Live sound nightmare! Please help!!!

[AV Guy]

Warning! This is a long post. I tried to be detailed...

Hello. I have been browsing this site for awhile and have decided to seek some advice from the pros...

I just started as an A/V tech for a major company and yesterday we had to do an important video conference with an important businessman in a packed 400 seat auditorium.

At the bottom of the auditorium was the stage where two people were seated and speaking, and there was to be another mic setup for the audience for Q&A. I was to be at the very top feeding these three mics into a Mackie 1402 VLZ and then my main out of that via XLR into our house PA system. Only one mic was open at a time when that person was speaking.

At the top of the auditorium I set up a Mackie 1402 VLZ mixer running a 25-50 ft. XLR cable from the main out into the house XLR mic/line input. From the Mackie mic inputs I ran a 100 ft. snake extension to the bottom stage. I connected that to an actual 25-50 ft. snake. From that I ran 4 25-50 ft. XLR cables to the three mic points (one was for backup). Two were to hardwired lavs, one was to an audience mic, an AT C87 mkII.

When I originally tested the audio in the empty room, I did not actually test this setup as I was alone and it was hard to be at both the top and bottom at the same time, so to alleviate this problem I instead hooked up a Shure SLX4 wireless lav to the Mackie (feeding into the house) and got great, healthy levels with no feedback. When I walked down toward the front I did notice it was more prone to feedback so to compensate I turned the volume down and still seemed to get a pretty decent, full sounding level.

I then tested two more wireless mics to be used as backup. The Sennheiser eW100 omni lavalier and the AT C87 mkII with the Sennheiser eW100 wireless handheld unit attached. Both systems were just like the Shure – healthy, full signals/no feedback.

On other tests I got great signal when plugging in hardwired mics up at the top of the auditorium (not running through the long snake run).

Right before the event, during our really tight 30 min. setup, I did some level checks with assistant speaking into the hardwired lavs to be used and could not get any decent signals out of them. The signal was thin and weak, and if I boosted it to any level that was audible it immediately fed back! With time ticking I dumped those lavs and decided to hook up some AT C87 mkIIs on tabletop stands instead (also running through the snake up to the Mackie). Upon testing these mics we had the same problem: weak, thin signal that fed back really easily. So as a third alternative I decided to use the wireless lavs, as they worked well during my initial tests. The assistant installed the Shure SLX4 omni lav on Speaker 1 and we did a level check. Sounded great. Full volume, no feedback. Then she put the Sennheiser eW100 on Speaker 2; weak sound/feeding back! It was showtime at this point. We decided to run with it. I hadn’t even had time to test the audience mic yet…

Speaker 1 starts to speak… no sound! It was working just a minute before but the receiver was not getting any signal all of the sudden (interference?) I ran down to check his transmitter. It was on and not muted. I don’t know what was wrong with the thing. I didn’t have time to figure it out. I decided to try one final, last resort – go back to the hardwired AT C87 mkIIs, but this time instruct the 2 Speakers to hold them up to their mouths to get a better level. I had them already hardwired underneath the table and ready to go from before. I handed the Speakers their mics and ran back up to get a level on the Mackie.

Speaker 1 speaks again; very thin and quiet. Every time I tried to get a healthy volume he fed back.

Speaker 2; sounded FULL and LOUD with no feedback!!!??? I really don’t understand this part at all. And then, when the students went up to the audience mic (once again a third AT C87 mkII already hardwired and set up) their signal was very thin and feeding back as well!

So for some reason 2 out of the three hardwired mics were very thin and feeding back, and the third, Speaker 2’s was how it was supposed to be. I tried to ride the levels the best I could the rest of the event, getting as much volume out of Speaker 1 and the audience mic as I could without it feeding back.

All of the mic inputs were set the same. No EQ on any of the channels.

I may be wrong but my only theory is that the snake run either had a short, had interference or was just too long? I just can’t understand why Speaker 2’s mic had such a healthy signal and the other two didn’t.

Question #1 is this: can really long cable runs (200+) cause this sort of problem (thin sound and tendency to feedback?

Question #2: Could a short in the long cable run or some other interference cause all the low frequencies in a signal to disappear and make it prone to feedback?

Question # 3: Do feedback suppressors help with speech audio?

I am desperate for some kind of advice or help on this as everything that could go wrong that day did! How embarrassing. I don't want it to happen again!

