Hi PRF. I come to you today with a mystery. Over the weekend, I was running sound at live theatrical/musical event, and kept getting horrible feedback from a piezo pickup on an upright bass. The situation became problematic when during a dress rehearsal I opened up the bass channel ”sound check having gone fine ”only to assault several celebrated performers (and an auditorium with freaking *children* in it) with screaming nightmare feedback. Three days later it feels like somebody punched me in the ear, and I feel bad about hurting the hearing of innocent bystanders. What happened?Here s what went down:The upright bassist wanted a monitor mix. During the events in question his signal was the only signal being fed to the monitors. Feedback occurred when the monitors were 20 feet away (needed to be offstage). When we moved the speakers closer, the problem persisted.This was the signal path: Upright bass with a piezo pickup that sits between bridge foot and soundboard (http://www.davidgage.com/store/product\_info.php?products\_id=28) > DI box to low-impedance balanced output > series of balanced connections > mic input of Soundcraft mixer (healthy prefader level) > Aux 1 line level out > powered JBL 317 monitor = PIERCING FEEDBACKWhile I was turning up the monitor send, the bassist said that he could barely hear himself. Above a certain level, feedback started instantly, and loudly.I didn t EQ much, and only subtractive. The feedback was more intense when I didn t have the 100 Hz high pass filter engaged.When I played music through a DI box into the same channel, I was able to turn in up to a reasonable level though Aux 1 easily. The solution we settled on was running his bass amp as a œmonitor. I took a balanced line out from the head, ran it into the mains, and called it a day. No feedback, despite high volume. So ¦ am confused about mic vs line level at any point in this chain? Are piezo mics just really finicky? Am I missing something obvious? Was something broken? What s up?

That's pretty strange.Two wild ass guesses:1. Something changed on his end between soundcheck and the dress rehearsal. 2. Somehow you ended up with a feedback loop in the mixer itself or in your 'series of balanced connections'.String bass with a pickup can be touchy but it shouldn't have been completely uncontrollable with the instrument 20 feet from the wedge.The pickup in question has an output impedance of 10M, running it through a direct box to balance and lower the impedance like you did was correct.or Gremlins. Could well have been Gremlins.

I m not an expert in piezo pickups, but don t they work more or less by converting the mechanical resonance of the instrument into an electrical signal? I wonder if something with the interaction between the monitor system, room, and instrument were causing a sympathetic vibration in the instrument.

Thanks, projectMalamute. Glad to hear that this is weird.The pickup doesn't have a level control, so I don't know what the bassist could have changed.I think you're right about gremlins in the cables. Thinking back, the final snake that carried signals to the mixer contained mixer aux sends and master outs. So maybe there was crosstalk between those outs and the mic ins.

I would buy that. Bass is a big vibrating box, and the problem definitely got worse when the speakers were louder.

Sure, these things are touchy but I have never found them to be completely uncontrollable in the manner the OP is describing. Especially if it was fine at sound check and with the guys amp. Sounds like something else is up to me.I'm still betting on Gremlins.

Adam P wrote:I m not an expert in piezo pickups, but don t they work more or less by converting the mechanical resonance of the instrument into an electrical signal? I wonder if something with the interaction between the monitor system, room, and instrument were causing a sympathetic vibration in the instrument.I am an expert in piezo pickups, and that's generally how they work. they produce current in respect to compression forces.If the monitors/mains in question vibrated the stage that he bass rested on, that mechanical vibration would translate to the body of the bass and the pickup, and the loop would feed back really quick.the bouncier the stage (ie: those terrible temp stages), the even worse this effect.The player could rehearse all they want, but unless they were on that same stage it's not going to react the same.

The whole bass amp = fine, monitor = bad makes me think the monitor was coupled to the stage surface better and transferred the vibration to the bass. If it sounds crazy, try taking an acoustic guitar with piezo pickups and then put whatever speaker you're playing into directly on the desk.

My thoughts are a little off, but could the monitor phase be flipped? It'd explain why the low end isn't coming through and all you get is high squeezing. Else the woofer is possibly blown.

Another vote for "the amp had powers that your pathetic console lacked."I don't play any acoustic thing, but I own a SansAmp Para Driver, and the instructions glow about how, once you identify the feedback frequency of the stage where you're playing your piezo-ified acoustic instrument, you can just set the parametric midrange to the frequency that's feeding back and turn it way down.I would imagine acoustic-oriented amps have some kind of intelligent way of accomplishing this. Multiband compressor or the like.