Cabinet Power Ratings and Other Questions

[Newf]

I've been thinking lately about getting a cab and wondering about their power ratings. I read online somewhere that you get go substantially over a given rating with little effect. In fact, I'm under the impression that it's worse to underpower than to overpower.

 

My amp is a Yorkville BM400 15" combo. The head potion is removeable/rackmountable however. 300 watts@8 ohms and 400watts@4 ohms. These are continuous ratings. Is continuous the same as RMS? It's also rated for 725watts@4 ohms 2-cycle burst. Does 2-cycle burst mean the same as peak? Does program mean the same as peak as well? Also, if Power = (E^2)/R shouldn't either the 4 ohm output be 600W or the 8 ohm output be 200W?

 

I'm looking at a SWR Big Ben (4ohm) on E-Bay do I have enough amp for this cab? I'd interested in a Henry the 8x8 (480watts@4ohm) too but would the amp hold up to a 2.67 ohm output as long as I didn't go crazy with the gain/power?

 

I played the head through a Peavey 2x10 TVX yesterday and it was nice but it's only rated at 175wattsRMS@4ohms.

 

I play 99% fingerstyle and I like blues, rock, and funk with some punk and metal thrown in. I like a nice fat bottom end with lots of punch and not too much high end (I don't like zingy). Any other cab suggestions that won't break the bank?

 

Cheers

Newf

[TimR]

Continuous and program are the same.

RMS is Root Mean Square, it is a figure used to describe the average power handling and is a mathematical calculation based on the fact that it is powered by an AC wave rather than DC.

 

Wikipedia as usual has quite a detailed explaination on this.

 

http://en.wikipedia.org/wiki/Root_mean_square

 

This is partly why the program or RMS values are not 600 and 800. When you connect cabinets of different impedances to the amplifier you are changing the impedances of the output circuitry as well.

 

We can use DC calculations as an example using RMS calcs.

 

"Power = (E^2)/R". E refers to the voltage and is constant. But as you drop R, I increases I=V/R. I can only increase to a certain point as heat is generated.

 

So in this example 300W=(48^2)/8 but (48^2)/4=600W however 48/8=6amps and 48/4=12amps which your amp cannot deliver.

[rumpelstiltskin.]

Newf -- don't confuse cabinet ratings with amplifier ratings. they are rated differently, even though they use some of the same words.

 

let's start with cabinets. they can be rated three ways, and usually at least two of those ways are used. there's RMS, program, and peak.

 

RMS ratings refer to the limit of power a speaker can handle before it begins to self-destruct thermally.

 

program ratings refer to the typical size of amplifier you should be using with the speaker.

 

peak ratings refer to the limit of power a speaker can handle before it begins to break down mechanically. too much power can push the speaker components too far.

 

commonly speakers will have RMS and peak ratings, but sometimes they use program ratings. a good rule of thumb, though, is to power a speaker with an amplifier rated no more than double the RMS rating of the speaker. for example, most 300W bass cabinets can be used with an amplifier capable of up to 500W. however, don't think that increasing the power by 100W will make you much louder.

 

it is not specifically better or worse to overpower or underpower a speaker. both can destroy your speakers. when you overpower a speaker, you can risk exceeding both its peak rating and its RMS rating. in some rare cases, you can exceed the RMS rating of a speaker by underpowering it. (basically, in an effort to get louder, you turn up the amp until it is very distorted. if the distortion is bad enough, you can actually source a lot more power than what you think you are, and it can damage a speaker, usually the tweeter or other HF components.)

 

now with amplifiers. amplifiers are rated for continuous, or RMS, power. while ohm's law says that if you halve the load impedance, the current will double, most amps are current limited, and therefore don't often double the power output going from 8 ohms to 4 ohms.

 

if your amp is not rated for 2 ohms, it is not safe to put less than 4 ohms on the output. i mean, we tested crest power amplifiers down to 1/8th ohm, just to make sure the protection circuitry worked. so your amp is probably tested. but at 1 ohm, our amps produced far less power than at 2 ohms. so you're typically not gaining anything, even if you won't blow up your amp. i don't know if your yorkville amp has protection circuits as stout as the ones in crest amps.

 

let me know if there's anything in this that needs clarifying. i've written up this kind of explanation a lot before, so i've tried to condense it to something that is both thorough, but also digestible.

 

robb.

[Newf]

Thanks Guys for the clarification. I was figuring that peak and program were the same and that RMS (.707 of peak AFAIK) and average were the same. I wasn't sure though and I finally had to ask even though I find it kind of embarrassing as I do have an electronics background but I've never worked on audio gear (ground radar systems - anyone need an unemployed radar tech out there? ) but the "program" term was the one in particular that I hadn't run into before and was confusing the heck out of me.

 

Cheers

Newf

[JonathanD]

Oh I see an oppertunity to highjack a thread!!!

 

I have heard people talk about compression in regards to an amp putting a lot of watts into a cab. Can you please explain this?

 

Also, would running 490Continuous into a 400 watt RMS cab be able to hurt the cab?

 

Thanks, Jonathan

[rumpelstiltskin.]

at some point, the voice coil of a speaker cannot dissipate any more heat. even though there is not enough heat to melt the voice coil, it loses it's ability to turn more power into more movement. the result is like compression -- you put more power (more amplitude) in, but you get no more power out. it levels out the peaks, like compression.

 

using a 490W amp likely would not hurt the cab. but i have seen very creative end users damaging equipment in ways i really did not think was possible.

 

playing bass guitar, i'd be surprised to see anyone put 490W continuous into a cabinet. if you put a 490W sine wave into a 400W cabinet, expect it to self-destruct. in the industry, we call them "thermal events".

 

robb.