[mohthom]

If it's anything like trade shows I've worked, there's been heavy setup until right on the show.

Assuming your mics work (I assume you checked them on the headphone output of your mixer), is it possible anyone ran over your mic/speaker cables? Given that speaker 2 works I'd suggest it was the cable to speaker 1 that may have problems. Were there any vehicles or heavy cleaning equipment (or overzealous carpet fitters) in the hall?

If extra lighting was brought in/turned on (dimmers!) between soundcheck and the show this can cause problems (though not the problems you necessarily talk about).

Were your mics balanced? It doesn't sound like interference, but hey.

For all of your wireless mics did you use fresh new batteries (!!!!!!)? Did you test them in the same place as they were actually used (i.e. did you extend the range for the gig?)?

What do your cables sound/look like now? Are any of them broken?

Did anyone else have access to your setup or was it locked away? Is there a chance an idiot could have had a play?

MohThoM

[SparkyCanada]

Just curious.. did you try plugging the mics into different channels on your board? I've had channels act up - much like you described...

SparkyCanada

[AV Guy]

Originally Posted by mohthom

If it's anything like trade shows I've worked, there's been heavy setup until right on the show.

Assuming your mics work (I assume you checked them on the headphone output of your mixer), is it possible anyone ran over your mic/speaker cables? Given that speaker 2 works I'd suggest it was the cable to speaker 1 that may have problems. Were there any vehicles or heavy cleaning equipment (or overzealous carpet fitters) in the hall?

No. All cables were run away from any kind of traffic.

Originally Posted by mohthom

If extra lighting was brought in/turned on (dimmers!) between soundcheck and the show this can cause problems (though not the problems you necessarily talk about).

Nothing like that present.

Originally Posted by mohthom

Were your mics balanced? It doesn't sound like interference, but hey.

All mics ran through about 200 ft of balanced XLR mic cable.

Originally Posted by mohthom

For all of your wireless mics did you use fresh new batteries (!!!!!!)? Did you test them in the same place as they were actually used (i.e. did you extend the range for the gig?)?

All brand new batteries and tested before event started and there were 400 + cell phone and wirless laptop users present. I think the wireless malfunctions were due to high concentrations of RF interference with all those wireless devices...

the main question now is, what would cause two mics running on the same snake to lose alot of volume and low end frequencies and be more prone to feeding back than a third mic on the same snake? Interference? Shorts?

Originally Posted by mohthom

What do your cables sound/look like now? Are any of them broken?

Originally Posted by mohthom

Did anyone else have access to your setup or was it locked away? Is there a chance an idiot could have had a play?

Fine. As I said, no one had access to them.

Thanks for your interest in helping me troubleshoot this mohthom!

[AV Guy]

Originally Posted by SparkyCanada

Just curious.. did you try plugging the mics into different channels on your board? I've had channels act up - much like you described...

SparkyCanada

No. i mean, I was swapping out channels in a frenzy between the wireless and snake connections, but I didn't notice one channel working any better than the other.

Just curious... what could cause something like that in two channels at once in such an otherwise reliable mixer like the Mackie?

Thanks for your help btw!

[SparkyCanada]

Just curious... what could cause something like that in two channels at once in such an otherwise reliable mixer like the Mackie?

Ya got me - could be anything from a flakey solder connection to a dirty connection to ....?

I don't have much experience in repairs - this type of stuff I usually leave to the experts.

SparkyCanada

ps - how do you guys get the poster's name to appear in the quote selection? I can't figure this out.... Thanks

[mohthom]

I wouldn't call Mackies reliable . . . certainly not in comparison to the big boys of A&H, Amek, Midas etc.

The cable may be balanced, but the mics may not have been . . . often Lav mics aren't.

Is your amp screwed? Exhibiting intermittent problems on both channels . . . I'd also check both power supply to the mixer and the amp. Was power regulated or just straight from the mains?

I wouldn't expect problems like the ones you've mentioned to be caused by 'phones and wifi. Just as a precaution, in situations with heavy RF interference, it's safer to use starquad cable.

It does sound like it was your snake or lavs - simply because it's only when you introduced them to the equation that you had problems. Wireless - fine. Mixer - fine. Speakers/amps - fine. Mics and snake - problems. Have you checked all the interconnects?

