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How may cabs to one head?

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Hey this might sound stupid to most but I was just reading some reviews and someone had said that you can connect as many as 4 8ohm cabs to one fender rumble amp head. Is this true? Also does that work for most. I was considering 2 cabs one with 4 10s and one with 2 15 but nice to know I can always add more if needed. Also would this be the same with other amp heads? How about Mark bass vintage? Thx for any inputs!!:)

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Well, the first thing to look at is the impedance "rating" of the amplifier.  The lowest impedance value the amplifier will happily play ball with. Now, if the Fender Rumble you speak of can handle a 2 Ohm load reliably, then yes four 8 Ohm cabinets can be attached. Impedance calculations are well documented on BC, but in this case, two 8 Ohm cabinets would give you a four Ohm load - and four 8 Ohm cabinets would present a 2 Ohm load to the amplifier.

So, the take away from this is to know what load the amplifier can happily run at. if it says 4 Ohms, then you can go higher but wise not to go lower. More and more manufacturers are offering amplifiers that "do" 2 Ohms, but as always read the manual in case the unit you are wanting to use specifies 4 Ohms minimum instead.

 

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My understanding is that there is a high spitzensparken probability here.  Most amps are rated at 4 ohm.  They will take one 4 ohm cab or two 8 ohm cabs. Any more may impact your house fire insurance.  But happy to be corrected.    

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Whilst we're here, there's always the "should I run a 4 ohm cabinet to get all the watts from my amplifier or would I be loud enough with an 8 ohm cabinet". Comparing two like-for-like cabinets with the same sensitivity, the difference in volume between an 8 ohm and a 4 ohm version will easily be cancelled out by nudging the volume control on your amplifier. :) I really cant imagine it would be a problem with either unless your 8 ohm cabinet also has a much lower sensitivity as well. 

 

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The Fender Rumble 800 HD will do down to 2 Ohms load

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The Fender Rumble 500 is no longer listed on the Fender website, but the suggestion from the specifications elsewhere is that 4 Ohms could well be its minimum load. 

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A cab may have a nominal 8 ohm impedance, but it will vary - above and below the 8 ohm point - across the frequencies. I would not connect more than three 8 ohm cabs to a 2 ohm head if I was running it at high volumes,  just to be on the safe side. Used power amps are cheap these days if you don't mind carrying one. You can run one to power additional cabs.

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Properly designed amplifiers take this into account. I have never experienced a problem running two 4Ω loads per channel on any of my power amplifiers - neither did I expect any! :)

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On 25/04/2020 at 21:21, BassmanPaul said:

Properly designed amplifiers take this into account. I have never experienced a problem running two 4Ω loads per channel on any of my power amplifiers - neither did I expect any! :)

You carry on then. I prefer to err on the side of caution, especially if I'm driving an amplifier hard. As an example, take a look at the spec' sheets for Kappalite drivers (popular choice for bass cabs). I'm not singling out the Kappalite - all drivers behave similarly. The impedance varies quite dramatically according to frequency. At certain frequencies, what is supposed on paper to be a 2 ohm load can be a lot lower.

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Posted (edited)

Well good luck to you too but you are missing out on some of your amplifiers' capabilities. Remember that you are not getting the full output from your amplifier all the time. Amplifier designers are cognizant about how impedance varies with frequency. It's one of the design considerations.

Edited by BassmanPaul

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I can't see anything in the original post that limits us to putting all the speaker boxes in parallel, so, if you connect each pair of 8 ohm speakers in series, and connect the two series circuits in parallel you will get a four-speaker,  8 ohm system.

According to Bill Fitzmaurice, adding a second speaker adds 6dB to your volume, partly due to getting more power by the halving of the impedance and partly due to the influence of each speaker on the other. With the speaker system described, you will not get the 3dB increase in volume due to dropping the impedance, however I think you will get a 9db increase in volume (8 times as loud?) by adding in three extra cabs. And your amp will run as cool as it did with one cab.

David

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46 minutes ago, Mottlefeeder said:

I can't see anything in the original post that limits us to putting all the speaker boxes in parallel, so, if you connect each pair of 8 ohm speakers in series, and connect the two series circuits in parallel you will get a four-speaker,  8 ohm system.

According to Bill Fitzmaurice, adding a second speaker adds 6dB to your volume, partly due to getting more power by the halving of the impedance and partly due to the influence of each speaker on the other. With the speaker system described, you will not get the 3dB increase in volume due to dropping the impedance, however I think you will get a 9db increase in volume (8 times as loud?) by adding in three extra cabs. And your amp will run as cool as it did with one cab.

David

This is the standard way to wire a 4x10” cabinet, so you’re effectively just putting a box around each of the drivers.

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An increase of 3db is supposed to result in a doubling of volume. However, it takes a 10db increase for people to perceive volume as having doubled. Barely any amplifier on earth will double the power output it gives into 8 ohms if the load is reduced to 4 ohms, because the power supply in most will run out of steam.

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You're all missing the point. How cool would 2 15s and two 4 10s look stacked up either side of the kit or next to one another.

The simple truth is each extra speaker increases the cool by 254% - you do the maths.

