How to work out 3-way cab Power Rating from different speakers? - diyAudio
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Old 30th April 2012, 06:44 PM   #1
Rafmike is offline Rafmike  Portugal
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Unhappy How to work out 3-way cab Power Rating from different speakers?

I wonder if you can help me, while working in Turkey we bought a power amp and 2*no-name, passive 3-way*PA cabs which worked well until the drummer left and took the amp with him. 2 years on, with no idea of what we'd had, I thought it'd be easy to choose a replacement amp for my cabs. Yeah, right...!

With no datasheet, as the cabs were put together by a little guy in a backstreet, I dismantled them to ID the speakers. After 2 hrs on Google images and lots of measuring, I am 99.9% certain that I have:

-*Compression Drivers - P. Audio BMD440s (60 watts, 106Db SPL/W/m, 8Ω).
- Mids - Celestion 818MR (100 watts, 99Db* SPL/W/m,*8Ω).
- Woofers - P. Audio IMF15LF (600 watt, 97Db* SPL/W/m,*8Ω).

But I now have THREE power ratings and THREE SPL/W/m figures for the speakers! Grrr... How do I know what the whole cab's overall *rating is, please?

A formula I found on the web, suggests the cab's overall power rating is the sum of the number of speakers x the lowest rated speaker's watts (ie 3x60 = 180 watts).*Though why would a guy build a cab without matching power ratings (watts) in all 3 speakers if the lowest is limiting?**If he's saving money, why not put 60w mids and woofers in too?! In the cab are some electronics - passive crossover? Is it possible this is wired to somehow restrict power to each speaker in line with its own rating so that 600 watts can be pumped in by the amp at the speakon connector but, with 600 watts still going to the woofer*, only 100 watts goes to the mid and 60 to the CD ensuring that nothing blows up? Essentially, should I consider the cab as a whole to be 600+100+60 = 760 watts? Hell, this is confusing!

Is my cab's overall or limiting power / sensitivity rating:

- 760 watts -*intuitively*(for an idiot like me), the average total power drawn by 3 speakers?
- 600 watts - the 'highest' draw of the speakers in the cab (the woofer)?
- 180 watts - the total of the draw from 3 speakers each capped at the lowest speaker's rating ( 60w in the Compression Driver)?
- 60 watts - the 'worst case' max power into the cab so I don't blow the tweeter (if I have no limiter)

- Can I assume worst case for sensitivity (97Db SPL/W/m)? I guess, the other speakers would just make more noise at a give distance?*

Crown amps' calculator says that for 15m listening distance, 95DbSPL, (using 97SPL/W/m) with 3 Db headroom I'd need at least 283 watts (per channel?). This suggests that I should buy something like.... (the ratings are the important bit)

- Crown XTi 200 or better (8Ω Stereo - 475W)
- Crown XLS 402 or better*(8Ω Stereo - 300W)
- Crown LPS 2500*(8Ω Stereo - 550W)

But without knowing if my cabs as a whole are rated at 60, 600 or even more watts, I'm either risking (in order of disaster!)*wasting money, having dreadful sound,*blowing the speakers or blowing up the amp. I can't find anywhere that helps with the maths to get me to a single power rating for the cab. From there is looks fairly straight forward.

Given the*3 speakers' ratings that*I have in my Cabs, what amp rating at*8Ω*would you suggest I look for?

Thanks in advance!
Mike
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Old 30th April 2012, 07:23 PM   #2
wg_ski is offline wg_ski  United States
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600 watts, if the builder didn't do something foolish with the crossover. Most of the power is between 50 and 500 Hz, which is fed to the woofer.
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Old 30th April 2012, 08:06 PM   #3
Rafmike is offline Rafmike  Portugal
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Thank you SO much... I've spent hours and hours on this!

So just to be certain that I've understood the internet right... And before I part with "hard-earned"... without a limiter, both my 600 watt speaker cab and its amp would be quite safe if I used a 600 watt per channel into 8ohm stereo amp... but I would do better with one rated between one and a half and twice the power rating to give headroom?


Wow! either way, that's big noise! 573 watts alone gives me 100dbSPL at 12m with 3Db of overhead. I'm never going to need that much so I can't see the amp knob ever going to 11 but is a 600 watt amp the minimum per channel? Several amps seem to be around the 500-550 watt per channel mark? Are they too small or should I really be spending the extra?Moreover, should I really be saving to get 900+ watts for that extra overhead?


In anticipation of confirmation of my understanding, so I'm as cast iron as I can be before finally buying an amp, thanks again - I really appreciate your help and for such a quick reply too.


Mike
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Old 30th April 2012, 09:10 PM   #4
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Depends on the use. If it's running bass and drums, then I'd go for the bigger amps, if it's just vocal support, a 500w jobbie would be more than adequate. Just keep an eye on the clipping indicator.
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Old 3rd May 2012, 07:03 AM   #5
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Hi Rafmike, keep in mind, if there is one 8 ohm speaker per channel for stereo operation, the amp will deliver approx 2/3 of the rated power. Firstly, yes, the power rating of your speakers is the power of the bass driver. Because the mid and high drivers are more sensitive they don't need to be as powerful. Speakers don't "draw"power from an amp, they "receive" it from the amp. If you have a 1000w amp (hmm sounds big), it will be 2 x 500w RMS into 4 ohms. Put an 8 ohm speaker and you will get 350w (in round numbers) into each speaker. That doesn't sound as impressive as the original 1000w but it is real world. It is a good idea to have at least as much amp as you have speaker so for a pair of 600w speakers you need a 1800w amp (2 x 900w). A clipped signal will do far more damage than too much power (remember music is highly dynamic so the maximum levels are very brief). Hope this helps.
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Old 6th May 2012, 11:29 AM   #6
Rafmike is offline Rafmike  Portugal
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Thanks again, everyone. I've been looking at the rated power levels at 8ohm (Peavey PV3800, CS3000, Yamaha P7000s, Crown XTi) and was (just) kind of OK with the prices but an 1800W amp would break the bank (probably via the divorce courts LOL!).So probably then my last question... what are the pros and cons of using 2 smaller amps to use in Bridged Mono to feed each cab rather than both cabs from a single, larger stereo amp. It strikes me that I can buy 2 smaller (=cheaper) amps from this end of the market where competition is keener than the expensive end aim at the 1800W commercial end of things. Or am I - as usual! - missing something here? In short, what is the cheapest way to do this properly without buying an ACMEPROSONIC Scratchmatic Distortaphone 2000! Mike
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