MohThoM

[shortyprs]

I don't have time to read through all the details today, so I shouldn't say anything. But, I just can't help myself.

1st, there are too many details that could be wrong. I don't know what you know about proper gain staging, how you set up monitors, whether you have any graphic EQ's on the mains or monitors, etc. , but...

2nd, If someone says to me, one mic is good and the other is thin, the first thing I'd do is switch polarity on the thin mic and see what happens. If you can't do that, pan hard left and hard right and see what happens. Assuming there isn't a problem in your gear, you've described mics that are out of phase.

3rd, after you've fixed the phase issues, go back and look at the gainstaging, EQ, monitor placement issues because the raging feedback is probably a separate issue.

[tINY]

1) Yes, but at mic level with dynamic mics, and they usually go the other way (muddy). If the mixer couldn't drive the capacitive load of the cable, you could get problems like this, but I wouldn't expect it.

2) No

3) They can really be a lifesaver when there are omni lavs walking around...





-tINY

[AV Guy]

Originally Posted by mohthom

I wouldn't call Mackies reliable . . . certainly not in comparison to the big boys of A&H, Amek, Midas etc.

The cable may be balanced, but the mics may not have been . . . often Lav mics aren't.

Is your amp screwed? Exhibiting intermittent problems on both channels . . . I'd also check both power supply to the mixer and the amp. Was power regulated or just straight from the mains?

I wouldn't expect problems like the ones you've mentioned to be caused by 'phones and wifi. Just as a precaution, in situations with heavy RF interference, it's safer to use starquad cable.

It does sound like it was your snake or lavs - simply because it's only when you introduced them to the equation that you had problems. Wireless - fine. Mixer - fine. Speakers/amps - fine. Mics and snake - problems. Have you checked all the interconnects?

MohThoM

This still doesn't explain why one of the three hardwired mics sounded good with no feedback.

I guess my main question would be can long cable runs cause this sort of problem? If not, can some kind of short in the cable do this?

I am at a complete loss here... there were so many variables. I am an experienced sound engineer which is why this is perplexing me so much. I covered all the obvious bases you would think to look.

My power supply was indeed coming from the wall, however. Is it possible I wasn't drawing enough power from the wall to send down the long cable run to the mics? Could that have caused these problems?

[mohthom]

Two lavs and one condenser - the lavs didn't work? Did you provide them with the correct power? Lots of lavs don't need the full 48v and just take c.7.5v.

MohThoM

[AV Guy]

Originally Posted by shortyprs

I don't have time to read through all the details today, so I shouldn't say anything. But, I just can't help myself.

1st, there are too many details that could be wrong. I don't know what you know about proper gain staging, how you set up monitors, whether you have any graphic EQ's on the mains or monitors, etc. , but...

OK. I set my input levels exactly how the Mackie manual says to (PFL Solo mode). I was getting really quiet levels to the mixer, even with my trim way up and my faders pretty high up.

There were no monitors at all. I simply ran the three mics into my mackie and one single XLR main out into the house sound mic/lin XLR input. From there the levels were all zeroed. I was controlling only the sound being fed to the house.

There were no graphic EQ's or processors before going to the house sound input. This was a university auditorium, so it was full of speakers inset into the ceiling.

Originally Posted by shortyprs

2nd, If someone says to me, one mic is good and the other is thin, the first thing I'd do is switch polarity on the thin mic and see what happens. If you can't do that, pan hard left and hard right and see what happens. Assuming there isn't a problem in your gear, you've described mics that are out of phase.

Only one mic was open at a time...

Thanks so much for your ideas and interest in helping me resolve this problem!

[AV Guy]

Originally Posted by tINY

1) Yes, but at mic level with dynamic mics, and they usually go the other way (muddy). If the mixer couldn't drive the capacitive load of the cable, you could get problems like this, but I wouldn't expect it.

This is the answer I am starting to consider. The mics were Small diaphragm condensers (AT C87 mkII) needing phantom power. Could the long cable run have had something to do with this? I was running straight out of the wall AC. No power conditioner or anything. Perhaps too low a power draw to push that much signal with 48V?

Thanks for your reply and interest Tiny!

[loujudson]

Sounds as though you were turning up the wrong mics. But maybe not, did you check carefully that everything was patched into the correct channels?

