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Jonny Walker

A quick question ?????

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If i get a (750w @ 2 550w @ 4 280w @ 8 Ohm) head would it be better to run it on a single (1000w 4 or 8 Ohm) cab or a 4x10 (8 or 4 Ohm) and 1x15 (4 or 8 Ohm) both of which are 600w ???

If you understand me please help!!!

Edited by Jonny Walker

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Dont keep getting hung up on power ratings. There is probably no difference between an 8x10 or 2x4x10, or even 4x2x10 as long as the impedence is right. There are some that say you should not mix up different sized drivers unless you are using a cross-over.

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There are no issues with any of these combinations. Just adhere to the golden rule....."If anything starts to distort when you don't expect it to, then turn it down".

The power supplied by the amp is determined by the impedance of the cabs attached. If you use two cabs it's usually better to use two with the same impedance, and these will be connected in parallel.

2 x 8 ohms in parallel = 4 ohms total

2 x 4 ohms in parallel = 2 ohms total

The total power rating for cabs in parallel is just the sum of the two single power ratings (and if they're the same impedance they will split that power equally).

Your single 1000 W cab will draw a maximum of 550W if its 4 ohms and 280W if its 8 ohm.

Your two cabs will draw a maximum of 550W (2 x 8 ohms cabs) or 750W (2 x 4 ohms).

These figures are all well within the cabinet ratings.

Do make sure the amp is ok running into a 2 ohm load before you choose that option (a quick question here or to the manufacturer will confirm that)

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[quote name='bass_ferret' post='141618' date='Feb 16 2008, 04:17 PM']Read the sticky on impedence. You cant set the impedence of speakers.[/quote]

Apparently it can be switched form 4 to 8ohms, isnt that the impedence?

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[quote name='Jonny Walker' post='141614' date='Feb 16 2008, 04:14 PM']So if i set the head to 2 ohms and the 4x10 and 1x15 to 4 ohms ill get the full 750w, but only 550w with the 1000w cab (which is a 1x15 in the middle of a 4x10) ?[/quote]

Yes.

It's the impedance (ohms) that determines the power drawn from an amp NOT the power rating of the cabs. Solid state amps always produce more power into lower impedances (e.g. 750W into 2 Ohms as opposed to 550W into 4 Ohms).

BUT, and it's a big BUT, note that these power ratings are manufacturer specs using test signals and only give you an idea of what the amp is capable of.

Also, Watts does not equal Volume. You need approximately 10 times the power to get twice the volume (all else being equal) so the difference between 750W and 550W is not as big a difference in volume as you might think.

Finally, the efficiency of the cabs (Sound Pressure Level - SPL) had a huge effect on the volume of a rig. It's not as simple as it seems at first.

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[quote name='BOD2' post='141621' date='Feb 16 2008, 04:31 PM']Yes.

It's the impedance (ohms) that determines the power drawn from an amp NOT the power rating of the cabs. Solid state amps always produce more power into lower impedances (e.g. 750W into 2 Ohms as opposed to 550W into 4 Ohms).

BUT, and it's a big BUT, note that these power ratings are manufacturer specs using test signals and only give you an idea of what the amp is capable of.

Also, Watts does not equal Volume. You need approximately 10 times the power to get twice the volume (all else being equal) so the difference between 750W and 550W is not as big a difference in volume as you might think.

Finally, the efficiency of the cabs (Sound Pressure Level - SPL) had a huge effect on the volume of a rig. It's not as simple as it seems at first.[/quote]

Thanks very much BOD2!
so even tho Mesa says that they are both 600w they could produce 750w right?

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[quote name='Jonny Walker' post='141620' date='Feb 16 2008, 04:25 PM']Apparently it can be switched form 4 to 8ohms, isnt that the impedence?[/quote]
Ah - these new fangled impedence switching cabs. A speaker moves as a result of electricity going through a coil of wire set on a magmet. It is the resistance of this wire that gives the driver its impedence (impedence is the term used to measure resistance in response to alternating current). I assume this impedence switching works by taking a tap out of the coil and only using half of it. I am not sure what the effect on power handling and sensitivity of

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Cabs dont produce power. Amps produce the watts, cabs turn the watts into decibels. If you give a cab too much beans it will either melt the coils (thermal overload) or destroy the suspension (mechanical failure).

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I've got to admit, I've never heard of cabs with switchable impedance, so I checked the Mesa website. And yes, they say 4 or 8 ohm. But that just refers to the cab options available - you just choose either a cab loaded with 4 ohm speakers or one with 8 ohm speakers. Tellingly there's no switch on the cab to select impedance.

What head are you thinking about - the SWR 750x? If so, the minimum impedance that amp will handle is 2.6 ohms - that's the equivalent of one 4 ohm and one 8 ohm cab, with the 4 ohm cab receiving more power than the 8 ohm.

Like bass ferret says, don't get too hung up over the wattage. Watts tell you very little about the volume. If you really want more volume you might be better off going for more efficient cabs.

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[quote name='Musky' post='141669' date='Feb 16 2008, 07:11 PM']I've got to admit, I've never heard of cabs with switchable impedance, so I checked the Mesa website. And yes, they say 4 or 8 ohm. But that just refers to the cab options available - you just choose either a cab loaded with 4 ohm speakers or one with 8 ohm speakers. Tellingly there's no switch on the cab to select impedance.

What head are you thinking about - the SWR 750x? If so, the minimum impedance that amp will handle is 2.6 ohms - that's the equivalent of one 4 ohm and one 8 ohm cab, with the 4 ohm cab receiving more power than the 8 ohm.

Like bass ferret says, don't get too hung up over the wattage. Watts tell you very little about the volume. If you really want more volume you might be better off going for more efficient cabs.[/quote]

I'm aiming to get the Mesa Big Block 750...

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[quote name='Jonny Walker' post='141770' date='Feb 17 2008, 01:39 AM']I'm aiming to get the Mesa Big Block 750...[/quote]

In which case you'll be fine with a pair of 4 ohm cabs.

It's worth bearing in mind that a doubling of power will give you a 3db increase in volume - that's the same increase you'll get as doubling the number of woofers, i.e. if you're using a single 4x10 at the moment, buying a pair will give the same increase in volume (assuming overall impedance remains the same). Likewise, using cabs with a higher sensitivity will give you a useful increase in volume - the difficulty here being that manufacturers are often less than forthcoming with meaningful sensitivity ratings or just 'economical' with the truth (as BFM often bemoans).

Volume seems to be your main issue here, but unfortunately there's more to perceived volume than simply the wattage. To complicate matters, amps with a valve power stage will often sound louder than an SS amp with the same rating.

This topic has cropped up a lot in this forum, and it might be worth your while trawling through a search on 'SPL'.

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[quote name='Musky' post='141866' date='Feb 17 2008, 01:20 PM']In which case you'll be fine with a pair of 4 ohm cabs.

It's worth bearing in mind that a doubling of power will give you a 3db increase in volume - that's the same increase you'll get as doubling the number of woofers, i.e. if you're using a single 4x10 at the moment, buying a pair will give the same increase in volume (assuming overall impedance remains the same). Likewise, using cabs with a higher sensitivity will give you a useful increase in volume - the difficulty here being that manufacturers are often less than forthcoming with meaningful sensitivity ratings or just 'economical' with the truth (as BFM often bemoans).

Volume seems to be your main issue here, but unfortunately there's more to perceived volume than simply the wattage. To complicate matters, amps with a valve power stage will often sound louder than an SS amp with the same rating.

This topic has cropped up a lot in this forum, and it might be worth your while trawling through a search on 'SPL'.[/quote]


Thanks very much !

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