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#31 dakos

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Posted 14 June 2012 - 12:39 PM

So I think we have gotten very useful info, the fact that you're not the only one blowing HF MRX drivers is very relieving, you now have better knowledge at what the problem is and replace those sensitive drivers with these Selenium D220Ti HF drivers. Now I'm not sure installing a bulb would even solve the problem because you will do all the calculations for the rated power of the original MRX driver, because it is more sensitive then the written specs, unfortunately I think you will still be blowing drivers but lets see what replies we will get...

#32 dakos

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Posted 14 June 2012 - 11:10 PM

Sorry for going back to this subject but I was thinking about this again, you might be overpowering those speakers, could you specify the QSC amp used on those (You need a continuous power of 1/2-1/4 of the continuous speaker rating for live shows)? what EV speakers? how did you limit/compress the signal to this amp?

#33 makis

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Posted 15 June 2012 - 05:59 AM

1/4 of the spkr rating? I think it's a very small amount of power. Anyway sx200, plx3600 and there was a Dbx compressor ratio 2:1 threshold -10 who wasn't even compressing the overall signal, some times maybe 1-2 LEDs top.

#34 dakos

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Posted 16 June 2012 - 08:17 AM

QUOTE(makis @ Jun 15 2012, 05:59 AM) View Post
1/4 of the spkr rating? I think it's a very small amount of power. Anyway sx200, plx3600 and there was a Dbx compressor ratio 2:1 threshold -10 who wasn't even compressing the overall signal, some times maybe 1-2 LEDs top.

I know, 1/4 of the rated power sounds very low, it all depends on how much headroom you want in the system, after all it's up to us to maintain proper headroom while having enough continuous power, 6 dB of headroom is built into the speakers (peak power = 4 x continuous power), it's the least amount of headroom needed for heavily compressed rock show, all the way to 20 dB and more for uncompressed shows. So compressing the sound at 2:1 ratio might not be enough if you're pushing it, combine all that with a speaker that has drivers that are sensitive to over power and it's a disaster waiting to happen. All that being said, even if you do everything correctly, still having those sensitive drivers in a live show setup is not a good idea even if you install those bulbs, the drivers will be saved but the show would sound pretty bad. As you probably read, when the bulb engages, it changes the crossover frequency and it also compresses the power to the HF driver so the system changes its sound dramatically. That's not the type of configuration you want in a live show cause it's not dependable enough.

So, I think you have the info you need to continue with this little project, the recipe is for the MRX driver, not the SRX, use a couple of these bulbs with a couple of BA15s generic screwable sockets, connected in series to each other and in series again to the HF driver, try to install them in a visible place (so you see their light through the port).

I also sent a PM to Curtis List (Too Tall) so I'll update when I get a reply from him.

#35 makis

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Posted 16 June 2012 - 08:44 AM

Lots of info to analyze. Thanks for everything, you are the best. If you don't mind, I would love to have some more of your wizdom on subject "HEADROOM"..
I also want to ask you if there is any kind of danger for the crossover or the amps ohmage when the bulbs engage?

#36 Deromax

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Posted 16 June 2012 - 01:00 PM

Bulbs in crossovers are a mean of cheap, simple and effective protection for HF horns against sustained overload, while allowing the briefs loud peaks to pass unaltered.  As it was said, it WILL change the sound balance if it engage more than just occasionally, in reducing the high frequency content of your output, which you will then tend to overcompensate for, compounding the issue even more.  This is obviously not desirable.  If you find this is happening, it's a sign that more HF output must be added to the system.  IE, more top cabinets.  There is no way around it.

Even if a bulb is a simple device in itself, I think it cannot be retroffited by the end user.  The manufacturer spent time and ressource to calculate what exact bulb is needed, using informations they have and that we don't have!

#37 dakos

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Posted 16 June 2012 - 02:16 PM

This is a good place to start reading about headroom so start to read and fire away those questions when you're all done.

Luckily, the impedance the amp is seeing doesn't change when the bulb engages.

#38 makis

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Posted 16 June 2012 - 06:31 PM

Very interesting, but I need more info about "compressing the signal/losing headroom"????

