Head and cab wattage maths

[Bill Fitzmaurice]

[quote name='28mistertee' timestamp='1482390667' post='3200067']
I'll look at ensuring the wattage of my head doesn't exceed rating of cabs.
[/quote]I repeat: [b]You can blow a speaker using an amp rated at half the speaker rating, you can use an amp rated for ten times the speaker rating with no issues.[/b]

[karlfer]

Excellent stuff Bill and Phil, many thanks.

Not going to worry too much about my Markbass and Schro, I don't do owt daft and they are way more powerful than I REALLY need. At least to my ears

Edited December 22, 2016 by karlfer

[28mistertee]

[quote name='Bill Fitzmaurice' timestamp='1482411588' post='3200280']
I repeat: [b]You can blow a speaker using an amp rated at half the speaker rating, you can use an amp rated for ten times the speaker rating with no issues.[/b]
[/quote]
Contradicts earlier advice given earlier on in thread. My head hurts 😞

[Bill Fitzmaurice]

[quote name='28mistertee' timestamp='1482431349' post='3200578']
Contradicts earlier advice given earlier on in thread.
[/quote]Not mine.

[28mistertee]

[quote name='Bill Fitzmaurice' timestamp='1482438382' post='3200648']
Not mine.
[/quote]
Would I be correct that we're talking singular cabs here Bill?
For example if I had a 500w head into a 500w cab and then added a second identical cab (500w into 1000w total rated cabs) surely doing this would reduce the chance of popping a speaker as you are spreading the load?

[Bill Fitzmaurice]

Amp power ratings are measured at low THD levels. If an amp will deliver 6dB more than it's rated for at high THD, which isn't uncommon, power output is quadrupled. Speaker power ratings are thermal, how much power they will handle before the voice coil melts. It's not unusual for their mechanical limits to be reached at half the thermal rating, especially with low frequencies. These are just two reasons why watts are possibly the least useful method to match amps and cabs. That said, a reasonable way to match them is to have the amp rating between one-quarter and twice the speaker rating.
Our hearing has a built in warning system that tells us when speakers are in danger of being damaged: they sound bad. When that happens turn it down.

[PaulWarning]

[quote name='Bill Fitzmaurice' timestamp='1482444104' post='3200732']
Our hearing has a built in warning system that tells us when speakers are in danger of being damaged: they sound bad. When that happens turn it down.
[/quote]this is true, until alcohol and adrenalin get involved

[Guest bassman7755]

[quote name='Bill Fitzmaurice' timestamp='1482438382' post='3200648']
Not mine.
[/quote]

Or mine. Think of it this way - using a 700w amp vs say a 900w amp into a speaker stack rated at 800w will not give you some magic guarantee that your speakers cannot be damaged under any circumstances.

The bottom line is that your speakers can either deliver the volume you need without damage or they cant. Yours probably can with plenty to spare so my advice is to quit agonizing over the paper power specs of the amp and get one that 1) sounds like you want and 2) can drive the speakers the volume you want.

Edited December 23, 2016 by bassman7755

[28mistertee]

Well said.... Thanks guys don't know where I'd be without this forum 😉

[Phil Starr]

[quote name='Bill Fitzmaurice' timestamp='1482411588' post='3200280']
I repeat: [b]You can blow a speaker using an amp rated at half the speaker rating, you can use an amp rated for ten times the speaker rating with no issues.[/b]
[/quote]

Whilst technically true this isn't helpful advice stated so baldly.

[b]You can blow a speaker using an amp rated at half the speaker rating, [/b]yes but only if you exceed the excursion limits of the drive unit. Most designers are aware of that and whilst a theoretical possibility it is less likely the lower the amplifier power.

[quote name='Bill Fitzmaurice' timestamp='1481819614' post='3195449']
If this was an issue reports of blown drivers would be rampant.
[/quote]

[b]you can use an amp rated for ten times the speaker rating with no issues.. [/b]But only if you keep it turned down. A 3000W amplifier is only a 300W amp if you turn it down to half (10dB) but turn it up full and in most cases you'll end up with a blown 300W speaker.

Mistertee the truth is that there is no way anyone can guarantee the reliability of an unknown combination of speaker, amp and crucially bass player. There's always a statistical chance of something failing as there is with any equipment. (our supposedly reliable VW Golf has just given us no end of trouble) if you do something stupid and use gear outside of it's design parameters it is more likely to fail but you might be lucky. Matching the amp to the speakers or having a little less power in the amp gives you a 90% or better chance of reliability for a long time.

I'd use the car analogy again; if you fit a huge engine into a car without modifying the brakes and suspension you might still drive it safely, some drivers can break even a family hatchback if they drive badly enough but drive the factory model as intended and you are probably going to be OK.

[PaulWarning]

I suppose this is where headroom comes in, if you get a rig more powerful than you're ever going to need you're not going to turn it up so much that you damage something, back to the original question, never mind the theory I always try and have my speakers rated a little higher than my amp, just seems to make sense

[chris_b]

This was my point a few pages ago. Design your rig so that the amp and cab will work together safely at the volume levels you require and you won't have to listen out for anything other than the great sound your band is making.

[chris_b]

[quote name='Phil Starr' timestamp='1482480798' post='3200861']
Whilst technically true this isn't helpful advice stated so baldly.

[b]You can blow a speaker using an amp rated at half the speaker rating, [/b]yes but only if you exceed the excursion limits of the drive unit. Most designers are aware of that and whilst a theoretical possibility it is less likely the lower the amplifier power.
[/quote]

Thank you Phil.

[Bill Fitzmaurice]

[quote name='Phil Starr' timestamp='1482480798' post='3200861']
You can blow a speaker using an amp rated at half the speaker rating[b], yes but only if you exceed the excursion limits of the drive unit.[/b] [/quote] An amp rated at 200w at 0.5% THD should be quite capable of delivering 400w at 2% THD, and therefore it would have no problem toasting a 300w rated voice coil. That's why you cannot assume that an amp rated for less than the speakers insures that it can't damage the speakers.

[quote]you can use an amp rated for ten times the speaker rating with no issues.[b]. But only if you keep it turned down. [/quote][/b]That fact should be self evident.