Difference between 4ohm and 8ohm

[mam1862]

Is there really that much difference between the two? I had someone
say the difference between the two is if you had the 8ohm cab you would just have to turn your amp a little louder to make comparable to the 4ohm. Just seeing what the feeling is out there.

[GeoffByrne]

Yup. Just turn up a tad.

[billfitzmaurice]

Originally Posted by mam1862

if you had the 8ohm cab you would just have to turn your amp a little louder to make comparable to the 4ohm.

Very little, maybe the difference between 5 and 6 on the volume control. It's an audible difference, but just barely.

[Red Planet]

8-4=4

[Darknut]

Actually there is BIG difference in how much power you will get from your amp between a 4 & 8 ohm cabinet ... you will get half as much power from the amp with the 8 ohm cab as the 4 ohm which means you will have to push the amp twice as hard to get the same volume.

This is assuming you are talking about just one cab at a time.

[billfitzmaurice]

Originally Posted by Darknut

Actually there is BIG difference in how much power you will get from your amp between a 4 & 8 ohm cabinet ... you will get half as much power with the 8 ohm cab as the 4 ohm which means you will have to push the amp twice as hard to get the same volume.
.

At full power you don't get half as much power into 8 ohms as you do into 4. The actual figure runs about 70%. In terms of what actually matters, decibels, the difference at full power is at best only 2dB. That's key, because it takes a 10dB difference to sound twice as loud. Those who think that doubling the wattage output by halving the load impedance will result in doubling the volume are always gravely disappointed. And as for having to "push the amp twice as hard to get the same volume", that's not the case. Halving the load impedance doubles the current draw, and that causes more heat in the amp components, which shortens their lifespan.

[Darknut]

Originally Posted by billfitzmaurice

At full power you don't get half as much power into 8 ohms as you do into 4. The actual figure runs about 70%. In terms of what actually matters, decibels, the difference at full power is at best only 2dB. That's key, because it takes a 10dB difference to sound twice as loud. Those who think that doubling the wattage output by halving the load impedance will result in doubling the volume are always gravely disappointed. And as for having to "push the amp twice as hard to get the same volume", that's not the case. Halving the load impedance doubles the current draw, and that causes more heat in the amp components, which shortens their lifespan.

Yes I know you are technically correct but who runs a amp at "full" volume?

I know in my experiences there has always been a big difference between the two in the real world at a gig especially if its a small amp to begin with.

[DaveAceofBass]

Volume correlates more to the number of speakers than it does anything else. However, you need more power to push more speakers without making the amp work too hard, or it will distort. If you're looking to add volume, add speakers. That's the whole point of 8ohm cabs, to allow you to add more when necessary.

A word about ohms--the higher the number, the higher the resistance. When you lower the impedance, the power flows with less resistance, therefore you get more power.

[1958Bassman]

Originally Posted by mam1862

Is there really that much difference between the two? I had someone
say the difference between the two is if you had the 8ohm cab you would just have to turn your amp a little louder to make comparable to the 4ohm. Just seeing what the feeling is out there.

Depends on what the amplifier design calls for. Impedance isn't constant- it varies with frequency and can dip below the amplifier's "safe" range. When the impedance dips, the current increases and that means the amp runs hotter. For some amps, that's a problem. The harder you play through a cab with lower impedance, the harder the amp has to work. If an amp has two output jacks and they're wired parallel, you don't want to use two 4 Ohm cabs- that will show the amp a 2 Ohm load and it won't be long for this world.

[Never_grew_up]

Here's an analogy I've used to explain watts/ohms to non-engineer types:
Think about riding a bike. The energy you spend pedaling (watts) to achieve the speed you want (output/decibels) is directly related to the slope you're riding on (resistance/impedance). You want to find the right gear on the bike for the hill you're riding on to get the most efficient use of the energy you're expending.

Try climbing a hill on a bike in too high a gear and you'll strain your butt off for very little forward motion.

Try riding down a hill in too low a gear and you'll have to peddle your butt off just to feel any resistance in the pedals. (Coasting is not an option).

Match the resistance/impedance of the cabinet and the wattage of the amp to achieve the output (decibels) you desire. When looking at cabinets, efficiency ratings have as much (maybe more) of an impact on output as does the rated wattage of the cabinet.

A cheap, heavy bike made with a welded steel frame is going to require more energy to pedal it without giving you any additional output for that energy spent. A super-tweaked bike with a lightweight magnesium alloy or carbon-graphite frame and helium-filled tires (I don't know if you can actually get helium-filled tires, but it's a cool idea) takes almost no effort to pedal so more of the pedaling energy translates to forward motion.

[billfitzmaurice]

Originally Posted by Darknut

Yes I know you are technically correct but who runs a amp at "full" volume?

Typically those with too small an amp or too few speakers for the job, and those also tend to be the ones looking to 'get all the watts out of my amp', thinking that it will help with their volume problems, which it almost never does.

I know in my experiences there has always been a big difference between the two in the real world at a gig especially if its a small amp to begin with.

I doubt that you're lugging about two identical cabs save for their impedance just to see which goes louder, so I assume that you find two 8 ohm cabs to go a lot louder than one. True, 6dB louder at small signal levels to be exact, when the cabs are identical. 3dB is from the halved impedance, 3dB from mutual coupling. That's the way you're supposed to go about getting more volume.

