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[Tim McCulloch]

I'm using Mackie C300z's which have a rating of 300/500/750 with Peavey IPR 1600's which have an 8 ohm rating of 330w.  If I bridge it then I'm looking at 1,060w which exceeds the program and peak rating of the speaker hence my desire to adjust the PA+ so that those levels could not be obtained.

I don't believe I'm over thinking this at all. It's a little dissapointing and frustrating to ask a logical question and basically be beat down over a legitimate technical question and be treated like I'm plugging in my first set of speaker wires.

Brett, you ARE overthinking this for the tools currently available to you.  Catastrophic failure caused by dropped mics, unplugged acoustic instrument pickups or even phantom powered mics is fairly rare.

Transducers die because they got too hot for too long, or because their physical excursion limits were exceeded.  To protect LF components, you might have a chance at protection if your limiters have an attack time around 3 -5 milliseconds, but for HF you need a limiter that clamps down in MICROseconds.

Long term RMS power limiting is beneficial in some circumstances, but nothing can insure system survival if the operator is determined to drive the rig with red lights on.  With extensive limiting, you pretty much eliminate any dynamic range; the speakers constantly get the maximum amount of voltage.  It might not go up more, but seldom goes down, so when do the voice coils get a chance to cool down a bit?

Limiting for system protection with devices like the DRPA and it's various DR-siblings is mostly marketing...  I *do* understand what you hope to achieve but what we've been getting at is that there is no "one number" solution that is guaranteed to fit.

[Mac Kerr]

Except that as we all know 3db is a double in power, I have a difficult time overlooking having that extra power when needed.

Here is the much more informative version of that statement: A doubling of power is only a 3dB difference in level. A 3dB change is the smallest difference in level that most people perceive.

You are splitting hairs. Use the amp in stereo. If your system doesn't get loud enough get a bigger system. To get a significant step up you should think in terms of a 10dB of increase, which is equivalent to 10x as much power.

Mac

[g'bye, Dick Rees]

I'm not putting low frequency content into them, I have separate 18" subs.

Second, they're not terribly cheap, they do have EAW drivers in them and they definitely better constructed than some plywood boxes I've worked on in the past. 

Finally, I never said I wanted to drive them to 750w, never.  I was simply looking at having the peak capability of 500 - 600w.


Anyway I appreciate the tip on the dB conversion out of the DriveRack

Horses Mouth Department:

Mackie recommends using a 300W amp for these speakers. 

[Bob Leonard]

Except that as we all know 3db is a double in power, I have a difficult time overlooking having that extra power when needed.

Perhaps, but 3db is a level of sound that most humans won't notice.

[Tim Perry]

I was wondering if someone could assist me with a project I'm trying to work the math on.

I can find plenty of tutorials on proper calculation of the amplifier gain, however, I cannot seem to find a way to calculate an accurate configuration of the output of the crossover and the limiter threshold.

As an example (just picking random numbers) if 1.2v into my amp gives me my 600w limit on my amp (which is capable of 1000w) how can I mathematically determine that lets say a -10db limiter threshold will limit the PA+ to 1.2v RMS?  Is the DriveRack talking dbm, dbu, dbv ?

Thanks!

If the DRPA+ has the same output click at power down as the DRPA I am in favor of setting the limiter all the way up (or off if possible) and driving the unit hard. then turn down the amp input to achieve the desired sound level with a reasonable amount of headroom.

I feel by reducing output via limiting or any other means that are included in the box I am more at risk of tweeter damage then otherwise.   

[Per Sovik]

If the DRPA+ has the same output click at power down as the DRPA <snip>

It doesn't click.

[Bob Leonard]

It doesn't click.

If the amplifiers or powered speakers are turned off first, as they should be, the only click you'll hear is the door being locked when you leave.

[Per Sovik]

If the amplifiers or powered speakers are turned off first, as they should be, the only click you'll hear is the door being locked when you leave.

Not allways possible, having a single non-clicking device in the chain is a good thing to have.

[g'bye, Dick Rees]

I feel by reducing output via limiting or any other means that are included in the box I am more at risk of tweeter damage then otherwise.

My point exactly.  The OP will be better off with a properly sized amplifier.  The best fix is always the one closest to the source of the problem.  Anything else, no matter how effective, is what I call a "band-aid".

[Mike Christy]

So to summarize eveyone’s replies…

•   Mackie recommends 300W because a clipped 300W amp is 600W ( 2X power ) and those who buy those cabinets will probably under power and drive the amps into clipping. ( 98dbSPL sens is not that loud)
•   What is interesting is the Mackie 300/500/750 spec, normally you see 300//600/1200, Mackie has de-rated those for some reason.
•   The easiest solution is to purchase a 300W amp, that way when it is clipped (output=600W) you have ½ a chance to save the $25 HF diaphragms, otherwise just expect to replace them, it’s not 12/21/12 situation.
•   The next solution if you want to bridge, as mentioned before, is to set your gain structure to accommodate the bridged amps - put a test sine wave through the system, cal to 50V ( 300w/8ohms) at the terminals of the amp then never exceed that average level on the mixer LEDS. Youll probably have better dynamic sound by allowing the +6/ peak transits propagate through w/o clipping.
•   Do not rely on the PA+ limiters.

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