Feedback from 260?

rschultz

Hi,
First time hooking up new mixer in new facility. Initially took mixer to powered speakers directly with iPod as ONLY input to mixer to get basic sound check. Worked good for 10-15 minutes.
Then I put the 260 in the chain - 2x2. I've been playing with the GUI for a few weeks now, setup was pretty basic, all from the wizard. I have a AFS module in there, but I'm 90% sure it was turned off. Custom passive speaker settings. I didn't adjust anything yet.
Within a minute after powering up the speakers, they started to feedback. Instinctively I killed the 260... that didn't help. So I ran up and killed the speakers. I restarted the 260, checked the board (all faders down). Turned speakers back on... same thing.
Restarted everything one more time and it worked fine. It's difficult for me to know where to start... is this a 260 problem, a speaker problem or power problem? I can't believe it has anything to do with the iPod or mixer. Both speakers did this at the same time, leads me to believe it isn't a speaker problem, but probably the 260. What would cause this?

Thanks.
Ryan

PS. Other thing I noticed is that the volume level was a lot louder without the 260 than with it. Any ideas?

Dennis

What was the frequency of the feedback?

Dennis

rschultz

It was low frequencies... maybe 100-150 Hz... just a guess.

Dennis

Was it low frequency feedback or low frequency hum?

Dennis

rschultz

I should say it was A low frequency, same frequency on both speakers. But it was feeding back, as in it started soft and then started to get louder and louder and louder.

Dennis

In addition to your speakers, you have three pieces of gear- an ipod, a mixer and a DR260. Without a microphone, actual feedback is unlikely. Set up the system and make it happen again. One at a time, physically move each piece of gear. Move the ipod away from the mixer and DR260. Move the 260 away from the mixer and ipod etc.,etc. We are looking for possible interactions between transformers in the various pieces of gear.

Dennis

Gadget

Pardon me for interrupting... BUT the 260 uses relays that DE-energize when the power is cut...SO lets verify this... you SHUT DOWN the 260... using the power switch on the BACK of the 260... right? So, with the 260 shut off.. the relays DE-energized the low howl continued...and got LOUDER? Are the speakers and the Driverack and mixer and ALL the components on the same power? I mean the SAME circuit? If not you likely have a ground loop...

If they are all on the same power try powering the speakers up with ONLY the cables that run from the Driverack... if quiet, plug one at a time back into an UNPLUGGED 260...if still no buzz...WITHOUT anything hooked up to the inputs... power up the Driverack... still no buzz.. plug in one input at a time.. otherwise report back on when the buzz DID return...

Sorry to jump in Dennis, thanks so much for your input...something just wasn't right here...

G

Dennis

Feel free to jump in, Gadget. I am running out of ideas. What Ryan has described does not sound like ground loop buzz. It sounds more like setting a microphone on top of an amplifier, right over the transformer. But it is something else.........a ground loop would not increase in volume.

Dennis

rschultz

Thanks for all your help. I may not be very seasoned with pro audio, but I am an engineer experienced in troubleshooting all types of technical issues. This problem is perplexing because I am having a hard time logically understanding WHERE it is coming from. I agree with gadget that if by shutting down the 260 should have stopped the problem if it was the 260. But why would the speakers do that on their own? Could the 260 start the problem and the speakers make it worse? Maybe my memory is bad on that detail... it all happened so fast. I'm definitely writing stuff down if this happens again.
The speakers are powered. I am reasonably sure the speakers are on a different circuit than the mixer/260. I will determine this. Either way, one 20 Amp circuit isn't enough to handle all the sound booth components AND the speakers. They have to be on different circuits, although preferably on the same leg of the electrical box.
The signal to each speaker is ran through an aux on 2 different snakes, which are in 4\" PVC under the floor. What if it ran past a HVAC unit or something like that, could the snake pick up interference that way?

Dennis

If it turns out this is nothing more than a ground loop, don't worry about using more than one circuit. A \"grounding scheme\" for a multiple circuit sound system isn't rocket science.

I am not clear on \"The signal to each speaker is ran through an aux on 2 different snakes\". Please elaborate.

You said when the problem happened before, you shut off the 260 but the problem continued. Perhaps you would have had better results disconnecting the 260's power cord which would have disconnected the 260's ground connection. Again, this is assuming the problem is a ground loop and I'm still not completely convinced.

You said \"What if it ran past a HVAC unit or something like that, could the snake pick up interference that way?\"
It's possible but not the first place I would look. If you have the time, you could check the 4\" PVC route.

Dennis

rschultz

32 channel board.
2 separate snakes, 16/4 each. Wall box on each side of stage. Speaker on each side of stage. So I ran the signal from the 260 through one aux on each snake to each powered speaker.

If it is a ground loop, what is the solution?
If this happens again, what should I do in what order?
Maybe mute first, then 260 power switch, then 260 power chord, then run up and kill the speakers... that takes a lot of time.

Dennis

If problem happens again:
mute outputs of 260
Disconnect mixer signal to 260 (inputs)
Disconnect 260 signal to speakers (outputs)
Take note of when noise stops
Report back to us.
All of these operations can be done at the DR260 and we are assuming that, once disconnected, the speakers wont make noise by them selves. Don't reconnect anything without powering down.

Dennis

rschultz

Yes, I agree. I should be able to stop it at the AV desk without turning off the speakers.
Thanks.
Ryan

rschultz

Played music and messed around with the EQ module through the GUI for about 45 minutes over lunch hour, never did feedback.

Dennis

It knows we're watching.......

