# 8 ohm to 4 ohm, more power?



## Bevo (Nov 24, 2006)

I have an Ashdown 15 inch combo 307 Watt amp that runs around 150-175 watts at 8 ohm.

The amp rating is 307 Watts at 4 ohm and presently has a 8 ohm speaker..So.. If I change out the 8 ohm to a 4 ohm will I get the full 307 watts?
I know if you add another 8 ohm it makes the load 4 ohms which is more powerfull..

I think it will but just checking..
Bev


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## Wild Bill (May 3, 2006)

Bevo said:


> I have an Ashdown 15 inch combo 307 Watt amp that runs around 150-175 watts at 8 ohm.
> 
> The amp rating is 307 Watts at 4 ohm and presently has a 8 ohm speaker..So.. If I change out the 8 ohm to a 4 ohm will I get the full 307 watts?
> I know if you add another 8 ohm it makes the load 4 ohms which is more powerfull..
> ...


It's like this, Bev!

With solid state amps, as you lower the speaker impedance the output power goes up. If you go lower than the rated minimum load then you run the risk of trying to draw too much power from the transistors and they may burn out.

If your amp is solid state and it says the minimum is 4 ohms then yes, changing the speaker will give you the full rated power. However, you won't be able to plug in an extension cab. The new load will be less than 4 ohms. If you don't ever plan to run an extension cab then what the heck!:smile:

Tube amps are different. They have different outputs available, either with discrete jacks or some kind of switch. You set things up for a 4 ohm output to drive a 4 ohm cab, 8 for 8 or whatever. You always get the same amount of power, no matter what you do.

That's why a tube amp will likely shrug off a shorted speaker cable problem but a solid state head will probably burn up. A short is the lowest possible speaker load you can have.

:food-smiley-004:


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## Robboman (Oct 14, 2006)

Wild Bill said:


> Tube amps are different. They have different outputs available, either with discrete jacks or some kind of switch. You set things up for a 4 ohm output to drive a 4 ohm cab, 8 for 8 or whatever. You always get the same amount of power, no matter what you do.
> 
> :food-smiley-004:


OK, but what happens when you connect an additional extension cab to a tube amp? What if the amp doesn't have discreet jacks or a switch, just a jack for adding another speaker? (like my old Fender Pro Reverb combo). Or another way to ask.. say you had a toggle switch on a head labeled 4 ohms / 8 ohms / 16 ohms. What actually happens if you have a 4 ohm speaker load and you set the switch to 8 or 16? Or vice-versa? What are the reasons to either do that, or not? I've never been clear on this, actually.


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## Bevo (Nov 24, 2006)

Thanks!

Its solid state and that explanation was exactly what I was looking to hear.

Its a bass amp and its great for our jams but have to run it up at 6-7 on volume...Not crazy but if we get a bigger room it won't keep up. With the extension cab I keep it around 3-4 with plenty of power left in the tank.

The idea to go with a 4 ohm 15 means I don't have to use the extra cab yet have all the volume and power I need..Not to mention a lighter load!!

Thanks
Bev


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## Wild Bill (May 3, 2006)

Robboman said:


> OK, but what happens when you connect an additional extension cab to a tube amp? What if the amp doesn't have discreet jacks or a switch, just a jack for adding another speaker? (like my old Fender Pro Reverb combo). Or another way to ask.. say you had a toggle switch on a head labeled 4 ohms / 8 ohms / 16 ohms. What actually happens if you have a 4 ohm speaker load and you set the switch to 8 or 16? Or vice-versa? What are the reasons to either do that, or not? I've never been clear on this, actually.


Old Fender amps don't give you any options about setting the output impedance. So if you have an 8 ohm internal speaker and you plug in an 8 ohm extension cab the total on the amp is indeed 4 ohms, from two 8 ohm loads in parallel.

If you have a switch and two speaker outs then if you plug in 2 speakers, one to each jack that are both 8 ohms then you again have a 4 ohm total and should set the switch for 4 ohms.

If you have only one jack for 4 ohms, one for 8 and one for 16 then you would have to "daisy chain" the loads, that is you plug the 1st 8 ohm cab into the 4 ohm jack and the second cab into a second jack on the 1st cab, if it's got one.

Now, what if you don't have such choices or simply forget? It's an old Fender or a newer amp and you simply forgot to change the switch?

The answer is, not much! You see, output transformers change the speaker load of 4 or 8 ohms up to several thousand ohms to be suitable as a load to the output tube(s). All transformers work in a fixed ratio. A pair of EL34's usually gets a 3400 ohm plate load. The OT would ratio it down to 4, 8 or 16 ohms by using a tapped secondary winding for the speakers. The higher loads have a higher number of wire turns in the coil. Any of the speaker outs should transform up to 3400 ohms, having different numbers of turns to change to the right ratio. If you put an 8 ohm speaker into the 4 ohm output jack the ratio can't change so double on the 4 ohm side means double on the tube plate side, or 6800 ohms instead of 3400.

