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Please Advice on amp selection


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#1 zulm

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Posted 24 February 2013 - 11:10 AM

I'll be using 4pcs. 500w(rms) 8ohm top and 2pcs 800w(rms) 8 ohm sub per channel for my sound reinforcement setup. Please advice on which crown amp most suitable and taking into account i might be using the same amp to set for 1 or 2 top and 1 sub per channel configuration.

Thanks.

#2 dakos

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Posted 24 February 2013 - 01:31 PM

Well, that's way too general of a question, I can answer your question quickly and shortly but I feel it's much better for you if you post specific links to the speakers you're trying to power if you already own them or to the speakers you want to buy if you don't.
Let me know if you want the short answer or the long one...
Avi

#3 zulm

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Posted 24 February 2013 - 11:15 PM

Thanks Dakos for the promt response.

The case is im getting the 500w8ohm and that 800w8ohm sub used speakers local brand from my friend for a very good offer that i really cant resist although im very interested in JBL SRX or MRX series and the spec of those speakers says that (500w8ohm) and still in good running condition. Im very keen to invest on crown amp for those cabs before i finally switch all those cabs to JBL or any other better brand and thats where i really in need of a second opinion so that the choice of the amp will suit my future setup of about the same config.

Thanks

#4 dakos

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Posted 03 March 2013 - 09:23 AM

You provided only partial spec for the speakers so I'm still having trouble answering you and I'll explain...

There is a real big misunderstanding of how much power speakers can handle and how to understand speaker specs. A curtain application may need much less power then the speakers can handle but that's a completely different subject.

Speakers are both electric and mechanical units that translate electric signals into a vertical mechanical movement of the cone. The electric unit inside the speakers that is in charge of this transformation is the voice coil. Basically the voice coil is a thin electric wire wound on a paper cylinder that's glued to the cone. When an electric signal goes through the voice coil it creates a couple of things:
1) It creates a desired phenomenon called magnetic flux, that's what pulls and pushes the cone to and from the speakers magnet and create the mechanical movement of the cone that generates the sound.
2) When an electric charge runs through any wire it also creates heat. This unwanted phenomenon, the heat in the voice coil takes little time to buildup (about 3 to 30 minutes), this heat buildup is in part what ruins our speakers and is why speakers are rated the way they are.

Speakers are rated in a way that represent the durability of the voice coil depending on how long the voice coil can endure a curtain amount of power or in other words heat. The RMS/Continuous wattage rating represent the amount of power a speaker can handle for a very long time (magnitude of hours) with no damage to the voice coil or other parts of the speaker. While peak power represent the amount of power/watts a speaker can take for a short amount of time (magnitude of seconds or milliseconds) which is usually +6dB of the RMS rating (four times the power). Program rating is 3dB between the two.

Another term we need to know in order to understand how speakers are rated and that's the Crest factor: that's the ratio between the RMS voltage of a signal (Average) to the max voltage of the peaks (Peak/RMS ratio expressed in decibels).
A DC power source has 0dB crest factor (no peaks).
A pure sine wave power source has 3dB of crest factor.
Pink noise generator typically has 6dB of crest factor.
Each increase of the crest factor by 3dB represents a decrease in the RMS value of the signal by 3dB (halving of the RMS wattage value).
In our world those peaks are transient higher amplitude in the music such as a kick drum and could have a crest factor of up to 24dB.
Headroom: More definitions I know... This term is very similar to crest factor but crest factor is inherent to the signal while headroom is a spec of the amp+speakers that we create.
If we take a 1000W amp and run it at 1000W we have 0dB headroom.
If we take a 1000W amp and run it at 500W we have 3dB headroom.
If we take a 1000W amp and run it at 100W we have 10dB headroom.
In a perfect world our rig would have as much headroom as the signal has crest factor.

So to sum all this data: Speakers usually have 6dB of headroom built into them. Nowadays amplifiers don't have allot of headroom built in them (1-2dB for the higher end amps) so it's up to us to create a rig that has enough peak power and at the same time, not too much continuous power so nothing would get damaged.

One more thing to take into consideration is Power Compression. When we feed a speaker its rated continuous power for a long period of time the voice coil heats up and its impedance rises a bit so at the same voltage we get less SPL. This phenomenon is called power compression, that means that in order to avoid that and get continuous stable SPL the actual continuous speaker power is 3-6dB below the rated continuous speaker rating.

So when you provide a wattage rating without telling me if it's an RMS value or peak or program then it's very limited data to work with.

On top of everything I just explained speaker manufacturers are not all rating speakers the same way and are also confusing buyers with bloated wattage ratings with no context of how it was measured

So I need the model of your specific speakers and maker or alternatively you can tell me the RMS/peak values yourself if you like.

For further reading here are two links:
understanding_the_nuances_of_crest_factor
we_need_more_power_captain_but_how_just_how_much_amplifier_power_is_needed

Why speakers fail:
LoudspeakerFundamentals.pdf

The infamous spec sheet game article:
THE_LOUDSPEAKER_SPEC_SHEET_GAME_2005.pdf

Wow this came out pretty long :)

Corrections are more then welcome :)
Avi

#5 joust

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Posted 04 March 2013 - 04:08 PM

Hey AVi, pretty cool explanation!

Alain

#6 dakos

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Posted 04 March 2013 - 05:00 PM

Thank you big J :)