[will33]

Originally Posted by billfitzmaurice

Halving the load impedance doubles the current draw, and that causes more heat in the amp components, which shortens their lifespan.

This carries more weight to me than some little bit of volume the speaker is going to struggle to produce anyway. It's about reliability and longevity. Your frying amp is going to sound like crap and when it ceases to make any sound at all your presence is no longer needed and you'll be opening your wallet while everybody else is playing gigs with amps they didn't squeeze all the life out of.

[BurningSkies]

Originally Posted by mam1862

Is there really that much difference between the two? I had someone
say the difference between the two is if you had the 8ohm cab you would just have to turn your amp a little louder to make comparable to the 4ohm. Just seeing what the feeling is out there.

The other day I was reading this site that had all of this awesome basic information about bass amps, speakers and how all this stuff works...ohms, watts, SPL, all sorts of stuff! It took me a while to find it again, but here y'go...try it!

AMPS FAQ!! Info on OHMS, Allsize RIGS-OverUnderPowerCabs DIY TechTalk-Links

[PhiDeck]

Originally Posted by Never_grew_up

Here's an analogy I've used to explain watts/ohms to non-engineer types:
Think about riding a bike. The energy you spend pedaling (watts) to achieve the speed you want (output/decibels) is directly related to the slope you're riding on (resistance/impedance).

Non-engineer types reading the ostensible analogy may want to note that energy is not measured in Watts. A watt is a unit of power. Energy equals power x time (unit: Watt-hour).
When you pay your electric bill you are paying for energy used. A 100 Watt light bulb left on for 10 hours uses 1 kilowatt-hour, which around here costs ~11 cents. If you leave a 60 Watt light bulb on for 16.7 hours, it also uses 1 kilowatt-hour of energy.

Recap: Power and energy are two different (related) things.

[BurningSkies]

Originally Posted by Never_grew_up

Here's an analogy I've used to explain watts/ohms to non-engineer types:
Think about riding a bike. The energy you spend pedaling (watts) to achieve the speed you want (output/decibels) is directly related to the slope you're riding on (resistance/impedance). You want to find the right gear on the bike for the hill you're riding on to get the most efficient use of the energy you're expending.

Try climbing a hill on a bike in too high a gear and you'll strain your butt off for very little forward motion.

Try riding down a hill in too low a gear and you'll have to peddle your butt off just to feel any resistance in the pedals. (Coasting is not an option).

Match the resistance/impedance of the cabinet and the wattage of the amp to achieve the output (decibels) you desire. When looking at cabinets, efficiency ratings have as much (maybe more) of an impact on output as does the rated wattage of the cabinet.

A cheap, heavy bike made with a welded steel frame is going to require more energy to pedal it without giving you any additional output for that energy spent. A super-tweaked bike with a lightweight magnesium alloy or carbon-graphite frame and helium-filled tires (I don't know if you can actually get helium-filled tires, but it's a cool idea) takes almost no effort to pedal so more of the pedaling energy translates to forward motion.

What if I drive?

[f.vega]

How does this translate into differences in tone between 8 & 4 ohms

[B-string]

Originally Posted by f.vega

How does this translate into differences in tone between 8 & 4 ohms

No difference at all.

[B-string]

Originally Posted by billfitzmaurice

At full power you don't get half as much power into 8 ohms as you do into 4. The actual figure runs about 70%. In terms of what actually matters, decibels, the difference at full power is at best only 2dB. That's key, because it takes a 10dB difference to sound twice as loud. Those who think that doubling the wattage output by halving the load impedance will result in doubling the volume are always gravely disappointed. And as for having to "push the amp twice as hard to get the same volume", that's not the case. Halving the load impedance doubles the current draw, and that causes more heat in the amp components, which shortens their lifespan.

This is the only thing I take an issue with in a otherwise awesome post (in bold).
With equally capable speaker cabs, one 8 ohm and one 4 ohm. You will use 30% less gain to achieve the same sound level. The "heat" produced in the amplifier will be the same, not more. In the real world we hear it being a little louder and push harder to see how far we can go. That is not a "load" issue to the amp design.

[Darknut]

Originally Posted by B-string

No difference at all.

Unless you have a low power amp that you are pushing to hard then the tone is most definitely different.

[JHAz]

Originally Posted by B-string

This is the only thing I take an issue with in a otherwise awesome post (in bold).
With equally capable speaker cabs, one 8 ohm and one 4 ohm. You will use 30% less gain to achieve the same sound level. The "heat" produced in the amplifier will be the same, not more. In the real world we hear it being a little louder and push harder to see how far we can go. That is not a "load" issue to the amp design.

I guess I'm lost. If the 2 cabs have the same efficiency (ie output the same spl at one watt, then if you need 100 watts for say 120 db from the 8 ohm cab, you'll need 100 watts to get 120 dB from the 4 ohm cab. Where you set your volume control to get 100 watts into the two loads is not relevant, AFAIK. 100 watts at 8 ohms will be about 3.5 amps (and a little over 28 volts). 100 watts at 4 ohms will be 5 amps and 20 volts. In short, according to Ohm's law, the same power will require higher current into lower resistances.