Tech crew blowing speakers too frequently...

[tenor_singer]

Hello!

I want to start off by saying that I have a very dependible tech director. I think she is smart, dedicated and wants to do the best job possible. She is also seeking outside help with various aspects of sound and lighting.

My question...

We tend to pop the tweeters on our speakers an average of two times a year and almost always on Wednesday of tech week. My friend, who is an engineer and TD for an area theater, said that it was because we were clipping the sound.

1. What is clipping the sound? I was under the impression that it meant turning down one wave frequency to almost zero while cranking the others.

2. I think that they are popping because some fool always trips on the plug and unplugs the system from the wall sending a loud "thwack" through the speakers. Could this be causing it? If so that was this fall's reason.

3. This spring, my TD turned on a body mic before checking it's volume on the board, which was pegged (some jack-off was messing around with the slides when she wasn't looking). The feedback that went through the system was amazing (unfortunatelly she didn't have the destroyer programmed yet). Would this cause the speakers to pop?

4. My system has a 1000 Watt Amp and two EV 300sx speakers. Are they too weak for the amp?

5. I will be showing my tech crew any responses to this post, so any additional advise you have for them would be great.


If we were to rate my knowledge of sound engineering and lighting on a scale of 1 - 10, it would be a 6. My crew's have been trained to the best of my ability, so their level is similar. My ability to translate answers will be limited, so layman's terms please :D.


Yours in theater,

Tenor

[TimoteusR]

I'll answer questions 1 & 4:

1. What is clipping the sound? I was under the impression that it meant turning down one wave frequency to almost zero while cranking the others.

here is a basic explanation. Your audio is being cut of at the tops (and bottoms ) of the sound wave this will cause your speaker to try and stay at the extreme positions (at forward or back) and switch quickly.

More info at http://www.prosoundweb.com/studyhall/studyjump.php?
pdf=clipping

4. My system has a 1000 Watt Amp and two EV 300sx speakers. Are they too weak for the amp?

I have been considering these speakers also. If you are talking about the EV sx300e their RMS power is 300W, this means you'll want to provide between 480W-750W per speaker. 600W would be the optimum. The basic calculation you can use is

RMS x 2 x 0.8 this is the minimum
RMS x 2 x 1,25 this is the maximum

http://www.prosoundweb.com/studyhall....php?pdf=watts

If you are running 1000W per speaker, that is a bit too much.


If you are overpowering your speakers and have these thumps and pops coming through without filtering that will surely blow out stuff. Your friend is right. If you are running clipped signal to speakers at that level (overpowering) it is likely something will blow.


Check the LAB archives at http:/www.live-audio.com/index.html for more info on sound stuff.

I can give you additional tips if you list your sound equipment.

--
Timoteus Ruotsalainen

[TimoteusR]

I forgot to ask about impendace. If you could post the amp's name and manufacturer it would help. If you are running the amp stereo with one sx300 per side you need to check how much that amp puts out at 8ohm.

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TImoteus Ruostalainen

[tenor_singer]

Thanks Timoteus!

I tell my crew never to turn any one wave frequency (my mixing board has a low, two midlevel, and a high) completely off. I will adjust that to mean that they should not turn the high and low too far down while keeping the midlevel high.

Will clipping occur if they turn the midlevel down while keeping the high and low too high?

I have given your request for information to my TD and she will get that to me asap and I to you. Thank you very much for your help!


Yours in theater!

Tenor

[DMXtools]

Clipping occurs when you ask an amplifier to put out more power than it can. As Timoteus said, it basically "clips" the top and bottom bottom curves off the sound waves, leaving very sharp corners from where the wave is rising or falling to the flat top or bottom where the amp hits its limit. These sharp corners are loaded with high-frequency energy, which is why it blows the tweeters.

A lot of more recent power amp designs have a built-in limiter (Peavey calls it DDT) to prevent this, but many of them let you switch it off (I don't know why). Look for a switch, probably on the back of the amplifier, labelled DDT or LIMITER or something similar and make sure it's on.

Heavy feedback is often at fairly high frequencies. Besides that, feedback can also push an amplifier into clipping very easily - and once again your tweeters get the brunt of it.

A neat trick is to put a lightbulb in series with your tweeters. An 1156 (used as a back-up light on a lot of cars and available at most auto-parts stores) works well.
Tungsten (the metal the filament is made of) has a very low resistance when it's cold, and won't interfere with the signal at normal levels. However, its resistance rises exponentially as it heats up. Power levels large enough to damage the tweeter are also large enough to light the bulb, raising its resistance. Then the bulb (under a dollar) takes the brunt of the power surge and protects the speaker. Note that this is just for the tweeter, not the whole speaker system. If you mount the bulb where it can be seen from the booth, the sound tech can watch and pull back the gain if the light comes on. In four years of running these tweeter lights in four cabinets I've burned out one bulb and NO tweeters.

John

[Mayhem]

Interesting trick John. You could also use "polyswitches" which do a similar job. Essentially, these components (which look like ceramic capacitors) go "open" when they get too hot and then as they cool down, they close the circuit again. You need to match them to the wattage and impedence of the speaker being protected and they work very well. The bonus here is that unlike the globe that needs to be replaced, backing off the power will cool down the polyswitch, allowing the gig to continue.

