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driver11

matching watts on head and cab

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i'm thinking of getting a setup consisting of an 830 watt head with a cab that's rated for 800 watts. will the cab be able to handle that? or is the cab supposed to be rated at least the wattage of the head?

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Originally posted by driver11

i'm thinking of getting a setup consisting of an 830 watt head with a cab that's rated for 800 watts. will the cab be able to handle that? or is the cab supposed to be rated at least the wattage of the head?

I ran twice the rated power to my Genz Benz for over a year with no problem. The extra headroom just makes it that much cleaner.

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how does that work? with the extra headroom making it sound cleaner, by the way what do you mean by headroom? i always hear people on bass boards talk about that, mentioning they have 2000 watt heads going through cab rated for 1000...headroom is always mentioned, i was under the impression that that setup would blow the cab...

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Cab ratings are in RMS, that's Root mean Square. Meaning that the power handling rating is a value measured as an average along the waveform instead of at the crests or troughs. Meaning you can power a cab with an amp rated much higher and still be fine. Conversely, UNDERpowering a cab can be bad, so make sure your amp is at LEAST 800 watts for powering cabs rated for 800.

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my setup will be two 1x15 cabs (8ohms each) rated at 400 watts each powered by qsc rmx850 power amp (830watts at 4ohm)...

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Headroom means you attain a particular volume level (which corresponds to a calculated power level) using a smaller percentage of the available power by using a bigger amp. This leaves you more available resources to deal with dynamic changes and transients and raises the distortion threshhold so everything is more clear and distinct. There is a downside to a lot of headroom.

 

I run my home stereo at 2.5x the RMS ratings of my speakers. My wife (who is remote challenged) had tried to get the stereo to work one day when I was out working in the yard. Along the way she hit the mute button on the remote so, of course, it did not make any sound. So, of course, she kept turning it up, ALL the way up, trying to get sound. But because of the mute she got nothing. She turned on the TV and said to me when I walked in, "I can't get the stereo to work!"

 

I looked across the room, saw the mute light was lit, picked up the remote and told her, "You've got the mute on," as I pushed the button. When I heard my speakers sizzling I quickly pushed it again. Too late. Recone time. But friends and visitors are always commenting on the souund of the stereo. Clear. Distinct. You can distinguish the drumstick hitting the cymbal or the fingers sliding on the neck even in loud passages.

 

YMMV

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Headroom generally refers to the fact that an amp running at its maximum output will introduce distortion, much the same as your car makes a lot more noise as your speed increases.

 

For example: If you need 250 watts of power and only have a 250w amp, you'd be running the amp at 100%. If your needs were the same, but you had a 500w amp, you'd run at 50%, and with much greater clarity, and so on.

 

The 'headroom' is a result of the fact that amps actually don't operate on the basis of how much they let out, it's how much they let IN, and therefore, THROUGH! Running more power at a lower volume gives you all of the rich fullness, and hearty flavor a powerful amp provides, but without the total POTENTIAL volume.

 

It is challenging to explain without getting into too much electronics theory, but in essence, a 1000w amp is STILL a 1000w amp even when it's only turned up to 2.

 

 

As for matching, you can cheat quite a bit, but it is VERY important to know what the stated power ratings of both amps and speakers are, as well as what formulation the stated power ratings were based on, i.e. RMS or Peak, and keep within those confines.

 

Also, know that the rating of an individual speaker is multiplied by the nuber of speakers in use, to reach a total pwere rating. Ever hear of a 200w Marshall running through Celestion Vintage 30s? Well, there are usually EIGHT of those 30w speakers (8x30=240 watts) plus Marshall uses the PEAK output when they rate the amp. So, it's a pretty bulletproof configuration.

 

 

 

 

g

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If you have a 600 watt head and a 1000 watt head you can blow up your speakers if you turn it all the way up. If anything is all the way up it is capable ofproducing power quite a bit beyond what it is capable of producing without significant distortion.

 

A volume knob is in actuality an attenuator. That means turning up a volume knob is not adding more power, it is increasing the sensitivity of the input. The amplifier will produce whatever power it is capable of making. Turning down the volume knob makes the input less sensitive.

 

When a 40dB signal (dropped mike, mike falls on monitor with FEEDBACK, short circuit on guitar cord, etc) gets applied to attenuator setting of -10dB. +30dB of signal still gets through. If the amplifier produces more power with a +30dB input than your speakers can take, well.....

