Head and cab wattage maths

[28mistertee]

Probably been done to death but finding lots of conflicting advice on matching heads and cabs wattage wise especially with more than one cab.
I was always under the impression that your head had to be rated less than the cabs as not to overpower the speakers but I'm also reading that it's also possible to underpower them. Some also say as long as impedances match then all is cool?
In my case I run a 212 and a 210 cab which are 500 and 400 watts respectively so this would be a combined wattage of 900watts??
Head wise am i better off to run a 300, 500 or 1000watt? Apologies again for a novice question but i seem to be finding lots of conflicting advice.
Cheers 👍

[PaulWarning]

depends on ohms and stuff like that, I presume with 2 cabs you're running at 4 ohms, I'd settle for a 500 watt head at 4 ohms, plenty loud enough for a medium size venue and you'd still be ok to use just your 500 watt cab if you wanted, basically if it's starting sound distorted you speakers will probably blow, well that's my opinion anyway, unlikely though with that amount of power on tap

[Phil Starr]

You need to know the ohms of your cabs. They are more important than the wattage. If they are both the same impedance (ohms) they will share the power evenly. If not the power will take the line of least resistance and go mainly through the cab with the lowest ohms. If both cabs are 4ohms than you can only use one at a time with the majority of heads anyway. More when you come back with the ohmage.

[28mistertee]

Hey guys thanks for the replies. I'm happy with impedances it's the wattage I'm unsure on. For the record they are 8ohm each (total load 4ohms).

[Bill Fitzmaurice]

Wattage ratings mean next to nothing. Cab power ratings don't consider driver excursion, which is the primary limiting factor in output. An amp can't be too large, assuming that the volume control is functional, along with the brain of the user. And since sound levels are logarithmic, meaning it takes ten times the power to sound twice as loud, the audible difference between most amps is modest in any event. Other factors, especially speaker response and sensitivity, are far more significant than watts.
And no, you can't under power a speaker.

[Acebassmusic]

[quote name='Bill Fitzmaurice' timestamp='1481754622' post='3194959']
.......assuming that the volume control is functional, along with the brain of the user......
[/quote]

I don't know if there is a "Quote Of The Year 2016" thread but this would be my nomination. I get the feeling I will be using this phrase in the not too distant future

[CrackerJackLee]

Don't forget the Sensitivity of the speaker...

Two small vids.

[url="https://www.youtube.com/watch?v=QaDO9aTFDdk"]https://www.youtube....h?v=QaDO9aTFDdk[/url]

[url="https://www.youtube.com/watch?v=Y9rur78FE1k"]https://www.youtube....h?v=Y9rur78FE1k[/url]

A police siren may be limited in the power it can handle (wattage) but be a very loud siren (decibels).

Power is the product of voltage applied and current. Too much power will burn out the speaker coil. Speakers are rated by the manufacturer in the amount of power you can apply to it.

The voice coil has resistance. Apply voltage across it and current will flow through it. The speaker moves. But it can only handle so much power. Power = current x voltage.

The unit for loudness is the decibel, named after Alexander Graham Bell (dB).
1 dB is the smallest amount of loudness increase/decrease that humans can perceive.
As our hearing is logarithmic, in order to make the siren twice as loud, you will need to apply 10 times as much power. BOOM! And if you double the power, you will increase the loudness by 3 dB as per the formula:

y=10logx
y=10log(2)
y=3



And be aware of this sales gimmick:

Edited December 15, 2016 by CrackerJackLee

[Phil Starr]

OK two cabs of 8 ohms is good that means 4 ohms load which will match any amp, just about.

That means both speakers will get the same power, so if you did go for a 1000W amp both speakers would get 500W and there'd be a good chance of blowing the 400W speaker. The actual thermal rating of the two cabs is twice the lowest powered cab. 800W in your case.

You are being given two other, possibly confusing, bits of advice:

All the stuff about decibels. What people are really saying is that if your amp is loud enough you don't need more watts. If you look you'll see that most manufacturers have settled at amps that produce about 300W into 8ohms and 500W into 4. That will give you enough poke to be more than loud enough. Going to 1000W will only make you one notch louder (5000w would make you twice as loud!) and damage your speakers so not a good idea. In fact with 4 speakers in two cabs you will probably have a very efficient system louder than most of us use nowadays.