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OK, I've trawled through several months of Bill's contributions here and I can't find what I thought I read. Adding a second speaker increases the sensitivity of the system by 6 dB, by halving the impedance the amp sees, resulting in the delivery of four times more power. I can't find any comment on gains by running two speakers in close proximity.

In my experience, adding a second identical cab gives me deeper, more effortless bass, but I can't find any science to back that up, (and we know how important it is to follow the science don't we). The alternative is to suggest that running a 1*12 louder would sound just as good as running a 4*12 which used the same drive units, and porting and volume per speaker.

David

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Posted (edited)
40 minutes ago, Mottlefeeder said:

OK, I've trawled through several months of Bill's contributions here and I can't find what I thought I read. Adding a second speaker increases the sensitivity of the system by 6 dB, by halving the impedance the amp sees, resulting in the delivery of four times more power. I can't find any comment on gains by running two speakers in close proximity.

In my experience, adding a second identical cab gives me deeper, more effortless bass, but I can't find any science to back that up, (and we know how important it is to follow the science don't we). The alternative is to suggest that running a 1*12 louder would sound just as good as running a 4*12 which used the same drive units, and porting and volume per speaker.

David

No, power = V^2/R.  R halves but V stays the same so power doubles.

Or looking at current, Power = I^2 * R,  current increases by 2  but resistance halves so 2^2 x 0.5 = 2, power doubles.

The other half of the volume increase comes because you've doubled the area of the speakers.

This is why a 4x10 can be louder than a 1x15, all other things being equal (they never are!) and a 2x12 is twice as loud as a 1x12 for the same power (assuming again that everything else stays the same).

 

Edited by Stub Mandrel

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Posted (edited)

Oh God this is going to turn into one of those threads.

Tin hats everyone.🧘🏼‍♀️

Edited by stewblack
Correcting the stud autocorrect

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Posted (edited)
5 hours ago, Mottlefeeder said:

OK, I've trawled through several months of Bill's contributions here and I can't find what I thought I read. Adding a second speaker increases the sensitivity of the system by 6 dB, by halving the impedance the amp sees, resulting in the delivery of four times more power. I can't find any comment on gains by running two speakers in close proximity.

Adding a second identical speaker in parallel gets 6dB additional voltage sensitivity, so with the same amp settings it's 6dB louder. It's not about the power going up by 3dB, that's just a side effect of the halving of impedance resulting in a doubling of current, which doubles power  Maximum displacement limited output is also 6dB higher, because the system displacement is doubled. Now, if you wire another set of parallel wired identical speakers in series with the first, as is the usual case with a 410, voltage sensitivity remains the same, so it won't go louder than the 210 at the same amp setting. However, since the impedance has doubled the 410 can take twice the voltage as the 210, so if the amp has the available voltage to give the 410 can go 6dB louder than the 210, 12dB louder than the 110.

Edited by Bill Fitzmaurice
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On 30/04/2020 at 08:01, stewblack said:

You're all missing the point. How cool would 2 15s and two 4 10s look stacked up either side of the kit or next to one another.

The simple truth is each extra speaker increases the cool by 254% - you do the maths.

But won’t the universe implode if we mix driver sizes? How reckless of you, Stew 😛

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8 minutes ago, Aliwobble said:

But won’t the universe implode if we mix driver sizes? How reckless of you, Stew 😛

Ah of course, sorry 😅😅😅

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On 30/04/2020 at 14:12, Bill Fitzmaurice said:

Adding a second identical speaker in parallel gets 6dB additional voltage sensitivity, so with the same amp settings it's 6dB louder. It's not about the power going up by 3dB, that's just a side effect of the halving of impedance resulting in a doubling of current, which doubles power  Maximum displacement limited output is also 6dB higher, because the system displacement is doubled. Now, if you wire another set of parallel wired identical speakers in series with the first, as is the usual case with a 410, voltage sensitivity remains the same, so it won't go louder than the 210 at the same amp setting. However, since the impedance has doubled the 410 can take twice the voltage as the 210, so if the amp has the available voltage to give the 410 can go 6dB louder than the 210, 12dB louder than the 110.

On a more technical note.  I’ve often wondered if having a bigger speaker array gives better projection. My 1x10 cabinet has a sensitivity of x dB at 1W at 1m, and the spl drops every time you double the distance from the speaker. However the calculated reduction  is based on the inverse square rule, and assumes a point source. If the source of sound is (say) a 8x10 fridge, how does that alter the reduction in Spl over distance?

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On 30/04/2020 at 08:01, stewblack said:

You're all missing the point. How cool would 2 15s and two 4 10s look stacked up either side of the kit or next to one another.

The simple truth is each extra speaker increases the cool by 254% - you do the maths.

Valid point about the cool factor though. We could stack all those glorious cabinets up either side of the drummer and not even bother hooking them up. Just run straight to PA.

EF8F043F-C72B-4815-8D57-E7B171243EF0.jpeg

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46 minutes ago, Aliwobble said:

 If the source of sound is (say) a 8x10 fridge, how does that alter the reduction in Spl over distance?

Not much. A line source drops 3dB per doubling of distance. To be a line source it must be at least three wavelengths high. That's easy at 5kHz, where a wavelength is around 7cm, not so much at 100Hz, where a wavelength is around 340cm.

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