effect of underpowering a cab?

[JimmyM]

Thanks Alex.

Please don't tell anyone, but when I first came here a year and a half ago I believed that square waves over-stressed speakers somehow... and I'd been a speaker manufacturer for years! What I wrote in my post is stuff I learned here.

Duke

and to think i'm buying two cabs off of you! what have i done?!?!?!



just kidding! duke is actually an extremely knowledgeable and respected guy who knows his stuff as well as anyone. compared to guys like him, bob lee and bill fm, i'm a dolt. he mostly designs high end home audio speaker installations and he's known as one of the bigger guns in that field. why he wants to slum with us lowly musicians when he could rake it in installing systems for rich guys and their mancaves i'll never know, but i'm glad he does.

 

[okcrum]

if a solid state amp is made with an output transformer, then it can be applied. however, there are extremely few solid state amps being made that way. the warwick hellborg power amp is one of the rare few, and honestly, i don't know of any others.

Old Macintosh power amps are the only others I know of. They used an tapped autotransformer. There are also direct-coupled tube designs with no output transformer, but I haven't seen any used as instrument amps.

 

[Bob Lee (QSC)]

Solid-state amps are usually designed to be voltage sources, with very, very low output impedances compared to their intended load impedances. (This is indicated by typical damping factors of 100, 200, and higher.) So the output voltage isn't significantly influenced by the load impedance (Z), as long as the load Z is equal to or greater than the minimum.

As a result, with most solid-state power amps, you get more power out of them when you reduce the load Z. This is because the voltage is mostly unchanged, so the current delivered increases in inverse proportion to the load Z.

There's only so far you can take this, though. You may have learned that the maximum power that can be delivered to a load is when the load Z is equal to the source Z. So if the amp's output Z is 50 mΩ, let's parallel 160 8Ω drivers and give it a 50 mΩ load! In theory, that could work if the amp could deliver the current without melting. But well-designed amps have current limiting to protect the output transistors and other components so they don't fry and release the magic smoke. So instead of putting out maximum power into the 50 mΩ load, the amp will throttle back in some way.

If the amp is designed to drive a load of 2Ω or greater, then you can hook up four of those loudspeakers to get the most power out of it. What if you only have one 8Ω speaker? Well, you could borrow the concept of a tube amp output and use a transformer. A 2:1 transformer would double the amp's output voltage and put that into the loudspeaker, and it would make the amp see a 2Ω load instead of 8Ω. Hey, more power! But there are some problems with this approach. Even the best transformers have some insertion loss and hysteresis distortion. And to reduce susceptibility to core saturation with high-power low-frequency signals, they would have to be really big, big, and heavy. The advantage gets a bit slimmer, and thus it's typically more useful to just hook up another loudspeaker.

 

[Eric Moesle]

There was mention in an earlier post of someone saying that you can put double the wattage into a cab than it is rated for. Its actually a misunderstanding and misquote.

You typically can use an AMP rated at double the wattaged rating of the cab (for headroom purposes) but even then you are never really DELIVERING 100% of that amp's wattage to the cabinet. Its called a volume knob, and people don't play with their pre-gain and master volumes both on 10 full-bore.

People would be amazed to see how little wattage a cabinet actually sees from their amps during normal playing conditions, probably one-fourth or less of its rated power!

 

[uhdinator]

in a PA system when you have a pair of speakers that are rated at 8 ohm 1000w peak
you want to run an amp that is a minimum if 250w up to 500 RMS at 8 ohms to each speaker. 500 is optimal.

What happens when your amp is say 150-200w is you end up running the Mixer output at its max to get the volume you need and reducing head room which puts you closer to clipping the amp input and that toasts your voice coil. (sends a DC current to speakers).

So say I have a 600w amp instead. Now I can turn down the mixer to say -5 db instead of say +3db and get the same volume I need and gain 8 db of headroom.

I have done some comparison with a PA cab running a cd player thru a mixer and listen to for a few minutes with say 100w of power then switched to an amp that had the proper power (500-600W) at the same volume levels and noticed the speaker sounded thin (lack of lower freq's as they need the most power) with it under powered.

Frequency throws another curve into it all. A sub woofer rated at say 4 ohms may have a freq range of 35-250 hz but that ohm rating is based on the lowest ohms rating within that band which is prob 4 ohms around 200-250hz. My crossover is ran at around 100hz. where I am pushing the freqs the most is prob between 40-80hz.
as the freq gets lower the ohms gets higher.

Sooo. At the range where I am using it the ohms the amp really see's is prob anywhere from 10-30 ohms. So I'm not really getting as much power as I think.

Because of this alot of sound guys may have a sub that is rated 800w peak 4 ohm and run an amp that is 600-750w RMS at 4 ohms. Just dont let the amp clip and use a limiter!

 

[Bob Lee (QSC)]

You typically can use an AMP rated at double the wattaged rating of the cab (for headroom purposes) but even then you are never really DELIVERING 100% of that amp's wattage to the cabinet. Its called a volume knob, and people don't play with their pre-gain and master volumes both on 10 full-bore.

The volume or gain controls don't limit how much power the amp puts out.

The reason for the rule of thumb of "amp power = 2 × speaker continuous power" is that normal audio—music, voice, etc.—is dynamic. An amp in a properly set up audio system may at times put out full power, but it will not be called on to do so for anything longer than short peaks and transients.