The second advice is about excursion and watts not mattering. The trouble is there are a number of ways you can blow your speakers. That 400w rating is what will happen with a constant test tone based on mainly mid-range sound. You can measure how hot a speaker gets with a test tone and if it doesn't get too hot then that is the power it can handle. It's a system that works well for most speakers but not for bass. The other speaker killer is bass, too much bass and the speakers coil is pushed out of the magnet and may even hammer against the back of the magnet. A speaker that can handle 500w of midrange may only be able to handle 100W of deep bass, maybe even less. The trouble is that no-one can predict how much bass you like or what effects you use so no-one can say how much the power rating of your amp should be to be safe. All I would say is that most people using a 500W amp with 800W of speakers isn't going to have a lot of reliability problems. Anything less than that will be fine, any more and you had better start reading. It's all very well for an expert to push the boundaries but if you just want to get on and concentrate on playing knowing your gear is enough then 3-500W is more than enough.

[28mistertee]

Some great advice on here thanks. I've had various wattage rigs over the years and always thought my old 350w Trace felt just as powerful as my SVT4 so the post about wattage makes sense. I'm currently in a transition of changing my setup so this is greatly appreciated, ta 😊

[PaulWarning]

[quote name='28mistertee' timestamp='1481796193' post='3195158']
Some great advice on here thanks. I've had various wattage rigs over the years and always thought my old 350w Trace felt just as powerful as my SVT4 so the post about wattage makes sense. I'm currently in a transition of changing my setup so this is greatly appreciated, ta
[/quote]Everybody knows Trace Elliot watts are twice as loud as anybody else's, my TE 250watt head is louder than my Fender Rumble 500watt

[28mistertee]

[quote name='PaulWarning' timestamp='1481798037' post='3195176']
Everybody knows Trace Elliot watts are twice as loud as anybody else's, my TE 250watt head is louder than my Fender Rumble 500watt
[/quote]
Oh yes do miss my old TE sometimes 😉

[alexclaber]

You only need as much power as you need to get loud enough - and the basic rule of thumb is that if your rig is twice as big then you'll only need half as much amp power to get loud enough.

[franzbassist]

[quote name='Acebassmusic' timestamp='1481764356' post='3195016']
I don't know if there is a "Quote Of The Year 2016" thread but this would be my nomination. I get the feeling I will be using this phrase in the not too distant future
[/quote]

+1

[28mistertee]

[quote name='alexclaber' timestamp='1481806272' post='3195290']
You only need as much power as you need to get loud enough - and the basic rule of thumb is that if your rig is twice as big then you'll only need half as much amp power to get loud enough.
[/quote]
I am looking for a new head (quite interested in the Orange stuff) but until now with my SVT4pro what is the best way to run these cabs then?
2 cabs (1 out of each power amp 350w 8ohm)
2 out of one power amp (625w 4ohm)
Mono bridge (1600w 4ohm)
Looks like the best route with a future head would be a 500w which would be closest match for my cabs?

[Bill Fitzmaurice]

On the issue of bridging:
http://billfitzmaurice.info/forum/viewtopic.php?f=10&t=19292

[Phil Starr]

The first two options are both good, really follow Bills advice on bridging and don't.

power wise I don't think you'd find any difference between one per channel and two. 312.5W and 350W aren't going to be noticeably different and frankly it is only 625W into 4ohms because the power supply limits your amp to 625W. I'd run one speaker per channel that gives you the chance to balance the outputs, If a channel went down you'd still have sound and you've reduced the current going through your output stage meaning that amp will run a little cooler.

[28mistertee]

Thank you both for the advice and point well made. Looking at changing my head anyway to maybe a 500w so it's just for the next couple of gigs. 👍

[Phil Starr]

Last bit of advice, choose an amp on the way it sounds not the spec, that's a nice amp you have so only upgrade when you are sue what you have is better.

Good luck, it's fun looking.

