Low input on 1 channel

[Dave71Participant]

Hi. Just setup for a garden party and I’ve lost a lot of gain on one of my xlr inputs. Instantly swapped cables, DI’s & source etc and narrowed it down to the xlr input on the desk. The trs input is fine, just need to boost the gain considerably when using the xlr. First noticed the problem when I had ch 15 & 16 linked and was getting very low signal on ch 15 when I had my phone connected for testing. Still have the channels linked but unlinked the gain to balance the channels for now.

Dry joint or something?
Cheers

[Mike CParticipant]

Do the PFL levels show the difference in gain or is it on the main outputs where you notice it?

What is the source?
About how many DB difference is there between channels?
Just to double check take a mic, set a handful of channels to all the same gain
and going by the PFL metering go down the channels one at a time watching each level.
Using the PFL level metering as a reference will eliminate level changes from channel
processing.

If the levels are about 6db lower you may have lost either the + or – side of the XLR input.
Aside from the lower level how does the input sound?

[Dave71Participant]

Thanks for the reply. Yes, pfl levels were down on channel 15 in comparison to 16 when fully linked. From memory the difference in gain to get the levels the same was about 6/7 DB. Sounded fine with the gain boosted and I only tried the input with a feed from a laptop or phone . Went straight in with a mini jack to xlr to rule out a DI or cable fault.

Guess it’s time for a trip to my local trusty electrical engineer. As I said, the TRS input is fine just the xlr.

[Mike CParticipant]

How is the mini jack to XLR wired?

When using the TRS input was it with a balance 1/4 TRS plug or a unbalanced
14 TS plug.

[Dave71Participant]

So…..now I’m home, have done some rigorous testing and found..

Booted desk up and set all 16 inputs to the same gain with all gates, eq etc etc off.

With nothing connected Channel 15 is crackling and hissing when I pfl the channel. This goes quiet after a couple of minutes.

Connected a mic via xlr and ran across all channels, channel 15 is a lot quieter.

Plugged a guitar into the TRS input using a standard guitar lead, channel 15 is again quieter than all the others.

Connected 2 active DI box’s on channels 15 & 16 and enabled phantom power. Still getting phantom power but a small amount of crackle and hiss on channel 15. Connect my phone to said DIs using a stereo mini jack to stereo xlr lead and the noise goes but the level is a lot lower on Channel 15 than any other channel.

What’s more confusing, I’ve rebooted the desk a couple of times and the channel noise has gone but the level is still very low. If I connect a stereo mini jack to 2 x xlr the noise went too.

To compare, the gains are set to 27db across all channels but 15 has to be 41db to get the same level.

Thanks.

[Mike CParticipant]

With that testing I’m going to say it’s the input preamp on ch.15.

[KeithJ A&HModerator]

@Dave71

Unfortunately based on your findings, that does sound like a hardware issue with either the socket or the preamp.
Without looking at it, it’s impossible to say for sure what’s causing it but to get this checked out and get your Qu back to full working order, please contact your local distributor.
In the UK, this is Audio Technica – https://www.allen-heath.com/dist_type/united-kingdom/

Thanks,
Keith.

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