Also, Mosfet power amps are prone to oscillation at higher frequencies whcih also will take out speakers. This is a lot harder to detect as it is past the audiable range. I do not know all that much about it but I am lead to belive that the polyswitches can offer protection from this as well.

John - are you able to add to this?

Another point regarding clipping - this actually means that you are sending DC or direct current to the speakers which is bad. The reason that I mention this is that you may hear someone suggest "checking if an amp has DC at the outputs". This is what they are checking for.

[TimoteusR]

Thanks Dmxtools. Nice explanation. Clipping can occur in any amplifier. It can happen on your mic preamp, buss amp or power amp. Clipping basicly happens when you ask an amplifier to do more than it can. Turning up the "band" controls or equalizers as they are called by the pros (hi,mid,lo) can ask the amp to do more than it can. You should always (Remember, exception proves the rule) cut with eq as opposed to boosting. Be especially careful of boosting the extreme low (and high) end. You might not hear a change, as the speaker will not produce any sound at these frequencys (like asking your sx300 to produce 30hz) but it surely will heat up the speaker as it trys to.

Do you know how to setup proper gain structure?


If you could provide more info on your setup, we could help more. here are a couple of questions.

What model speakers are you using?
What is the make and model of your amplifier?
Is there a mixer, if yes what make and model?
Are there any other outboards such as effects or graphic eq's?
Any mics or di's?
What kind of cabling?
How much power, or how big circuit brakers are you using?
Any specific problems other than tweeters blowing?

If you go with the light bulb trick, be sure to cover it up, and/or tell others about it. There's a story on LAB about a fellow, who was a little drunk, that got scared and ran away when he saw the lamp light. He thought the monitor wedge was possesed by demons...


Ok, I re-read your post's and i would like to clear something up. Clipping happens when your amp (be it pre, buss or power) can't produce what it is asked to do. This happens to the whole signal (the amp doesn't sort the signal based on what frequencies it has. It does it's things to the whole signal). The "wave controls" (EQ) separet the signal into different bands and you can amplify them separetely. So turning some bands completely off will not cause clipping. It will just make the sound suck most times. BUT boosting any one of those controls might cause clipping. If any red lights are flashing, you are nearing the clipping point. Indicators marked PEAK are usually used.


One way to demonstrate clippping is to use a piezo and a 440hz (A) signal. You can also use this to check for proper gain structure. Just put the signal through your mixer (with amps off or speakers disconnected, the sound get's annoing after a while). Then put everything on the channel your feeding the signal to to 0db. then connect your piezo to your mixer outs. Now turn the gain up on the channel trim until you hear a "sound" from the piezo.

--
TImoteus Ruotsalainen

[TimoteusR]

A quick search on google gave me the following info. A clipped sine wave (square wave) isn't DC as it doesn't have the properties of DC. Whatever that means.

The propeblem with clipping is that it make sthe speakers try to extend all the way to the front. Then after being there for a while jump all the way back. This causes stress to the speakers. It also heats the voice coils. When the speakers heats up, the impendace grows. This makes you push the fader even more thus heating the speaker more. This can also be caused by feeding signals (frequencies) above or below the speakers production range. Like 30hz to a mid speaker capable of 400hz-3khz. Amps should have DC protection. DC damages speakers as it has some properties of a clipped sine wave (square wave).

I can't believe i wrote all that ;)

--
Timoteus Ruotsalainen

[DMXtools]

Originally Posted by Mayhem

You could also use "polyswitches" which do a similar job.... The bonus here is that unlike the globe that needs to be replaced, backing off the power will cool down the polyswitch, allowing the gig to continue.

The problem with polyswitches is excessive hysteresis - once they trigger, you have to cut the power down to almost nothing to get them to reset. The lamp, as long as it doesn't blow, acts more as a compressor/limiter, with the resistance dropping rapidly as the power falls below dangerous levels.

Also, Mosfet power amps are prone to oscillation at higher frequencies whcih also will take out speakers. This is a lot harder to detect as it is past the audiable range. I do not know all that much about it but I am lead to belive that the polyswitches can offer protection from this as well.

John - are you able to add to this?

Not really, most of my experience has been with bipolar power stages.

Another point regarding clipping - this actually means that you are sending DC or direct current to the speakers which is bad. The reason that I mention this is that you may hear someone suggest "checking if an amp has DC at the outputs". This is what they are checking for.

Actually, not true. DC at the outputs is generally the result of the failure of one or more transistors in the amplifiers. Also, being that it's a two-channel amplifier driving a pair of speakers with passive crossovers, DC is effectively blocked from the tweeters by capacitors in the crossover. Such a fault is much more likely to smoke the woofers.

Originally Posted by TimoteusR

He thought the monitor wedge was possesed by demons...

ROFLMAO!

John

[Mayhem]

Thanks for clearing those points up John.

I am happy to say that I have never caused a pollyswitch to activate (touch wood), so I cannot comment on the required power reduction to bring them back online.

You are right with the DC / crossover / tweeter scenario but my comment was more aimed at clipping in general. Although, it would still seem that I am somewhat off the mark still. Am I correct in my statement that clipping causes DC voltage to appear at the Amp outputs? I know that you are correct in your comment about DC often being casued by a faulty output device but I sure that I read/was told that clipping produces DC.

Confused :?