Tubes are forgiving creatures and will tolerate a wide range around their "perfect" load impedance. The power output changes a little bit, the distortion might get a little better or worse, and/or the tone also gets tweaked a bit. A worst possible mismatch like a 16 ohm cab into a 4 ohm jack might stress the tubes a bit if you're playing on 11 for an all night gig but it's not likely anything will blow up!

You'll just find that if you keep doing things like that your output tubes will last a few months less than you usually get from them.

Hope that makes things more clear...

:food-smiley-004:


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## Hamm Guitars (Jan 12, 2007)

If you add another 8 ohm speaker in paralell you will get a 4 ohm load with each speaker getting roughly 153 watts with the amp wide open.

If you change your existing speaker to a single 4 ohm speaker you will get approximately 307 watts through that speaker when the amp is wide open.

Each way has its advantages, two speakers give you more cone area which equates to better air coupling so more air will be moved. A single, more powerful speaker has less phasing issues, and will move smaller volumes of air with more force (all of this depends on the specs of the speakers of course). The single speaker enclosure may also be physically smaller.

with all things equal:

1 speaker - less air volume is moved, but with more localised pressure.
2 speakers - more air volume is moved with wider dispearsed pressure.

Two speakers also allows for coupling, which can be controlled and directed in certain areas based on placement.



Bevo said:


> I have an Ashdown 15 inch combo 307 Watt amp that runs around 150-175 watts at 8 ohm.
> 
> The amp rating is 307 Watts at 4 ohm and presently has a 8 ohm speaker..So.. If I change out the 8 ohm to a 4 ohm will I get the full 307 watts?
> I know if you add another 8 ohm it makes the load 4 ohms which is more powerfull..
> ...


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## Wild Bill (May 3, 2006)

Bevo said:


> Thanks!
> 
> Its solid state and that explanation was exactly what I was looking to hear.
> 
> ...


Well, hopefully that works for you but you won't get as much increase as you might hope!

You see, there are a couple of factors going on with your setup. First of all, doubling the power doesn't mean twice as loud to the human ear. Our ears need a jump of 4 times the power to sound twice as loud, then 4 times that, and so on. Like this: start at 4 watts and you need to go to 16. then 64, then 256...!

This is why a little Champ can sound surprisingly loud if you plug it into a big 4-12" cabinet!

Cabinet efficiency is perhaps even more important! Running an extra cab means pushing twice as much air! It's moving air that actually counts - that's what sound is, after all.

What's more, modern bass cabs tend to be too small to have much efficiency. You need hundreds of watts because small cabs waste most of the power. The cab may be lighter and smaller but it just can't convert as much of the amp power into sonic power, period! The salesguy will give you a pile of technobabble about space age cones and magnets but it's all crap! Physics is physics and you can't cheat the way the Universe works. It's why those lame ass little hifi speakers your woman made you buy need a receiver that can pump 300 watts to sound as loud as that old Pioneer 50 watt receiver did back in the 70's, when you used reasonably large speaker cabs.

In the glory days of rock bass players used cabs as big as some refrigerators! They had wheels along the bottom edge so they could be "dolly-ed" around. With a 100 watt Traynor tube head we blew the walls out!

If you put one of those classic cabs onto YOUR solid state amp you could probably launch small payloads into orbit!

:food-smiley-004:


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## Ripper (Jul 1, 2006)

Wild Bill said:


> In the glory days of rock bass players used cabs as big as some refrigerators! They had wheels along the bottom edge so they could be "dolly-ed" around. With a 100 watt Traynor tube head we blew the walls out!
> 
> If you put one of those classic cabs onto YOUR solid state amp you could probably launch small payloads into orbit!
> 
> :food-smiley-004:


Oh do I remember those and so does my back. Our bass player years ago ran two 8x10 ampeg cabs with two old SVTs. It was like moving two volkswagons everytime we went to a gig. Sounded killer though!


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## Soupbone (May 17, 2007)

Bevo said:


> I have an Ashdown 15 inch combo 307 Watt amp that runs around 150-175 watts at 8 ohm.
> 
> The amp rating is 307 Watts at 4 ohm and presently has a 8 ohm speaker..So.. If I change out the 8 ohm to a 4 ohm will I get the full 307 watts?
> I know if you add another 8 ohm it makes the load 4 ohms which is more powerfull..
> ...




Just use which one sounds the best to you.


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## Midas (Dec 20, 2007)

I couldn't agree more with you guys. You move way more air with more speakers. You got to be careful with ohms. Its very subjective because we don't really know how loud you need to push your amp to get what you want. The amp will distort faster if it needs to push more. Distortion could lead to destroying your speakers as the amp starts clipping out DC current. 

Today I was testing a pair of Ashdown 410T cabs with a Mag 300 head. The head was able to push both cabs but it would distort at a much lower level than with just one cab. I could get along with it a lower level but it might not cut it for you. Figure out what you really want to achieve by adding another cab or if need be replace a speaker (as in lower tones or different tone).


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## james on bass (Feb 4, 2006)

Listen to Wild Bill.

I will say you are far better off adding an 8 ohm extension speaker. Use just the combo for rehearsal and add the extension for gigs. You'll perceive much greater volume moving more air (speakers) than you will just throwing more watts at the one speaker.


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## Midas (Dec 20, 2007)

Exactly....


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