What Ohms Cabinet should I get?

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beagle
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What Ohms Cabinet should I get?

Post by beagle » Thu Feb 22, 2007 9:28 pm

Hey this topic came up in another post and it's a really helpful thing to know when buying speaker cabinet(s).

All bass amps are optimised for a certain Ohms rating (Ohms = resistance) and to get the most volume output from your amp you should match the total speaker Ohms with the amplifers optimised rating.

Here's a practical example...
My amp outputs 200watts of power at 4 ohms.

This means I can plug in 1x speaker box rated at 4 ohms....
or 2x speaker boxes rated at 8 ohms each.

Duh you say? how does 8ohms + 8ohms = 4 ohms?? is this guy nuts?

As said before... Ohms is a measurement of resistance.

Amplifiers with dual speaker outputs are generally wired in parallel. When you connect two 8 ohms speakers in parallel you actually half the resistance and get a total output of 4 ohms.

So in summing up... If you want the loudest possible sound from your amp and don't want to carry more than one speaker box.... buy a 4 ohm cabinet for your 4 ohms amplifier etc...
or buy 2x 8 ohm cabinets for your 4 ohms amplifer.... which gives you the ability of mixing different speaker combinations for a wider tonal range.... i.e. 1x 15' + 4x 10".

And a warning... Never run a total speaker Ohms rating lower than your amplifier is specified for. Higher is fine as this will just result in a volume drop. If you're not sure on the wiring then just ask first ;)

Hey guys please feel to correct me on anything here.

Steve ;)

p.s. I've just bought a second 1x 15" 8ohm cabinet to go with my 4x10" 8ohms cabinet and am over the moon with the result. I've been missing out on the extra power for way too long.
Last edited by beagle on Fri Feb 23, 2007 11:12 am, edited 1 time in total.

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Post by Jazzbass » Fri Feb 23, 2007 8:45 am

We all know that more speakers = more air pushed = perceived louder/better tones (most of the time). But many SS combo amps are already optimised to matching impedance (such as 4Ω as beagle says) and don't have extension speaker outlets. This also works with heads and sep cabs, of course.

Sometimes we want to add more speakers, but can't find a way.

Here's a little trick I use:

Add a jack outlet somewhere suitable (I use the metal surround around the handle area in one side of the combo). Now, if you wire this in series with the speaker, so that when a jack plug is inserted it effectively open-circuits and there will be no sound - unless you plug it into another cabinet, when a series circuit will be established.

Now a series circuit will ADD to the impedance, so let's say you have a 4Ω extension cab and you run it with your 4Ω combo. This will give you 8Ω total load and that will reduce your amp's output. Thus your 100W amp will now be about 80W.

You will NOT notice this power drop at all, other than maybe a need to go up one notch on your master volume, since we already know that to double your loudness you need 10 times the power (if you have a 100W amp and want to be twice as loud, you must have a 1000W amp! - or if you have a 200W amp and want to be twice as loud, you need 2000W!). However, you will have a deeper/fuller sound, because you are using more speakers.

Despite what the salesman will tell you - there is almost NO perceived loudness difference between a 150W and a 200W amp - the human ear can't detect it. What you do have is more headroom and more transients allowances. There will be louder sound, but if you were to put a 150W and a 200W amp side by side - YOU wouldn't be able to hear the difference in volume, but you might hear/feel more 'balls', because of that.

Do remember with extension cabs though, that if you run them in parallel, every one you add will DECREASE the total impedance. So if your amp has a lower limit of 4Ω and you plug another 4Ω cab in parallel, you will have an effective total impedance of 2Ω. This is bad and will damage your amp.

Rog

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frequency

Post by nzsimon » Fri Feb 23, 2007 9:55 am

running cabinets side by side ( tightly) also decreases the attainable lowest frequency due to the coupleing effect

Ie two cabinets together will go lower than one

you also gain up to 3 db for every surface the box is touching ie if in a corner you can gain up to 9 db or nearly double the volume particuly useful with subs not much use on an open stage but in some small clubs there are gains to be had
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Post by Jazzbass » Fri Feb 23, 2007 9:58 am

Agreed - but bear in mind running bass cabs along a wall will transmit deep bass all around the place and people will complain - often loudly, IME! :)

It can cause imbalances in stage mix.

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Post by beagle » Fri Feb 23, 2007 11:14 am

Hey great comments guys. Some ex points there I never thought about ;) Cheers!

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Re: What Ohms Cabinet should I get?

Post by timi » Sun Feb 25, 2007 4:51 pm

beagle wrote: Never run a total speaker Ohms rating lower than your amplifier is specified for. Higher is fine as this will just result in a volume drop.
Assuming that bass players have solid state amps......

A tube amp should ALWAYS be matched, to the best of my knowledge running a higher rated cab will be quieter AND put a lot of strain on the output transformer.

Also for tube amps, do not turn them on without a speaker connected.

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Re: What Ohms Cabinet should I get?

Post by Crazykiwi » Sun Jan 27, 2008 9:55 pm

We have regular battles over speaker cab specificiations on my forum. Its like a honeypot for geek flies there! :)

There's bit of a guide and some discussion if you're interested:
http://basschat.co.uk/index.php?showtopic=135

I would have preferred to cut and paste here but there are some user posted images which I can't replicate, unfortunatley.
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Re: What Ohms Cabinet should I get?

Post by foal30 » Tue Feb 05, 2008 11:30 am

yes, great link and comments.

Here is the bit I struggle with, you should use the same number (8ohm for instance) for both cabs.
Not say 1 x 4ohm and 1x 8ohm.

but if I use 4 + 4 then it's running at 2 which is bad for the amp?

so what am I missing? apart from common sense?

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Re: What Ohms Cabinet should I get?

Post by john » Tue Feb 05, 2008 12:15 pm

foal30 wrote:yes, great link and comments.

Here is the bit I struggle with, you should use the same number (8ohm for instance) for both cabs.
Not say 1 x 4ohm and 1x 8ohm.

but if I use 4 + 4 then it's running at 2 which is bad for the amp?

so what am I missing? apart from common sense?
you could use a 4 ohm and 8 ohm together. but only if your amp goes to 2 ohms. same with the two 4 ohm cabs.
if your amp only goes to 4 ohms you could wire two 4 ohm cabs in series to make there load 8 ohms + to -

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Re: What Ohms Cabinet should I get?

Post by foal30 » Tue Feb 05, 2008 12:24 pm

god Lord I have trouble with shoe laces ...I'm not wiring anything....
Wired, thats a different story.

if your amp only goes to 4 ohm...
but thats like why have an extension on a 4ohm combo? whatever you plug in will be wrong.
maybe?

the link suggests you do not mix 4 with 8 (or whatever number) due to power being sent unevenly to each cab.
at least thats what I think it says

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Re: What Ohms Cabinet should I get?

Post by beagle » Tue Feb 05, 2008 2:22 pm

Crazykiwi wrote:We have regular battles over speaker cab specificiations on my forum. Its like a honeypot for geek flies there! :)
Ha ha :D
That's me!!!
foal30 wrote:the link suggests you do not mix 4 with 8 (or whatever number) due to power being sent unevenly to each cab.
at least thats what I think it says
Yeah I would have thought mixed ohms cabinets would have been ok ... assuming both speaker outputs from the amplifier were simply running in parallel.... and the resulting resistance of the speakers didn't go below the amps rating.
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