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Spirit 8 output noise - any regular issues to cause this?


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Hi all


I have a 32-ch Soundcraft Spirit 8 sat in our main installed system, which yesterday started going quite noisy. There was enough mains hum to mask speech which was metering at about -20dB on the desk master meters - but the hum doesn't show up on any metering. If it helps, all faders were operating in the marked 0dB position.


There are two modes of hum:


One is really unpleasant and (by ear) has most of its output at 100 and/or 150Hz, sounding somewhere between a triangle and square-wave. This sound is below metering range but very distracting, present only in groups 2, 4, 6 and 8, even with nothing plugged in or routed to them. This buzz can be heard when AFL'ing the group faders. Pulling the fader(s) down stops the noise. The noise got slightly (1-2dB) worse in testing yesterday when the group was routed to Master outputs. There is some similar noise in groups 1, 3, 5 and 7, but significantly lower. As our system is currently running mono we were able to get through yesterday evening by routing to the odd-number channels only, which kept the noise to (barely) tolerable levels.


The second noise is mostly 50Hz (Sine wave, with some 150Hz buzz in there too) in the main outputs and headphone monitor circuits only - channels, matrices, aux and group outputs seem immune.


I've tried cleaning the jack connectors for the group and master inserts, output etc, and am planning looking at the desk in more detail later today.


Wonder if anyone else has one of these and has encountered the same issue?

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With any serious hum problems - the first thing to suspect is the power supply.

However, first prove this by disconnecting everything from the mixer - and listen with headphones.

Normally if a smoothing capacitor in the PSU fails - you would hear the result on the main L+R outputs with the faders down - so your reported differences on the groups are a little odd?

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Okay, well that all confirms this morning's thoughts about where to start. Well, at least it's not appearing to be a common failure narrowed down to one part, so this should prove an interesting afternoon.


Going in with the screwdriver shortly - will shout up with any helpful questions/findings as I go.


Thanks guys!


Edit: More info


So I just unplugged everything from the desk and it's quiet as new. Curiously, adding one of four cables from one particular room in the building to any of the groups sets off the problem. Yet the desk remains quiet even when plugging in that same cable to any other MXLR on the desk chassis.


That there is a grounding issue between two different rooms doesn't surprise me. That the issue only affects anything from that room being plugged into specific outputs on the desk is a mystery.


Also - Anyone happen to know here, is it normal for the Solo LED to remain dimly illuminated even when nothing is being PFL or AFL'd? It flickers more brightly for a moment whenever the Mute Group 4 button is toggled. Wonder if that's odd or just a "feature" of this design - or both?!

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So I just unplugged everything from the desk and it's quiet as new. Curiously, adding one of four cables from one particular room in the building to any of the groups sets off the problem. Yet the desk remains quiet even when plugging in that same cable to any other MXLR on the desk chassis.



I am not conversant with the Spirit 8 but the symptoms are still pointing towards a grounding problem around the group outputs, I would still start with 1 and 2 above. The Solo LED problem may well be related.


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So after a day spent taking the desk apart, and with the help of some VERY helpful tech-support guys on the Soundcraft end of the phone line, we've solved the problems, or so it seems from testing this evening.


Resolving grounding in the desk:


Step 1: Make sure all pots are securely fastened to the control surface - the metal chassis is used to provide grounding and/or screening.

Step 2: Turn the desk over and take the back plate off.

Step 3: Re-seat all ribbon cables.

Step 4: The central support pillar (seems to be made of aluminium) was loose - tightening this improved both structural and electrical connectivity of the chassis components.

Step 5: Tweak the internal grounding. The grounds (incoming from PSU, and internal from desk) so far as I could tell were commoned to the PSU connector on the chassis - but the paint between the chassis and the connector prevented the chassis becoming a part of that bonding. Perhaps intentionally - my impression was that the Soundcraft guys I spoke to couldn't be sure of this. So as an experiment, I removed the PSU connector and the adjacent lamp connector from the chassis and scraped some paint away from the screw holes for the lamp socket. Each of the (two) bonding cables were moved from the PSU socket to between the lamp connector and the chassis - then both connectors were screwed back to the chassis. Before doing so I made sure to remove the metal/paint swarf I'd scraped off - no need for that getting into anything!

Step 6: Scrape off some paint around two matching screwholes on the back plate and the mating surface on the sides of the console - then put everything back together.


Putting everything back together and testing seemed rather daunting given that I was doing this with less than an hour before the next event was starting that would ideally require use of the desk! Unbelievably, all 518 pot-caps and the 42 fader caps were all still where I left them and I even managed to remember where they all went without having to check!


Testing the desk without anything else connected showed that the long-known issue we had found, where a very low-level mains hum in both main and headphone outputs, is no longer with us. The mains hum resumed in the groups as soon as the offending cables (found over the weekend) were plugged back in, albeit at a much lower level than experienced over the weekend - low enough in fact to not be heard in the residual background noise in the building. Even so, the mere presence of hum still bothered me so...


Resolving ground loop between systems:


This was a simple case of lifting the ground connection in the XLR connectors feeding the second system. As I was sat at the desk it was easiest to do at the sending end - technically I'm not sure whether it would be any better to do this instead at the receiving end - but either way the hum is now gone and the signals are apparently still present and correct wherever they're needed.


The alternative was to use our spare 1:1 isolation transformers left over from a previous radio mic installation - but this would have taken up more space and cabling than we could free up this evening - and electrically I've achieved the same thing with the grounding that would have needed to be done anyway - I'd tested these yesterday before deciding to spend time on the desk itself.




Aaaanyways - unless someone can tell me I've done something silly with all of this, I'm happy as we no longer have any mains hum in our system(s) and my world is a happy place. Thanks to all those who helped with knowledge both here and at Soundcraft.

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Apparently the solo light thing is not an uncommon thing according to Soundcraft, caused by leakage of current through the number of diodes that can drive the AFL/PFL main LED. It's a small issue that's bugged me for ages, but it doesn't affect sound or function in any way I can find so I'm happy to leave it for now.
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