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!

You've reminded me I use to have a Peavey amp with hot and cold transposed!

The mixer is an Allen and Heath PA20 (unpowered):http://www.allen-heath.com/UK/DisplayProduct.asp?pview=40

The induction loop is by "Current Thinking" (I dont know which model):http://www.current-thinking.com/professional.htm

Ok.. thanks for the links.
The PA20 - not a bad unit (think A&H make them in China)- thought about getting one for a local band, but the fixed low mid eq put me off, other than that it covers all the bases.
I've had a look at the pdf manual.
It's wired the 'normal' way. In fact it has a nice cables & connectors section, worth a look as it shows every combination of connector and how to wire it for use in a balanced / un-balanced way.

It would appear that the stagebox at the stage end may have been 'home made'.
The box at the mixer end.. all XLR's yes?
I'm wondering if that was the multi-cores original stagebox, does it have male (pins) XLR's or female (holes)? If it's a 'stagebox' it will be female, and thus to connect to the desk you would have 12 female-to-female XLR leads! like wise the returns would also be a bit 'backwards' and I'm woulding if that's why the wiring may a bit 'iffy'.

A tiny problem with having all XLR's at the desk end means that the desk is 'stuck' in mic level, as the line level input on the desk is on a balanced jack. There is no 'pad' button so it might be very easy to overload and distort when plugging in stuff at the stagebox.
Something to be wary off.

The returns..
4: 1 & 2 Main outs to the PA amps?
3 - Foldback 1?
4 - now here is where things get interesting..
It might be coming off Foldback 2, or it might be coming of the aux send, and this might be your send for the loop amp.
Without seeing it, 3 and 4 might be the other way around.
The foldback sends from the desk will be 'pre-send' that is to say they offer an independent mix. The channels faders will not have any effect on these signals. That's why they are used as foldback sends. This would not be ideal for you loop amp.
The aux send however will be controlled by the channels fader. Normally these are used for send signals to reverb / echo units. Be it would be ideal for your loop amp, as it mirrors what actions are being sent to the main PA. If the loop amp needs less music, and more vocal, you adjust the aux sends levels accordingly.

Has this made any sense? I know it's sounds complicated, but it isn't really.


I don't think the guitar / amp / loop amp is too much of a problem, as this would happen more or less straight away. The feedback only occurs when you add the Laney into the equation. There's a slim possibly that the pick-up for the spring reverb (Horrid sound) might be getting the loop amp. Try killing the reverb in the amp - not that you should be using it!
I still think it's a gain, and / or cabling issue.

I also looked up the loop amp link.
All the units offer 3 XLR in's. However only one of the three in's offers a mic/line input. It's this input you should be using with the desk.

Another desk issue, and worth looking at, not so much to do with your current problems, but may improve the whole sound of the system.
Each input has a 100Hz lowcut filter. These should be mostly turned ON. It will reduce bass feedback, and 'popping' problems on the vocal mics.
The only channels I wouldn't put the filters on, would be bass guitar, kick drum, possibly snare (try with / without, and keyboards (again try with / without).

Well that should give you plenty to get your teeth into.
Would I be right if I guessed you are UK based? If so where-a-bouts?

Good-luck,



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The Sonic Energy Authority - a sound investment

As suspected, and with thanks to you all but especially "Sonic" for such a compreshensive trawl of the possibilities, it's all down the the induction loop being picked up by the gear.

Turn off the loop and everything is fine. I'm looking to re-route the loop wire to try to give a "dead" area around the "stage" area, at present I thing the loop goes straight underneath. Also the intermediate stage box is currently directly on top of the loop amplifier, and this cannot be helping, so I'm looking to move things about but I'll have to make some new wires up to give the necessary freedom of movement.

I have not actually found any miswiring of connectors so far, but that's now a work in progress.

For information here's the setup now.

The wiring is using commercial multicore cabling (all cores are twin and earth, with unique idents) with XLR/Jacks at one end and a stagebox at the other.

The "intermediate" box provides 12 feeds to the mixer (1-12) and receives 4 feeds back (A,B,C,D). A and B are main PA L/R, C is now a foldback line fed from the "summed mono out" of the desk, and D is the Induction loop drive from the "Aux 1" feed of the desk. The loop drive uses the line level input of the loop drive amp, I haven't dragged the PA amps out to check them but I'm confident that they work as expected.

The "stage" box may have 16 "ins" and 4 "outs" but only 8 of the "ins" and one of the "outs" are wired up into a multicore that comes out next to the intermdiate box to give 8 XLRs and a single TRS jack respectively. The 8 XLRs go straight into 1-8 on the intermediate, and the jack goes into "C" on the intermediate box to route the foldback signal to the stage box. This is also now working as expected.

So I think this situation, if not fully sorted yet, is at least in a position where we know the issues and can take some steps to improve things.

Thanks again!

John

I forgot to mention, we are using the 100hz filters on anything microphonic and by default on everything else; as you say turned off for bass, keys, piano, drums as necessary.

And I'm in the UK, Preston Lancashire!