#39 Deromax

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Posted 16 June 2012 - 08:34 PM

The bulb is placed in serie with the HF driver.  When the threshold is reached, the bulb begins to illuminate and its resistance increase, hence limiting the power delivered to the driver.  So yes, the impedance of the HF branch will go slightly up.  Which is a non-issue.

#40 dakos

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Posted 17 June 2012 - 12:50 AM

QUOTE(makis @ Jun 16 2012, 06:31 PM) View Post
Very interesting, but I need more info about "compressing the signal/losing headroom"

First I have to ask what was the after compression gain in the compressor?

I don't know your knowledge about compressors/limiters so let's start with the basics:
http://www.youtube.com/watch?v=eXCaKStVAEs
One note that when you set the threshold level, it's supposed to represent about the same level on the mixer that the compressor would start to do its magic, that happens only is you have proper gain structure on your mixer.

Second, a compressor is not used to controll the headroom, that's up to us to create using our gear. What it does is it takes down the dynamic range in the music and adjusts it to the amount of dynamic range our rig is able to handle. Lets say you need 110dB at 30 ft from the speakers and you have 16,000W peak power to the speakers and you have the amps to supply that power, the continuous speaker power is 1/4 that so it's 4000W continuous. Up till now we have only 6dB of headroom. Now we are in a rock show with peaks of up to 20dB, that means that at that +20dB you will utilize all that 16,000W. Now for the continuous power, if you need 20dB headroom that means the continuous power will be the maximum power devided by 20= 160W only. That's when the compressor gets in the picture, it takes down the dynamic range (that 20dB) to a number our system can handle while keeping the desired continuous SPL. So if you set a ratio of 2:1 and a threshold of -10dB that means that all the dynamic range + continuous signal over -10dB is going to be compressed at that 2:1 ratio. So now you will need only 10dB of headroom for the peaks plus all the continuous signal over -10dB point, that means 1600W continuous power and 16,000W peak. Please correct any wrong things I wrote smile.gif
Look here for more info.

Another good video tutorial about compressor usage and how different setting effect the sound:
http://puremix.net/video/tooling/compresso...n-overview.html

#41 dakos

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Posted 17 June 2012 - 11:11 AM

Have you seen the zener solution to your problem?

Here is a rundown of tweeter protection history (taken from here, a post by djk):
Drivers fail for two reasons:

Excess average power
Mechanical damage

Clipping will not hurt anything if it does not cause the above problems.

After larger amplifiers (above 50W) became common, problems with high frequency units self-destructing became a problem.

A tweeter like the EV T35/Klipsch K77 could handle 5W continuous, 50W for 10mS peaks, so normal program material wasn't the problem.

The tweeters were dieing from mechanical failure. The voice-coil was wound with aluminum wire that ran out to the terminals on the frame (self-termination). Shallow slopes in typical crossovers (6dB) combined with higher drive levels produced failures. Copper wire was tried instad of aluminum (aluminum work-hardens in a very short time).

Still, the failures continued.

Klipsch switched to the 18dB crossovers in the very early 70s. This helped with the excursion failures.

Now, 100W+ amplifiers became common.

Klipsch went to cathode-to-cathode connected zener diodes. These clipped off the peaks that were mechanically destroying the tweeters. Things went well for about ten years.

In the early 80s it was time to try and get rid of the expensive band-aid (the zener diodes). EV changed the lead-out wire from the self-terminated copper voice-coil wire to a flat BeCu wire like used on expensive JBL and Altec type compression drivers. A new network was designed with an elliptical filter with 50dB of attenuation only a half-octave away from the crossover point. A fast- acting instrumentation fuse was added.

Problem solved?

The new version of the tweeter used a UV cure adhesive vs the old thermo-set adhesive. After the fuses blew from modern program material (about the same time as the introduction of the CD), they got replaced with fast-blow types (which offered reduced protection). The special instrumentation types were very expensive, and very hard to find.

The new UV cure adhesive got soft, bubbled, and failed quite easily.

A PolySwitch was tried. Too slow.

The old thermo-set adhesive came back.

The combination of the super-steep crossover, the flat BeCu lead-out wire, and the PolySwitch seemed to work (with the old adhesive).

The zener diodes were retired for mainly two reasons: cost, and limited dynamic range. The AA networks used a pair of 5.1V 10W zeners. These only allowed about 2W RMS through before they started clipping off the peaks (a 4W peak square-wave) .

