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ohms and speaker cabs


Graham
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im looking at an ampeg bass combo on amp on ebay which has an extra output on the back for another speaker cab. the back pannel on the amp says the max load is 4 ohms. the speaker cab i have just now (ashdown 4x10) is rated at 8 ohms. does that mean its not possible to use the cab with the combo? if its not possible would it just not be able to power the cab or would it damage it?

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im looking at an ampeg bass combo on amp on ebay which has an extra output on the back for another speaker cab. the back pannel on the amp says the max load is 4 ohms. the speaker cab i have just now (ashdown 4x10) is rated at 8 ohms. does that mean its not possible to use the cab with the combo? if its not possible would it just not be able to power the cab or would it damage it?

yep, ideally you want to match the impedances, if you cant match, the 4ohm output impedance rating on the amp means it can drive the current required that a 4 ohm speaker would draw. in your case the impedance you are driving is larger (8 ohms) meaning it will not draw so much current, ie should be ok.

i'm not familiar with ashdown cabs, by any chance does it have a small selector switch on the rear? this might give you an option of 4, 8, 16 ohms.

the other point, can you tell me what the impedances of the four speakers are?

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yep, ideally you want to match the impedances, if you cant match, the 4ohm output impedance rating on the amp means it can drive the current required that a 4 ohm speaker would draw. in your case the impedance you are driving is larger (8 ohms) meaning it will not draw so much current, ie should be ok.

i'm not familiar with ashdown cabs, by any chance does it have a small selector switch on the rear? this might give you an option of 4, 8, 16 ohms.

the other point, can you tell me what the impedances of the four speakers are?

i dont think the cab has a selector switch but i'll have a look. this is my cab here Ashdown Engineering | Bass Amplification im not too sure what the impendances of the speaker are. had a look at the specs of it on the website but didnt see anything.

I think the 4 ohm max is more to do with possible damage to the amplifier if you attach a higher load than damage to the speakers.

so i shouldnt do it then? wouldn't want to get the amp then to damage it.

this is the amp im looking at by the way Ampeg bass combo SVT-100T 100W 2x8" + tweeter on eBay, also Bass, Amplifiers, Pro Audio Equipment, Musical Instruments (end time 23-Apr-08 21:13:19 BST)

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I think the 4 ohm max is more to do with possible damage to the amplifier if you attach a higher load than damage to the speakers.

ian:

have you ever seen a higher load than 4 ohms on a cab, ie 2 ohms? i haven't.

i do agree with you though, the 4 ohm rating is to protect the amp, not the speakers.

graham:

quick way to blow an amp, attach no load at all ie no speaker and you will probably cook your output transformer... or do it slowly by connecting a 16 ohm amp output to, say, a 4 ohm speaker (load)

do you or dont you? i think it will be ok, may sound brighter/ toppy, i am guessing though, give me a sec, i'll see what i can find.

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You will be able to run two 8 Ohm speakers just fine. Two 8 Ohm speakers in parallel give you a nominal resistive load of 4 Ohms.

Tube (valve) amps have an output transformer that should be matched to or be higher than the resistance of the speaker. So you can run a tube amp set to 4 Ohms into an ordinary 8 Ohm speaker, but it is not a good idea to do this the other way about, as this will damage the EL34s and possibly the output transformer as well.

Solid state amps (FET) just have to work harder into a lower resistive value and 2 Ohms could make the amp see a short circuit. The better PA amps are able to work with 2 Ohms.

This is only the nominal resistive value when measured with a small DC voltage. When driven, the speakers behave very differently. I'm sure there's loads on this subject at Wickipedia.

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surely a 2 ohm load is a higher load than a 4 ohm load, as the 2 ohm load draws more current? ie you need a higher compliance source to drive a smaller resistance

Nah, load is impedance. Think of it as a higher "burden" on the circuit because,as it name implies, it impedes the current.

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Nah, load is impedance. Think of it as a higher "burden" on the circuit because,as it name implies, it impedes the current.

that doesn't make sense, a crappy knackered battery at say 1.3 volts will barely drop its own voltage if it sees, say a 1 megaohm load, a big load impedance drawing f'all current, a small load. power dissipated at load is barely 2 u watts

conversely, a perfectly healthy power supply set to 1.5 volts will either current limit or crash and burn if faced with a 0.00001 ohm load (essentially short circuit). power dissipated at load is 225 k watts (assuming the power supply could handle it)

the greater load is the smaller impedance, agreed?

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  • 2 weeks later...

Ok i dont get any of this...?(

i'm looking to get an amp that has a rating of

# RMS Power Output (8 Ohms) : 275

# RMS Power Output (4 Ohms) : 450

does this pretty much mean i can hook up a 8ohm cab and get 275 supposedly outta it?

or likewise a 4 ohm one and get 450?

the cab i'm now looking at is supposedly 500watts/4ohm....so these two would work well together?

ta for any help..

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Ok i dont get any of this...?(

i'm looking to get an amp that has a rating of

# RMS Power Output (8 Ohms) : 275

# RMS Power Output (4 Ohms) : 450

does this pretty much mean i can hook up a 8 ohm cab and get 275 supposedly outta it?

or likewise a 4 ohm one and get 450?

the cab i'm now looking at is supposedly 500watts/4ohm....so these two would work well together?

ta for any help..

No, it means that the amp will happily throw out 275W>8 ohms:450W>4 ohms all day without breaking sweat. How much you get out of the cab depends on other factors.

It all depend on whether your cab is rated at 500W RMS or not.

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