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Originally posted by driver11

so if you had a 1200 cab and a 1000 head, you can still blow the speakers if you turn it way up?

 

Yes, if the amp is 1000w RMS and the cab is 1200w peak.

 

There was a 'Joke Thread' on here the other day. Someone put up the joke: 'Why did Bono fall off the stage? He was dancing too close to The Edge.' Believe it or not, this is related.

 

What J the D did was cheat -- AND get caught doing it! As I said before, there are lots of ways to cheat, but the safest and most reliable setup -- the one that gives you the most lifespan for the money spent -- will be the one that pays the most attention to the correct technical details.

 

There are actually tube amps that were intentionally designed with underrated output transformers and/or rectifiers. Either of these features creates a very good effect, but if one little thing elsewhere in the circuit goes significantly off-spec, P*O*O*F! It's off to the shop with it!($$$$$) Same thing goes for tube biasing; you can bias them hot, and it'll sound reeaallllyy goood... for a while.

 

If you are gigging regularly, you will want the most reliable setup to get the job done. It's NO FUN AT ALL when your gear craps out on stage! J the D probably had a beatbox, or something else stashed away in the garage that he was able to hook up for the night and still have tuneage. But for playing out, you can avoid a lot of aggravation by following the rules, at least where the electronics are concerned!

 

Trust me on this. I make money fixing things that never should've broken in the first place!

 

 

g

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There have been similar threads to this in the past, and among these threads, you'll read posts where people claim that it is bad and/or potentially harmful to underpower a cab...and some posts will claim that you can underpower it without much problem, as long as you don't underpower it too much...

 

What gives? :confused:

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Underpowering is volume releated

 

If you had a cab rated for 300w and you used a 30w amp to rehearse in your bedroom with then you should be fine as you won't be cranking the volume

 

take the same setup into a rehearsal room with a drummer and guitarist and you'll need to wind the volume up to be heard.

 

When the amp runs out of puff to power the speaker it clips - sending raw DC current to the speaker.

 

This is what causes most speaker deaths

 

Power the same cab with a 400w amp and it's less likely to reach clipping as it can keep up the speakers demand for power.

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alright, i don't know anything about the technical specs, etc., so can someone tell me if the following setup will be safe to use without blowing speakers, etc.:

 

two mesa boogie 1x15 cabs, 8ohms each, 400 watts each (running them together, 800watts, 4ohms)

 

qsc rmx850 power amp (830 watts 4ohms)

 

ampeg svp pro

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I can run from 1950w to 750 watts to my 700w cab depending on the amount of head room i want and how loud i need to get. You need about 50 or so watts of headroom. A lotof combos come with speakers overrated for the amp, like my ampeg Balls(BA115)

it will do 100w out of the amp and the speaker is 130watts. I think it will throw out a little more than 100w on the low end though.

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Yes, as you can see speaker power rating is not an exact science. Many variables affect the manufacturer's power ratings, including the low frequency cut-off of the cabinet (below the cut-off, the speaker is no longer correctly loaded mechanically with the enclosure... like impedence matching), peak to average ratio of the signal (kick drum is vastly different from bass), amount of distortion used (intentionally or otherwise), amount of compression/limiting and many other items.

 

Speakers actually have two different power ratings, mechanical and thermal. For guitar speakers, thermal is probably the most important number, for slapping bass the mechanical number is more important. Unfortunately, most manufacturers give only one number, which is more-or-less a wild ass guess of what the speaker can take while holding together for most users. It is a gross simplification that works ok for most users, but definately not all.

 

I have seen damage from both underpowering and overpowering... nowadays itis mostly exterme overpowering that brings a lot of expensive drivers into the shop, and it is almost always mechanical damage on subs!

 

I have posted more detailed info before on the live sound forum, a search should turn up.

 

Andy Field

Product Development Engineer

Genz Benz

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Originally posted by the_big_geez



J the D probably had a beatbox, or something else stashed away in the garage that he was able to hook up for the night and still have tuneage. But for playing out, you can avoid a lot of aggravation by following the rules, at least where the electronics are concerned!


Trust me on this. I make money fixing things that never should've broken in the first place!



g

 

 

Very true. I understand the stereo speakers are at risk and knowingly choose to overpower them to get the headroom benefits at normal listening levels.