With the advent of digital program material, 2W of undistorted program material no longer seemed adequate (about 97dB at 10 foot). The zeners limited the maximum distorted output to about 100dB at 10 foot.

Removal of the zeners allowed exploitation of the 50W/10mS rating of the tweeter, about 14dB more output capability (referenced to the 2W RMS zener clamp).

What did EV do for products sold under their brand?

The STR tweeter protector was developed for this use. Later it was modified by adding a lightbulb in parallel with the relay contacts.

I hope this give a little insight into what is needed for tweeter protection, and how we got to where we are.

Vifa, Dynaudio, and others offer a choice between self-terminated tweeter lead-outs and a braided (or tinsel) type lead-out.
The difference in cost for the braided type is worth it in my book. Most manufacturers of lower-priced product do not spend the money for this, or better crossovers either (although I am seeing lighbulbs and/or PolySwitches in some inexpensive product).

Crossovers and tweeters must be designed to avoid mechanical damage in normal use (correct slope, frequency, and lead-out wire for the intended use).

Long-term thermal protection is worthwhile. Some sort of switch device (relay, PolySwitch, fuse) in conjuction with a lightbulb seems to be the most cost-effective. Due to the long time constant of lightbulbs, most use does not seem to demand the switch (which shorts out the lightbulb in normal use).

Suggested current levels:

For 1" coils (tweeters), about 1A. The 561 or 211-2 automotive lamp has worked well in this application.

For 1-3/4" coils (1" compression drivers), about 2A. The 1156 type automotive lamp has worked well in this application.

For 3" (kapton) to 4" (nomex) coils (2" throat 16 ohm compression drivers), about 2A. I have found a 1.5A AGC (or 3AG) type fast-blow fuse will pass 400W of program material in normal use, and blow almost instantly if bad feedback is encountered. A pair of 1156 automotive lamps wired in series with each other, and then wired in parallel with the fuse will allow the show to go on when the fuse blows, and still offer some protection. If the fuse blows in the course of normal use, you need additional HF drivers, horns, and amplifiers.


#42 makis

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Posted 18 June 2012 - 05:26 AM

After compression gain was 0.
4000 continuous, 16000 peak power = 6 db of headroom. 1600 continuous, 16000 peak power = 10 db of headroom? How is headroom db estimated?
Also, is it safe to use bulbs and fuse in parallel? Doesn't it create some kind of capacitor?

#43 dakos

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Posted 18 June 2012 - 07:28 AM

QUOTE(makis @ Jun 18 2012, 05:26 AM) View Post
After compression gain was 0.
4000 continuous, 16000 peak power = 6 db of headroom. 1600 continuous, 16000 peak power = 10 db of headroom? How is headroom db estimated?
Also, is it safe to use bulbs and fuse in parallel? Doesn't it create some kind of capacitor?

The calculation is very simple, just a dB ratio formula:
10 X Log(peak/continuous)= headroom in dB.

I don't understand why would that create a capacitor, none of them has any capacitance in them. In parallel to each other or to the driver? The bulb is supposed to be connected in series with the driver also the fuse. Did you mean the dual Zener solution?

#44 Deromax

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Posted 18 June 2012 - 08:36 AM

Doubling the power = +3dB
Half the power = -3 dB
Multiplying the power by ten = +10 dB
Dividing the power by ten = -10 dB

It is then easy to quickly do calculations.  Example, if a driver have a sensitivity of 99dB at 1 watt, how much output will it put out at 200 watts?  Going from 1 watt to 100 watts is a hundred time more, so 20 dB.  Then, going from 100 watts to 200 watts is 3 dB.  So the driver will produce 99 + 20 + 3 = 122 dB at 200 watts, at a distance of one meter.

#45 makis

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Posted 18 June 2012 - 09:51 AM

QUOTE(dakos @ Jun 18 2012, 03:28 PM) View Post
I don't understand why would that create a capacitor, none of them has any capacitance in them. In parallel to each other or to the driver? The bulb is supposed to be connected in series with the driver also the fuse. Did you mean the dual Zener solution?

I mean in parallel with each other. I just don't like the idea of splitting a simple conductor into two separate ones and then join them back together, is that perfectly safe?
Does a fuse sorts out a light bulb in normal use and why?