 

[Bob Lee (QSC)]

No. Come on, man. Read. Don't come in here with myths that we've gone on for page after page debunking.

Clipping does not send DC to your speakers. Clipping is not what toasts speakers, etc.

 

[bongomania]

There was mention in an earlier post of someone saying that you can put double the wattage into a cab than it is rated for. Its actually a misunderstanding and misquote.

You typically can use an AMP rated at double the wattaged rating of the cab (for headroom purposes) but even then you are never really DELIVERING 100% of that amp's wattage to the cabinet. Its called a volume knob, and people don't play with their pre-gain and master volumes both on 10 full-bore.!

Hmm--I'm not sure which post you're referring to, but could it have been one where somebody was describing the scenario where somebody can send almost double the wattage into the cab (unawares) by driving their amp fully into clipping?

Also FWIW the volume knob doesn't regulate wattage.

 

[billfitzmaurice]

(sends a DC current to speakers).!

It does no such thing. Read the FAQ, learn how amps and speakers actually work, and don't give advise about things which you have no understanding of, all you're doing is perpetuating this piffle.

Again, why is this thread still open?

 

[Korladis]

Hmm--I'm not sure which post you're referring to, but could it have been one where somebody was describing the scenario where somebody can send almost double the wattage into the cab (unawares) by driving their amp fully into clipping?

Also FWIW the volume knob doesn't regulate wattage.

Yeah, it's a bit more complicated than that.

As for the wattage doubling thing, my (elementary) understanding is that amps' ratings are usually how much power they put out "clean," that is to say, before they start distorting. That is, that's about how much power they can put out without noticeable distortion in the signal.

However, pretty much all amps are able to put out much more power than this after running out of clean headroom. Perhaps twice as much power as the ratings given. Normally, especially with solid state amps, people like to have clean headroom so that when peaks happen, they aren't too distorted, as the general consensus seems to be that when SS amps "clip," they don't sound very good. Hence why you see people with 1000 watt amplifiers and such. They're not using all 1000 watts all the time, they just want the sound to stay clean while still being loud.

With tube amps, a lot of people (including myself), run them into distortion levels pretty much all the time because many of us find a distorted tube amp sounds good to our ears.

But it's not the distortion from clipping that hurts speakers. It's putting more power into them than they can handle.

You're probably not going to blow a 500 watt speaker with a 5 watt amp no matter how hard you drive it into clipping.

I think how this underpowering myth probably got started was someone using an amp that was rated at less wattage than the speakers were rated at, but was still capable of putting out enough peak power to kill the speakers, or perhaps dialing in so much low end that overexcursion was a problem.

I'm probably mistaken about some of this, as most of my knowledge comes from reading Bill Fitzmaurice's posts, and JimmyM's translations of those posts into language an oaf like me can understand.

 

[bongomania]

Jimmy's very fluent in Oaf.

 

[seamonkey]

Keep in mind that a loudspeaker is a linear motor, and voltage applied across it causes a current to flow, which interacts with the magnet to create a force.

When you apply a constant DC voltage across a motor does it stop? No. So why would the flat top of a clipped signal cause the loudspeaker to stop moving? Answer: it doesn't. It is just a constant accelerative force for that minuscule fraction of a second.

Two things blow loudspeakers: excessive power (thermal failure) and overexcursion (mechanical damage--e.g., bottoming out--from too much low-frequency energy).

If you hook up a battery the cone moves out and stops.
The spider and suspension is pulling back so work is being done.
Old voltage meters were the same idea. But true, there are no square waves just in bass frequencies, so this could never happen with a signal.

 

[JimmyM]

Jimmy's very fluent in Oaf.

not really, bongo. i have a genius-to-oaf dictionary.

 

[uglyrumor]

If the fundamental is 50 Hz, the first harmonic in the Fourier Series is 50 Hz. The second is 100 Hz, the third is 150 Hz, the fourth is 200 Hz, the fifth is 250 Hz, and so on.

Let's take "A". frequencies there are; 110, 220, 440, 880, 1760. How does that translate to 50, 100, 150, 200, 250?

 

[1n3]

Second time I've seen this, once from Sundogue, this time from SurferJoe. If the note you are playing has a frequency response of 50 Hz, the first harmonic will be 100 Hz. The SECOND harmonic will be twice that...200 Hz., NOT 150 Hz.

You are describing octaves, not harmonics.

 

[silky smoove]

not really, bongo. i have a genius-to-oaf dictionary.

So that's how you do it! Well played sir, well played

 

[uglyrumor]

ok...I gotta go back to school. Thanks! I thought the octave WAS the harmonic! Dammit, everytime I think I know something! So, the harmonic of A-110 would be A-220, and then a slightly sharp E-329? Then A-440 again? Then back to E-659 to A-880 to E-1318 (again, slightly sharp)?

 

[B-string]

There are odd and even order harmonics, many harmonics lay within the octave above and below.

 

[Devo-lution]

wow, surprise....

It's not the "might-or-might-not-be" chance of blowing a speaker that made me open this thread at all...

I just wanted do know if it would make for sonic differences?

 

[billfitzmaurice]

I just wanted do know if it would make for sonic differences?

That's easy: No. It just won't go as loud. That can result in a different perception of tone, see: Equal Loudness Curve.