 

My primary bass rig is a GK 400RB putting 120 watts into an 8 ohm Eden cab rated for 200 watts. When the money is on the table reliability is everything. If I need to get real loud I go through the PA.

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Here's my understanding of the problem - and agedhorse is welcome to correct me if there are huge holes in my logic :)

 

Audio power amps don't put out DC or anything close to it - even when clipping.

 

As the amp clips the top and bottom of the sine wave flatten out but it still doesn't get anywhere near DC. As clipping (and distortion) increases the amount of RMS power applied to the driver also increases. When you exceed the thermal limitations of the speaker the speaker fails - but it fails from being overpowered, not underpowered.

 

The shape of the wave isn't what causes the speaker to fail - it's the amount of power sent to the speaker.

 

If you have a 300w speaker it's not possible to cause thermal overload with a 30w amp. With a 250w amp that's clipping it may still be possible to exceed the thermal limits of our 300w speaker - but that's because you dumped more than 300w into the speaker - not because of the shape of the waveform.

 

Of course, mechanical damage is another thing.

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Allan's got the clipping thing nailed. A watt is a watt, and assuming accurate ratings, a 100w speaker can safely handle no more than 100watts.

 

The "underpower" thing is very deceiving. Lets say you have a speaker rated for 150w RMS, and a tube amp rated at 100w RMS at

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To get back to the original question, Driver, you should be fine with the setup you described, just don't get in the habit of running full-bore. It's safer that way!

 

Much the same as all the great advice you're getting here, the warranty is limited. As I said, you SHOULDN'T have a problem, but...

 

And now you probably know WAY more than ever thought you would on the subject!

 

 

 

g

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Originally posted by allan grossman

Here's my understanding of the problem - and agedhorse is welcome to correct me if there are huge holes in my logic
:)

Audio power amps don't put out DC or anything close to it - even when clipping.


As the amp clips the top and bottom of the sine wave flatten out but it still doesn't get anywhere near DC. As clipping (and distortion) increases the amount of RMS power applied to the driver also increases. When you exceed the thermal limitations of the speaker the speaker fails - but it fails from being overpowered, not underpowered.


The shape of the wave isn't what causes the speaker to fail - it's the amount of power sent to the speaker.


If you have a 300w speaker it's not possible to cause thermal overload with a 30w amp. With a 250w amp that's clipping it may still be possible to exceed the thermal limits of our 300w speaker - but that's because you dumped more than 300w into the speaker - not because of the shape of the waveform.


Of course, mechanical damage is another thing.

 

Absolutely correct, and well stated also.

 

That's why the shape of the waveform is so important in calculating out the thermal power.

 

Really squared off waveforms cause some additional mechanical damage from the accelerations involved, and often times an amp can oscillate around the leading and trailing edge of the square wave. This can cause some unpredictable results. Also, when a power amp is clipped (rather than a preamp signal) the power amp will lose control of the speaker since once you hit clip, the amp has no control of the cone until it re-enters normal operation. This can really cause damage at high power levels!

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Craig, Allan, Agedhorse:

 

Well said. I'm in exam mode right now and didn't have time to write one of my trademark clipping and speaker damage rants;)

You guys have taken care of it for me. Heheh....

Here's a couple of recent observations, one related to Agedhorse's last post and another that came to me when I was doing some reading on cab design: A lot of people say that when an amp clips, the flat top of the waveform locks the speaker in place, imitating DC. I've always contended that a square wave of a certain frequency will move a speaker in and out the same number of times per second as a sine wave, and it does. But on top of that, the speaker's inertia (due to cone mass) will keep the cone moving while the wave top is flat. It doesn't actually get "locked into place." This is where the amp doesn't actually have control of the cone.

It's also quite possible to thermally damage a speaker with clean power well below the rated power of that speaker. Power handling varies with frequency, so cab design come into the equation. In a ported cabinet, speaker movement is damped by acoustic suspension. At the port tuning frequency the driver barely moves at all. So the cooling efficiency of the driver is compromised. This means that it's power handling is also compromised. Therefore, it's possible to do thermal damage at that frequency with perfectly clean power well below the rated power of the driver.....

 

I find this subject fascinating. It's very complex and there are a lot of factors at play. I'm actually considering doing the senior project for my EE degree on the subject....

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