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Cooked Voice Coil with Itech HD


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#1 JPM.

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Posted 08 October 2013 - 06:26 PM

Hello,
I have burnt out the voice coil on some drivers using Itech HD amps. I set the limiters as per the guide on the Crown website using the speakers RMS and Peak voltage ratings. .
A lot of the time I am not driving the system so I do not know exactly how it has been run. I do have my sound guy there with amp monitoring who claims it isn't flogged.
Anyway, if I was to better protect my drivers from being cooked should I lower the RMS voltage or the THERMAL voltage?
Thanks

#2 dakos

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Posted 08 October 2013 - 11:00 PM

Hello and welcome,
I'm sorry to hear about that.

I'm guessing you are frying the HF drivers on your speakers and not the LF driver? Are you Bi amping your speakers (I'm guessing not)?
It would also help if you could post the limiter settings (Watts/Volts, Attack and release times) and which speakers you're trying to protect.
Avi

#3 JPM.

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Posted 09 October 2013 - 12:16 AM

Thanks for the reply.

Speaker is a Peavey 500 watt Rms 12" woofer, 100hz to 2k.
Nominal Impedence 8 ohm, min 6.5 ohms.
Rms limiter @ 57volts, Peak Limiter @ 114volts attack not adjustable, release set by amp @ 0.193 sec and 0.0600 respectively.
Thermal limiter setting I cant remember exactly but set using the Crown guide for setting Limiters on HD amps. It was slightly lower than the Rms limiter and 10 seconds response.


#4 dakos

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Posted 09 October 2013 - 01:23 AM

We're almost there...
Would you be willing to post the exact model of the Peavey speakers you're trying to protect or a direct link to the product page? Also please specify which driver was fried,  HF driver or the LF driver and whether it's a bi amp configuration or not.
Avi

#5 dakos

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Posted 09 October 2013 - 03:11 AM

I'm asking for all this specific data from you because limiters ONLY protect the biggest driver in the speaker, so if the HF driver was the one fried then the limiters were probably doing their job properly and you would need to fix the damage and better design your system by using a bi amp configuration. If it was the LF driver that was hit then we need to recheck all your limiter settings to see what was wrong.
Avi

#6 JPM.

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Posted 09 October 2013 - 05:11 AM

Sorry, I assumed you picked up that it is biamped since it was a 12 inch woofer with a bandpass of 100hz to 2K.

The driver is a Peavey TC 12, rated to 500 Watt Continuous, 2000 Watt Peak, Minimum Impedence is 6.5 Ohms, Nominal Impedence is 8 Ohms.
I have been entering 57 Volts for RMS Limiter, 0.193 Release and 114 Volts for Peak Limiter, 0.060 Release and 56.5 Volts for Thermal Limiter, 10 Sec response.

#7 dakos

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Posted 09 October 2013 - 05:54 AM

First, the terms used for the limiters are RMS and Peak, sometimes when an RMS limiter is coupled with long attack time it's called a thermal limiter so I assume you mean the RMS limiter when you say "thermal".

I also need to tell you that no rig, no matter how well you protect it, is not bulletproof, a bad operator can always cause damage.

Now to your settings, I don't know how you calculated them but that's not what they should be, some are good but some aren't, you may read more about the theory behind the values I'll give you in a moment here:
http://www.crownaudi...st=0#entry16886

The Peak limiter accepts wattage values so just enter 2000-2dB=about 1300W. I'm taking it down 2dB more as an additional precaution since less than 3dB difference isn't audible.

For the RMS limiter, you have a 500W RMS speaker, with nominal impedance of 8 ohms and minimum impedance of 6.5 ohms, what you did was this:
V=SQR(WATTS X IMPEDANCE)=SQR(500X6.5)=57V

The problem is first you didn't account for power compression, to do that you need to further limit the voltage another 3-6dB but that alone shouldn't have damaged your speaker but still this needs some attention, so 500W-6dB is 125, subtract another 2dB from that for the same reason as before and you get about 80W of continuous actual power (sorry if it sounds too low). Let's do the math again for the new numbers:
V limiter=SQR(80X8)=25.3V
Even with what seems like super aggressive RMS Vs Peak limiting you only get about 12dB of headroom!!

Now to the second problem I see with your settings, now I didn't completely understand your settings so from what I see is you have very short RMS attack time, a more acceptable setting for the RMS limiter is 4 second attack and 6 second release. Also Peak attack and release times, a good starting point is attack 0.010sec, release 1 sec.
Avi

#8 dakos

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Posted 09 October 2013 - 09:58 AM

I couldn't find the manual for your speakers, could there have been a mistake in the details you gave me?

#9 JPM.