This is the school of hard knocks - all of us have gne thru the wringer this was as we learn our craft. Can you gte a more experienced tech tp work with you on it to find out what went wrong? It would be very difficult to do this on a forum or even email...

Speaker positioning, gain settings, phase issues, wiring defects, there are just a lto of things that can happen. I have experienced having a monitor patch I did nto know about make things behave exactly as yours did. Many variables in the process!

Good luck,

L

[loujudson]

You have posted the same question in two threads. Please delete one of them!
I resonded on the other one.

L

[AV Guy]

Originally Posted by loujudson

Sounds as though you were turning up the wrong mics. But maybe not, did you check carefully that everything was patched into the correct channels?

This is the school of hard knocks - all of us have gne thru the wringer this was as we learn our craft. Can you gte a more experienced tech tp work with you on it to find out what went wrong? It would be very difficult to do this on a forum or even email...

Speaker positioning, gain settings, phase issues, wiring defects, there are just a lto of things that can happen. I have experienced having a monitor patch I did nto know about make things behave exactly as yours did. Many variables in the process!

Good luck,

L

No, not turning up the wrong mics either. I am a fairly experienced engineer so the obvious ones were already factored out.

Indeed there were many, many variables...

Now when you say monitor patch... what is that exactly?

[AV Guy]

Originally Posted by loujudson

You have posted the same question in two threads. Please delete one of them!
I resonded on the other one.

L

Thanks, I know. I did this because I wasn't sure which forum was most appropriate. Also, it seems more people visit the other forum, so I thought I would widen my snare a bit. I think I will just leave this up as i am getting different responses from both sides.

Thanks!

[mohthom]

Another re-read later . . .

You've only had problems with two channels at a time, though one channel appeared to change. This problem has only been with the hardwired mics - the wireless mics simply didn't work in the show.

I wouldn't rule out problems with the amps or power, but this is a long shot. Were you bi-amping? This kind of problem can occur when the bottom half of a biamped pair goes down. If your amp wasn't happy this could be the result.

I think it's likely to be cable. The reason that problems were only with speaker 1 and then subsequently only with speaker 2 may be that you switched cables when changing from the lavs to the ATs i.e. cable 1 plugged into lav 1 (and c2 to l2), and you chaged to cable 1 to AT2 (and c2 to LT1). Does this sound possible? You don't mention whether the audience mic was ever working correctly (you say you didn't have time to test it).

If your setup worked flawlessly with the wireless mics then I can't see that they are the issue - or the amp, mixer, speakers and speaker cable. When they didn't work, they simply didn't work. Your mics worked when wired in without the snake (as did the rest of the system). Long cables (200m) aren't a problem - bad cables are.

I'd put my dollars on it being a cable issue.

MohThoM

[aussie_techie]

my guess would be a cable problem. so test all you cables both your own and install cables.if you dont have on get your self a behringer cable tester, it will make life alot easier and get the job done alot faster.

beyond that it is very possible its the channel(s) on the desk. all desk develop problems over time, wackies often have more problems developing quicker than higher end desks.

ive had wireless gear not work at show time when it worked 10 minutes before with no logical solution, the only failsafe is to have a wired backup that you can be sure works.

wifi works in a different band and shouldn't cause interference with analogue transmitters.

if you are having any problem with a specific frequency feeding back your best bet is to notch it out with a GEQ rather than try to setup automatic feedback suppressors.

[ba1377]

im new and i dont know how to start a new post and its 3am so im tired but i have a question if anybody knows let me know. i do live sound and my subs dont work. the signal goes into the crossover and then into the sub amp then the subs. i tested the amp and that works fine, i tested the cable and that works fine, i looked at the speakers and they arent blown. the weird thing is both sub boxes dont work and they are both on separate channels of the amp, what could be the problem?

[loujudson]

Go to the top of any subforum and press the "new thread" button. It is almost as simple as setting up subs.

[SparkyCanada]

Originally Posted by ba1377

im new and i dont know how to start a new post and its 3am so im tired but i have a question if anybody knows let me know. i do live sound and my subs dont work. the signal goes into the crossover and then into the sub amp then the subs. i tested the amp and that works fine, i tested the cable and that works fine, i looked at the speakers and they arent blown. the weird thing is both sub boxes dont work and they are both on separate channels of the amp, what could be the problem?

