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I have a Laney bc75 for house practice but someone had swaped the speaker for a rubbish one. So I swapped it for a 400w 8 ohm so I could also use the cab with my Behringer Ultrabass 300. What's confused me is my 300 is 300w at 4 ohm or150w at 8 ohm. Am I right in thinking if I put two 8 ohm I will still get 300w or could I mix and put a 4 and 8 ohm together. The 400w speaker seems to struggle which I didn't think it would if my amp is only 300w. Any ideas

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The power an amp can drive is roughly halved if you double the resistance.

If you connect two 8 ohm speakers in parallel they will act like one 4 ohm speaker with each getting half the power. So yes your amp will give 300w into 4 ohms

You say home practice so is it worth the cost for a little extra volume you probably don't need?

 

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7 hours ago, Gary1963 said:

Thanks Bill. That's very technical for me

You're the bassplayer, that makes you the smart one in the band. Really. The majority of top tier engineers and designers who also play an instrument are bass players. My personal experience, that many others have also stated, is that since the equipment demands of bass are so challenging they more or less had to learn the science. It's a more than worthwhile endeavor. I made a tolerable enough living as a musician when that was my primary source of income, but I made my fortune as an engineer. 

To be more helpful with your issue I'd need the particulars on the driver and enclosure.

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16 hours ago, Gary1963 said:

. . . .  Am I right in thinking if I put two 8 ohm I will still get 300w or could I mix and put a 4 and 8 ohm together. The 400w speaker seems to struggle which I didn't think it would if my amp is only 300w. Any ideas

Define "struggle". Define "house practice"! Sounds like you are playing a lot louder than I do at home! The difference between 8 ohms and 4 ohms at home shouldn't be so noticeable. Are you sure the 400 is the watts and not just the model number?

You can connect 2 8 ohm cabs or 1 4 ohm cab and your amp is at it's max. That's fine, but you can't mix 4 and 8 ohm cabs because that will look like 2.67 ohms to your amp and that is beyond what it can handle. I don't know Behringer amps but depending on how clever the designers were, but at 2.67 ohms, you might see the amp just power off when it gets too hot or you could cause damage to the amp.

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2 hours ago, chris_b said:

Define "struggle". Define "house practice"! Sounds like you are playing a lot louder than I do at home! The difference between 8 ohms and 4 ohms at home shouldn't be so noticeable. Are you sure the 400 is the watts and not just the model number?

You can connect 2 8 ohm cabs or 1 4 ohm cab and your amp is at it's max. That's fine, but you can't mix 4 and 8 ohm cabs because that will look like 2.67 ohms to your amp and that is beyond what it can handle. I don't know Behringer amps but depending on how clever the designers were, but at 2.67 ohms, you might see the amp just power off when it gets too hot or you could cause damage to the amp.

Thanks for that advice. Yeh it is definitely 400w speaker, but there seems to be sent sort of fuzz there especially if I longer on a note. Maybe my cab needs venting as there is nothing to let the bass out. Don't know if that would make the sound better

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Venting the cab may help, but you can't just willy nilly cut a hole in it. The area and depth of the vent is critical to a good result, as is the cabinet volume, and both are determined using the driver Thiele/Small specs with speaker modeling software. Or the driver specs may not be suited to a ported enclosure at all.

 

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1 hour ago, Gary1963 said:

Yeh it is definitely 400w speaker, but there seems to be sent sort of fuzz there especially if I longer on a note.

Fuzzy notes doesn't sound right. Maybe the driver has a problem or there might be an issue in the amp.

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1 hour ago, chris_b said:

Fuzzy notes doesn't sound right. Maybe the driver has a problem or there might be an issue in the amp.

Does it with both amps think it could be speaker or venting

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Also it's a lottery putting a different speaker into a cab. I like you don't understand the science but am happy to believe those that do. Each speaker needs a different amount of air or something. 

I have got lucky swapping out a knackered speaker for one of a different make and power rating but I'm aware it may not have been all that efficient. 

Good luck however you decide to play it. 

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9 hours ago, Gary1963 said:

Thanks for that advice. Yeh it is definitely 400w speaker, but there seems to be sent sort of fuzz there especially if I longer on a note.

Maybe you have a loose wire resting on the cone? Have yo taken the driver out and had a look?

 

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It's probably worth going over the whole driver and the cab itself. If it's an old speaker sometimes the dust cone in the middle or even the corrugated surround  start to come loose as the glue hardens and breaks over time. anything loose in the cab will sound and anything not screwed down properly or even the speaker grille might be making a noise.

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8 hours ago, Phil Starr said:

It's probably worth going over the whole driver and the cab itself. If it's an old speaker sometimes the dust cone in the middle or even the corrugated surround  start to come loose as the glue hardens and breaks over time. anything loose in the cab will sound and anything not screwed down properly or even the speaker grille might be making a noise.

Thanks I will check everything over the weekend

19 hours ago, chris_b said:

Maybe you have a loose wire resting on the cone? Have yo taken the driver out and had a look?

 

Yeh not that

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