pritch Posted June 20, 2011 Share Posted June 20, 2011 Sound isn't my forte, so go easy with me! I've set up a feed into our desk for a bassist; there's a Behringer DI100 sitting between the bass and his amp, with a balanced line going off to the desk. After I'd set this all up (but with no instrument connected), I noticed a tendency for the house PA to start squealing. If I pressed the ground lift switch on the DI, the squealing was cut back quite a bit, but not completely. I then stuck a -20dB pad in, and that eliminated it. All of this was done without the bass connected - unfortunately the player and I haven't managed to both be there at the same time just yet. Last night, the player was in, and I've had reports back that there's been a bit of squealing coming through - not huge amounts, but it did apparently sound like feedback was just about to kick in. I've suggested that they put in the other -20dB pad on the DI, but I'm not sure just how much signal that's actually going to leave! I'm fairly sure that it's not a mic picking this up as when I first noticed it, there were no mics connected at all. This is going to be a case of me doing something dumb, I'm certain of it - but what? Link to comment Share on other sites More sharing options...
MarkPAman Posted June 20, 2011 Share Posted June 20, 2011 Is there an induction loop system nearby? That's the only way I can think of feedback getting into the system as you've described it. I know those DI boxes are good at picking up mains hum if you put them on top of the transformer in the bass amp - usually picking them up & moving them to the other side of the amp does the trick in these circumstances. Link to comment Share on other sites More sharing options...
pritch Posted June 20, 2011 Author Share Posted June 20, 2011 There is an induction loop, yes; I might try turning off the loop and seeing if that helps - no idea what I'll do then, though - with the average age of our patrons, we'd have a (very gentle) riot on our hands if I did that. I would add that we've got another, identical box feeding an identical cable right next to it, but this one's plugged into a keyboard and is behaving itself. It doesn't sound like mains hum, but it is sitting on top of the amp; I've only got it connected with a short lead at the moment, so I think I'll dig out a longer one and try moving it. Link to comment Share on other sites More sharing options...
MarkPAman Posted June 20, 2011 Share Posted June 20, 2011 Try moving the DI box first - it's a nice easy solution! If you get no joy there, some things you may be able to try with the induction loop; depending on how you're driving it you may be able to: Turn off the bass feed into the loop, which may mean just muting the bass channel. If the loop is the problem, then the feedback should stop. Then play something (CD etc) so that it does go into the loop & PFL the bass channel. If you can hear the CD that pretty well confirms that the loop is being picked up by the DI (or something connected to it). Not sending bass to the loop is probably going to be the best solution if it is the problem - loop uses probably don't want it anyway. Let us know how you get on with that. Link to comment Share on other sites More sharing options...
pritch Posted June 20, 2011 Author Share Posted June 20, 2011 Unfortunately, our loop is fed from an overhead mic, and doesn't actually link into the stage PA in any way (don't look at me - I wasn't consulted on the install!) I'm hoping it'll be picking up the transformer from the amp. If not... well, we'll burn that bridge when we come to it! Link to comment Share on other sites More sharing options...
MarkPAman Posted June 20, 2011 Share Posted June 20, 2011 Unfortunately, our loop is fed from an overhead mic........ I was hoping that you wouldn't say that! Let's hope it's the transformer, though the word "squealing" suggests to me that it's not <_< Link to comment Share on other sites More sharing options...
timd Posted June 20, 2011 Share Posted June 20, 2011 Tried swapping the DI's? Remote possibility it's picked up some internal noise or the like. Link to comment Share on other sites More sharing options...
J Pearce Posted June 20, 2011 Share Posted June 20, 2011 Be systematic. Is that desk channel fine with nothing plugged in, with the multi plugged in, with an XLR in the stagebox, with a DI on the end of the XLR, with the bass plugged into the DI, with the amp connected but with no bass. Strip it right back and add until something goes wrong. I've had a few desk channels go into oscillation before, caused by an ageing capacitor. Link to comment Share on other sites More sharing options...
Solstace Posted June 21, 2011 Share Posted June 21, 2011 It's not unusual in my experience for such "borderline" feedback to come from induction loops fed by distant/overhead mics - usually I find the signal is being picked up in the unbalanced cabling between instrument/DI/amp and any pedals used, but moving one or all of those can help kill the worst of the problems. I also find "humbucking" pickups in the instruments can help here, as can changing the polarity of either the loop or the mic feeding it - but benefits from polarity changes are usually undone as soon as part of the physical setup changes position. Ultimately, change the induction loop feed and I think you'll solve the problem without having to turn it off. Link to comment Share on other sites More sharing options...
pritch Posted June 21, 2011 Author Share Posted June 21, 2011 Cheers, people. I am suspecting the loop more and more. I do wonder why the keyboard that's sitting right next to the bass doesn't have the same issue, though. In fact, the keyboard is using a longer run of unbalanced cable than the bass is! Now, the DI box for the keyboard is on the floor, whilst the one for the bass was (until it got moved last night) on top of the amp. It was also a few feet further away from the wall, and hence a bit further from the loop that runs directly above where the band are seated. Controlling what goes into the loop is a non-starter due to the setup we've got. I think trying to increase the distance between the loop and the bass is what I need to be concentrating on for the time being. Link to comment Share on other sites More sharing options...
Al M Posted June 21, 2011 Share Posted June 21, 2011 It is probably the bass pick-ups picking up the induction loop, hence why the keyboards are unaffected Link to comment Share on other sites More sharing options...
pritch Posted June 21, 2011 Author Share Posted June 21, 2011 It's not the pickup; it happens even with no bass plugged in. Link to comment Share on other sites More sharing options...
timsabre Posted June 21, 2011 Share Posted June 21, 2011 This situation happens at our church due to earthing of the instrument/amp. I would think the keyboard is not earthed, but the bass amp is, and that's the difference.You should earth lift the bass DI (I think you already tried that) and play with the position and orientation of the DI box. We have to run everything earth lifted and sometimes put the DI boxes on their sides or ends (I suspect due to cheapy not very well shielded DI's). And that's before the fun starts with the instrument pickups. Link to comment Share on other sites More sharing options...
fliggygeek Posted June 25, 2011 Share Posted June 25, 2011 As mentioned, you have a second DI nest to the squealing one, try swapping cables/positions/DI's until you find the defective/malfunctining part. Also, Don't trust Behringer. Link to comment Share on other sites More sharing options...
pritch Posted June 26, 2011 Author Share Posted June 26, 2011 Distance between loop and DI/amp increased. Feedback gone. Huzzah! Thanks for the suggestions, folks. I'd never have looked at the loop if you hadn't suggested it. Link to comment Share on other sites More sharing options...
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