I'm an occasional member of a worship group and last night we were running through some hymns for a forthcoming song service.

Our guitarist and flautist are both miked up and going straight into the wall box that feeds to the mizer and hence the hall's PA system, which isn't set to a very high volume anyway.

I'm running my PSR3000 via a small Laney amp, to get some local foldback of the keyboard. The amp has a "DI" output which I'm feeding into my Behringer UB1002 mixer purely to use the balanced main output as a feed into the PA wall box. I do not have anything else plugged into the PSR3000.

If I now turn up my mixer I get audio feedback!

However if I feed my mixer from the PSR alone, I don't get feedback. I can in fact completely overload the entire system to chronic distortion levels without any feedback.

But if I now send the amp a signal out of the PHONO output of the PSR, I get feedback... and I can here the flute coming out of the little amp!

All the wires are checked and wired correctly.

Anyone got any idea whats going on here? Could it be some strange earthing behaviour?

I'm completely baffled!

and I'm sorry about the spelling

Try disconnecting the Laney amp. Run the 3000 directly into the mixer then use a mixer out to the Laney you can control it all from the mixer.
I don't know maybe too many preamps involved the other way.

I'm a PA guy..
You've got something cross-wired somewhere!

Think about it..
If you are getting flute in your keyboard amp, something which is not even connected to you.

I'm guessing there's a return back from the FOH desk, possibly for foldback.

Another issue..
Keyboard goes to keyboard amp - yes?
DI out of keyboard amp goes to your small mixer, and then to stage box for PA mains - yes? (as if you are using the Behringer as a DI box)
The DI out of the keyboard amp;-
Jack or XLR?
Balanced or unbalanced?
What level (guessing line level)
How is it connected to your mixer? (jack-to-XLR, jack-to-jack, XLR-to-XLR)

If it was me I would go;
Keyboard out (stereo if required)--> to small Behringer mixer --> to PA mains stage box.
For your local foldback, use the Behringer's 'control room' outs, and send that to the keyboard amp. Would have been better if the Behringer had a separate Pre-fade aux send, but at least there is a separate control knob for the control room level.

The desk for the main PA may need to 'pad' your level.. XLR's are normally microphone level. The output from your Behringer will be line level, so a little 'hot'.

BTW - hope you are using shielded cables!

no I haven't any spare gaffer tape


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The Sonic Energy Authority - a sound investment

This might be a silly question, but ...... do you have a mic plugged into the psr3000? Sounds like that's the culprit. Also ...... why use the mixer at all, if the Amp has a DI - that should be balanced.

Thanks for the idea's people. I can confirm that there was nothing plugged into any inputs the PSR.

However I've been looking at the system. There are some "interesting" wiring anomalies to do with the way the induction loop system is wired up. Apperently there were "difficulties" with that unit some time ago and whoever came in to do the job may have got it working, but now the loop system only works when both the "Aux 1" and "Foldback 1" outputs of the desk are plugged in, even though the loop system should be driven solely by Aux1. If I plug the small amp into the Foldback output of the stage box, then any gain applied to the output gives feedback.

On this basis I suspect that the loop system, which is in a cupboard separate from both the stagebox and the desk, but which also contains all the amps, is cross-wired to the foldback somehow. I can mute every input and output of the desk and not alter the feedback; the only way to stop the feedback (other than turning off the amp) is to remove either the Aux1 or Foldback1 from the desk.

Hopefully tomorrow I'll get some time to check each circuit in isolation.

?????
Shouldn't that be the Foldback INPUT of the stagebox!

As far as I'm concerned the foldback signal is coming out of the desk back to the stage, therefore so to me that makes it an output from the stage box.

Anyhow, had another fiddle today and suspicions point at the Induction Loop system, or the wiring in the vicinity. If the Loop amp is turned off we don't get feedback.

The induction loop amplifier is in the same area as the main PA amps. Found out some more about the wiring. Theres a box by this gear which is the one hooked up to the desk; the stagebox that is by the stage is connected into this intermediate box by a load of XLRs.

Still think it's a crossed wire or two somewhere. At this intermediate box, if I unplug the balanced jacks that go to the amps for the main speaker system all goes quiet, if I touch the jacks to any exposed metal in the stagebox that is connected to the mixer the signal reappears!

What fun. I'll be noseying further on Thursday.

induction loop amplifiers..

a well known cause of grief to electric guitar players, usually in the form of feedback.

Does your Laney amp have a spring reverb unit in it, and do you use it?
Are the leads connecting everything up screened?

Did you try my suggestion of keys-->mixer-->stagebox, and using the control room outs of you local monitor?

Reading you latest message:
I'm wondering if the intermediate box is a passive DI box working backwards.. that is taking the balanced out of the main desk (aux 1 going from what you said), and converting to a unbalanced high impedance source for the loop amp.
Unplugging the leads to the main PA amps will make the system go quiet!
Touching them onto any metal work of the stagebox (odd the main returns being on jacks, and not XLRs) and the signal re-appearing tells me that someone before you doesn't know how to wire a 3 pin XLR from the main mixers main outs.
Pin 1 Screen ------> Jack screen
Pin 2 Signal hot --> Jack tip
Pin 3 Signal cold -> Jack ring

I think Pin's 1 and 2 have been switched.

If you can provide some info of the main mixer it would be very helpful, as some mixers made in the US of A (Peavey, Mackie) have been know to swap 2 and 3, but that would only effect the phase of the signal , as pin 1 is still screen.

Does the venue have an in house engineer?
If so I suggest you get together, possibly re-wire the lot!




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The Sonic Energy Authority - a sound investment

I like the idea that the induction loop is being picked up by the guitar and the amp; the amp has a spring reverb. That explains the wierd signal pickup. I check that tonight.

So now we are left with the strange wiring behaviour, and I will now be checking every XLR and jack in the place!

The intermediate box looks like a normal unit with 12 xlrs feeding the desk and 4 "outputs" coming from the desk as (spooky music here) Jack sockets. The stage box is at the "other end" of a load of XLR plugs which are plugged into this intermediate box. For reasons unknown the stage box is a "16 in 4 out" all XLR unit with a big label next to inputs 13-16 of "Not in use". This leads me to wonder whether the stage boxes were purchased separately from the multicores and have therefore all been wired on-site.

The system was installed by a contractor who may be contactable but I don't know if the same guy was dragged in when "the loop went funny" some months back. By total coincidence, it turns out that I work at the same place as the bloke is the sound engineer for another local church, and whilst I don't want to lumber him with this setup on a permanent basis I'm going to seek his advice as well

Once again, thanks for your help. I think we are getting somewhere!