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Posted 09 October 2013 - 05:01 PM

No, I actually do mean thermal limiter.
With respect, it's apparent you do help to a lot of people on this forum but I don't think you are able to help in this case.  I'm not chasing a lesson in speakers 101 but someone with extensive Itech HD knowledge.
I can tell you are not familiar with this particular amp.
Thanks



#10 dakos

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Posted 10 October 2013 - 03:06 AM

Dear Justin,
Sorry for writing this as a "Speakers 101" tutorial, I'm doing this for a couple of reasons, one I didn't know how experienced you are and what background you have. My second reason is that there are many other people reading these lines long after you and I will forget about this thread and it's important for me that even newbie sound engineers will understand what we did here. I'm just like you and them, prowling the net for answers when I need them and trying to help others when I can.

Now for the thermal limiter issue, I was mistakenly talking about first gen itech, my mistake.

Now for your settings, if you're still interested in discussing the subject. I see your thermal voltage is just 0.5V less then the RMS voltage. I didn't find the manual for your speakers like I told you before but I didn't see long term specs from Peavey in other speakers manuals made by Peavey anyway so I'm assuming yours wouldn't have that spec either. JBL has a 2 hour continuous wattage spec and a 100 hours wattage spec on some speakers but I couldn't find an example of that today, IIRC the 2 hour spec is about 25% higher then the 100 hour spec. The 2 hour spec is supposed to be in the RMS limiter voltage setting, and you're supposed to enter the 100 hour spec in the thermal limiter voltage setting, you only put 56.5V which could have led to the cooked VC. Since Peavey doesn't supply the 100 hour setting and looking at what JBL provides, I would guess that 400W (51V) would be the proper value to input the thermal voltage setting. Just to note, the 51V still doesn't take power compression into account. Do you remember why you put 56.5V in the thermal limiter voltage setting?

Sorry again for the misunderstanding.
Avi

#11 JPM.

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Posted 10 October 2013 - 04:38 AM

Hey Avi,

Its all cool. After reading through my posts I'm not quite explaining it right. Such as the 100hz to 2k bandpass could still mean a passive network etc. My config is clear in my head but I deal with it all the time. I'm still irritated that I send out systems with operators that have wireless monitoring of limiter action on all amps yet damage still occurs.
Another thing is I don't like bringing brands and models into it on a public forum. It disputes advertised ratings which can be circumstance dependent. I have for example seen the Crown Itech HD specs put under scrutiny on a public forum by someone who works for another brand. I don't think that is right. Lets just deal with ratings.  I have stated everything that is available.

Any way, you should have a read of this. It shows how I come to some of my settings. I do not use auto mode except to set the release times. There is a limiter wizard there to but it doesn't fully follow the guide for some reason. It only deals with nominal impedence, not minimum.

www.crownaudio.com/media/storagebk/pdf/support/ITech%20HD%20LevelMax%20Limiter%20Readme%20File.pdf

As for power compression, sure it has an effect but we are not talking about subs here. Its a mid driver and it really only drags down performance. It is more of a side effect of voice coils heating up but not really the cause of them burning out. But maybe accounting for it might provide the protection needed. Still, with a 3 step limiter system, do you need to allow for it? Itech HD guru may be able to answer that.

#12 dakos

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Posted 10 October 2013 - 04:57 AM

Thank you for understanding.

I read the manual you posted throughly before my last post, I found it on the thread you started about two years ago.
http://www.crownaudi...?showtopic=4016

I'm only interested in the specs that help you get the job done, I definitely won't put the specs up for debate. I understand that not everyone that joins this forum will have a Crown/JBL combo, even I don't.

Back to our topic, did you open up the driver to visually inspect the damage? Is the crossover in tact?
Still about the specs, are you sure the the 56.5V thermal setting is low enough?

#13 JPM.

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Posted 10 October 2013 - 05:20 AM

Bi amped sorry. The 12 inch woofer has its own amplifier channel and processing. I have looked at the nice brown voice coil. The new ones also have some brown in them but this one is way more brown and open circuit.
The thermal rating is set as the guide says to. Lets say the 2 hour rating is 500 watt. Square root of (0.8 of 2 Hour Rating x Nominal Impedence).
It is the first thing I'm thinking of changing with maybe less response time.

#14 dakos

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Posted 10 October 2013 - 08:19 AM

Just one thing, the manual says to use minimal impedance when available and not nominal, that's how I got 51V (=SQRT(0.8X500X6.5)).

As for the response time, since it's a parameter of the driver, I would try to get it from peaveys support or even talk directly to Marty on the peavey forum. My logic tells me shorter response time as well.
Avi

#15 dakos

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Posted 10 October 2013 - 02:49 PM

I'm guessing this is the limiter wizard you used:
http://www.crownaudi...izard_v1.0.xlsx
So just to validate our math, when I input the values we discussed (2HRS RMS=500W, 100HRS RMS=400W, minimal impedance=6.5 ohm) you get 57V and 51V for the RMS and thermal limiters.
Avi