +4 dBU "pro" one), sometimes you msut manually select the right one two.
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