2 Things to check yet:

1. Is your crossover working properly?
2. What frequency is your crossover set to send signal to your subs?

SparkyCanada

[rcrowley]

my subs dont work.

How do you know they don't work? Did they work before but not now? What changed between when they worked and now?

the signal goes into the crossover and then into the sub amp then the subs.

Is that a statement of the design, or is that an affirmation that you actually tested that the signal gets to the inputs and then the outputs of each link in the audio chain?

By withholding the identity of the crossover, amp, speakers, etc. you severely restrict out ability to offer you any specific advice.

i tested the amp and that works fine

Do you mean specifically the sub amp? HOW did you test it?

i tested the cable and that works fine

WHICH cable? There is more than one involved here. HOW did you test it?

i looked at the speakers and they arent blown.

How can you tell they aren't blown by looking at them?. Are you Superman with X-ray vision?

the weird thing is both sub boxes dont work and they are both on separate channels of the amp, what could be the problem?

Pretty suspicious that BOTH sides of the amp aren't sending anything you can hear in the subs. But you said that you "tested" the amp (but not HOW you tested it.)

The common-sense debugging principles apply. Substitute cables, speakers, amps, etc to determine exactly which piece is broken.

[audibell]

Originally Posted by AV Guy

This still doesn't explain why one of the three hardwired mics sounded good with no feedback.

I guess my main question would be can long cable runs cause this sort of problem? If not, can some kind of short in the cable do this?

I am at a complete loss here... there were so many variables. I am an experienced sound engineer which is why this is perplexing me so much. I covered all the obvious bases you would think to look.

My power supply was indeed coming from the wall, however. Is it possible I wasn't drawing enough power from the wall to send down the long cable run to the mics? Could that have caused these problems?

Did you ever figure out whether coming out of just one side of the mackie had something to do with the attenuated mic outputs?

[DannyL]

It is of course too late to save the show that already happened. But it should serve as encouragement to follow a few basic protocols for the next one(s).

1) Test the setup that you will be using. EXACTLY. There are many places a signal can fail:mic, cable, snake, input, output, processor, amplifier speaker. You need to have ALL these elements in place before you can consider any test complete.
2) "Thin sounding" balanced signals are most often caused by what is called a half pair:
In an XLR cable pin 1 is shield, and the signal is carried between pins 2 and 3. If either 2 or 3 has some problem: broken wire, cold solder, etc. only half of the signal will be present and it will characteristically be devoid of any bass response.
3) All mic signals should be in phase: All cables and equipment must be wired correctly so that pin 2 and pin 3 are always in the same relationship. To test this take 2 mics of the same type (ie 2 lavs) and speak into both at the same time. When you open up both faders the signal should increase. If it decreases, then one of the 2 signals is out of phase. Push the ø button to correct. You need to check every mic against the same pattern....ie: use mic 1 to check mic 2, use mic 1 to check mic 3 and so forth. If every mic but the first mic appears to be out of phase, then mic 1 is incorrect, change the phase of mic 1 and leave all others as normal.
3)When using an Equalizer for room correction never add and gain to frequencies, cut only. Listen to your voice with a mic and to a piece of music you are familiar with. Everything should sound natural and clear with no obvious holes or extra emphasis.

There is a lot written about this subject. Business meetings are not very exciting but the clients are often paying a premium price and therefore are very demanding, the rooms are often quite restrictive (hotels have their own sets of priorities that have nothing to do with your requirements) and scheduling is based more on financial concerns than technical ones. You need to prebuild everything in the shop to test it (see point #1) and then reassemble everything in the room as quickly and neatly as possible.

Regards;
Danny

[ba1377]

yea i checked everything out and they all seem to work but as it turned out i checked each speaker with an ohm meter and all 4 subs didn't measure any kind of impedance, so even tho the speakers actual woofers were in place (not visibly torn) they all went out at the same time, its been at least ten years so i guess it was about time, but i think i need some sort of limiter so when bands bring there own sound guy who knows nothing about the limitations of the place and gear that he's mixing in, and he just pushes faders up and boosts eq, there will be more protection than just me yelling the whole time to stop clipping my amps!!!

[ba1377]

hey rcrowley thanks for letting me know how to better ask questions on here, it was super late and i just buzzed through it, i never registered for a forum before so im a super newbie, but next time i will better phrase my questions so guys like you can better assist guys like me! thanks bro!