[Marty Forrer]

One thing that needs stating. There is a big difference between a clean signal and a dirty signal. Most speakers will handle a pure signal many times their rating, but not if the signal is dirty. I went to a Peavey clinic once where 30,000 watts of power amps were cranked full on into a 500 watt speaker with a pure signal running, and it was fine. As soon as some dirt was introduced the speaker started farting and our ears started hurting. I have run my GK MB800 (500w at 8ohms) into my Barefaced One10 cab (200w) no problem, but I run the cleanest signal I can get.

[Bill Fitzmaurice]

[quote name='Marty Forrer' timestamp='1481855735' post='3195782']
Most speakers will handle a pure signal many times their rating, but not if the signal is dirty.
[/quote]Speakers can't tell the difference, and they're unfazed by clipped signals. Were that not the case guitar players would have to change drivers as often as they do strings. Tweeters can be toasted by clipped signals, because the high frequency power density is greater when the signal is clipped. Woofers, never.

[28mistertee]

[quote name='Phil Starr' timestamp='1481848064' post='3195759']
Last bit of advice, choose an amp on the way it sounds not the spec, that's a nice amp you have so only upgrade when you are sue what you have is better.
I couldn't agree more although it won't be necessarily better but just different. My cabs are Glockenklang which i really love and are very natural sounding so I'm just looking for a suitable head for these. Just fancy a change really.

Good luck, it's fun looking.
[/quote]

[Phil Starr]

[quote name='Marty Forrer' timestamp='1481855735' post='3195782']
One thing that needs stating. There is a big difference between a clean signal and a dirty signal. Most speakers will handle a pure signal many times their rating, but not if the signal is dirty. I went to a Peavey clinic once where 30,000 watts of power amps were cranked full on into a 500 watt speaker with a pure signal running, and it was fine. As soon as some dirt was introduced the speaker started farting and our ears started hurting. I have run my GK MB800 (500w at 8ohms) into my Barefaced One10 cab (200w) no problem, but I run the cleanest signal I can get.
[/quote]

I don't believe it. That equates to 60A into an 8 ohm load, far too much to be carried by the thin copper wires in a speaker for more than a few miliseconds. you'd not find a mains supply capable of producing that power in most buildings. I've just run a check on a cab with a Peavey Black Widow and the sound levels would reach 150dB, that would have damaged the hearing of anyone within 10m of the speaker. Unless you've misremembered the figures some salesman was telling porkies.

If the figure is 3,000W then there is nothing remarkable. A few seconds at that power wouldn't heat the coil enough to damage it and if the pure tone was above 200Hz the speaker wouldn't exceed it's maximum excursion. It would still be enough to damage peoples hearing without some sort of ear protection and would be impressively loud. Dramatic but proving nothing.

[alexclaber]

Clean signals are no less damaging than dirty signals if the average power is the same. The key thing is that for a signal to remain totally clean the amp needs to have some headroom left - and if it has headroom left, and you're playing uncompressed bass guitar, the average level can't be more than about 10% of the amp's power rating because the crest factor of bass guitar is so high.

[karlfer]

[quote name='alexclaber' timestamp='1481882922' post='3195888']
Clean signals are no less damaging than dirty signals if the average power is the same. The key thing is that for a signal to remain totally clean the amp needs to have some headroom left - and if it has headroom left, and you're playing uncompressed bass guitar, the average level can't be more than about 10% of the amp's power rating because the crest factor of bass guitar is so high.
[/quote]

This is beyond me. Most things are beyond me.

Am I to understand that a Markbass LM800, run with no effects, no compressor (I never use either) all EQ points noon, VLE/VLF (whatever they call them) off, that the maximum average ? I'm achieving is around 80 watts?

Edited December 16, 2016 by karlfer

[alexclaber]

[quote name='karlfer' timestamp='1481886392' post='3195925']
This is beyond me. Most things are beyond me.

Am I to understand that a Markbass LM800, run with no effects, no compressor (I never use either) all EQ points noon, VLE/VLF (whatever they call them) off, that the maximum average ? I'm achieving is around 80 watts?
[/quote]

Yes, if the preamp and power amp are never clipping. Sorry about that!