Dra

Feed back can be caused by a hard loop in the electronics, not just from overly hot mics. Did you possibly have a send/return, aux, Efx, etc routed to \"over-lap\" on itself in some area of the mixer? For example, signal from channel 1 routed to aux 1 for proccessing and back into ch-10, aux 1 pot up on channel 10... hard loop.

DRA

rschultz

Nope. Only thing I had going into mixer was iPod. Only thing coming out was LR to DR260.

Dennis

What input(s) was the iPod connected to?

Dennis

rschultz

I'm currently using stereo input 4, which is channel 16 on GL2400. The GL2400 has dual stereo inputs on ch 15/16 for a total of 4 stereo inputs. 1/3 are unbalanced, 2/4 are balanced (I'm not sure why). All 4 have a pair of TRS connectors.
It's pretty easy to set the gain structure on the mixer to get unity on the channel and master meter. But I haven't done that on the 260 because I didn't understand how. I think I have a clue now... hopefully.

Dennis

Not that this has anything to do with your problem, but your mixer has two stereo channels, 15 and 16. ST1 and ST2 are right and left of ch 15 and ST3 and ST4 are right and left of ch 16. These two channels can also be used as mic channels (with slightly reduced features) which are controlled by the \"mic gain\" pot at the top of the channel strip. This explains the XLR connector on ch 15 and 16.

Dennis

Dennis

Sorry, that was incorrect information. There are 4 independent stereo channels with ST1-ST4 being level control. What I said about ch 15 and 16 doubling as mic channels is correct.

Dennis

rschultz

Over lunch I attempted to set the gain structure properly. First, I set the HPF at 50 Hz, slope of 18 db/oct (not that I needed to, speakers have own DSP limiter on each of the 3 amps/speakers). Then I turned on the pink noise from the console and set it such that the main fader levels (4 LED) read +6 with just barely clipping. The main console meters (12 LED) read solid +9, no clipping.
The 260 saw the exact same input level. The output level was probably +15-18 db. I'm not sure where clipping is on the 260, but there were no read lights being lit up so I left the 260 channel gain alone at unity. Why is the output level higher if the gain is set at unity?

So this seems to be about right. The problem now is that when I play something on the Yorkville U15P speakers, it POPS with anything past unity on the console. What causes speakers to pop? The gain on the speakers is set at unity... I also tried it at one notch above unity. I think there is something wrong with the speakers, the volume just isn't there from what I think it should be. Any ideas?

Thanks.
Ryan

Dra

Back the x-over gains back until the output meters read the same as the input meters. 6-9db is not a little bit.
How did you set unity on the channel with Ipod?

DRA

rschultz

The iPod channel has it's own meter, I gain it up/down so that unity on the fader is unity on the 4 LED meter.

But when I was looking at the gain in the 260, I was using a pink noise generator from the console, directly to the LR meter, no channel associated with it. I gained it up/down such that the main faders were at unity and the main console 12 LED was at +9 db.

Although maybe it would be better to do everything at 0 db. Set the console reading at 0, the 260 output to 0, etc.

Ryan

rschultz

So I guess my question is why does the DR put out more than is going in? I never adjusted hardly anything, just custom settings from the wizard. To gain down the output, I would have to do it at the Post Xover, right?

Oh wait. If I had +9 coming in, but the M was set at unity, then that is the problem right? If +9 coming in, shouldn't I set the master to +9. I'm confused, what is the M used for?

Dennis

I don't have an answer to your last question at the moment. I would need to fire up my 260 and figure it out and I won't have time before tomorrow but I am sure someone else here can answer it sooner.

Do you feel there is a down side to setting your entire gain structure at unity?

Dennis

rschultz

I'm not an expert, the only down side to setting system to unity is possibly not as much headroom as setting to clipping. Still should be plenty of headroom. But with powered speakers, it is difficult to set to clipping because you don't have any way of doing that with the amps/speakers.
So for my situation, setting to unity makes more sense. But as I said, I'm not an expert... closer to a novice.

Dennis

I just skimmed this thread looking for the make/model of your speakers and either I missed it or you didn't share it. Please refresh my memory.

Dennis

Dra

What you are doing, is setting the channels to unity (one at a time) and the main to unity when the channel output is at unity (PFL), then the DR260 input at Unity (yada yada) and then the outputs at unity (yada yada yada). All this with amps off. The next is tricky because you have powered speakers, so I recommend the unity setting for them and be done with that part.
Now (amps still off), you would set your limiters. Shoot pink noise through the board and bring the meters into pretty steady but bouncy max (maybe 50% cycle). Now, check the 260's input meters (GUI is better because the graduations or smaller). They should be absolutely no higher that +18db (I think +22db IS clipped (+20 max) and that is BAD). Shoot for +16 or less. This is where the input gains are used to bring down the level coming into the 260. The crossover gains control the output levels (also +20 max). The input gains also are used to reduce the noise level when needed. I don't feel comfortable advising on the limiter setting with powereds, because there are so many things to consider. Maybe Gadget will chime in.

Does that help?

DRA

rschultz

Yorkville U15P's, mono LS700P (not installed yet)

Dennis

I am going to suggest a slightly different route to get to the same destination.with the speakers off, set all stages of the mixer and driverack to unity with pink noise. all meters should be around unity. Put earplugs in. Turn level all the way down on U15P. Power up U15P. Slowly raise level of U15P until clip light flickers and then back it down until clip light goes off. Kill pink noise and run music through system and judge if level seems adequate. Now go to the last page of the manual and appendix A.10 will tell you how to set limiters. When none of this works out for you, do what Dra said.

Dennis