4-Ohm load vs. 8-Ohm load - benefits to amp?

[esa372]

Is there any benefit - to the amplifier - to run a higher or lower impedance rig?

That is, does an amp which is capable of running 4- or 8-Ohm loads run any better/cooler/more efficiently with one or the other?

It seems to me that a higher impedance rig would cause the amp to work harder, thus generating more heat, thus needing more cooling. Is this so? ....or are there counter-intuitive electronic mysteries at work that cause the amp to be happy with either?

Thanks,

~esa

 

[Bass_Pounder]

With solid state amps, the lower you go, the harder the amp works and more heat is produced.

 

[countrybass007]

I've never noticed much difference over the years. The power output of most amps increases some with a 4ohm load over an 8ohm one, but for actual performance it always seemed the same.

 

[BassmanPaul]

I've never noticed much difference over the years. The power output of most amps increases some with a 4ohm load over an 8ohm one, but for actual performance it always seemed the same.

As it takes ten times the power to get twice as loud this is very true. A SS amp will try to output twice the power into a 4Ω load as it does at 8Ω. Some can, most cannot.

Paul

 

[Adam Bomb]

Dude, I have totally been wondering the same thing. But from a slightly different angle: Can I be more comfortable cranking a head that is going into just one 8-ohm cab, knowing that it isn't approaching its limits? With all the usual caveats about EQ, using my ears, etc. I've guessed that an amp with only an 8-ohm load is only working half as hard. But, I know nothing about the engineering side of this. So, 'scribed.

--Bomb

 

[mulchor]

It seems to me that a higher impedance rig would cause the amp to work harder, thus generating more heat, thus needing more cooling. Is this so? ....or are there counter-intuitive electronic mysteries at work that cause the amp to be happy with either?

Think of this going the other way. To get the same power with lower impedance, you have to squeeze out more current.

Which gear does your car go faster easier? Low impedance 1st, or high impedance 5th?

There's no counter-intuitive in electronics!

 

[SanDiegoHarry]

As it takes ten times the power to get twice as loud this is very true. A SS amp will try to output twice the power into a 4Ω load as it does at 8Ω. Some can, most cannot.

Paul

Curious: Why the mention of "Solid state"? Do not tube power amps treat loading the same way solid state does?

Can the same be said for class A vs. Class A/b amps?

 

[KJung]

Curious: Why the mention of "Solid state"? Do not tube power amps treat loading the same way solid state does?

Can the same be said for class A vs. Class A/b amps?

No, tube amps are totally different. They have output transformers, and must be matched to the impedance of the cab or cabs. This is either done with a switch or more typically, multiple speaker outputs for different cabinet impedance.... i.e, an 8ohm tap, a 4ohm tap and a 2ohm tap. The power output is identical each different impedance.

A solid state amp (regardlless of topology... class D, class A/B, whatever) will put out more power the lower the impedance of the cab or cabs. Many have a limit of 4ohms, below which the cooling system cannot keep up with the heat generated, among other things. Some can go all the way down to 2ohms, but the really good designs (like Genz and Mesa) have a switch that, I believe reduces the voltage or something (fewer windings on the power transformer or something of the sort) so that the heat and power and THD stay relatively close to 4ohm operation when driving a 2ohm load. I think that sort of switch is a bit similar to the output transformer thing on tube amps.

There is little reason to run a solid state head at 8ohms that is engineered to have good specs and safe operation (i.e., adequate cooling and THD) at 4ohms. The additional power will add headroom and a bit of volume if the cab can make use of it (a little 112, not so much, a big 410, sure). Of course, if you have some monster amp that puts out 700 watts at 8ohms, then very few cabs could benefit from more power than that. However, going from 300 to 500 into a big 410 or 212... you might really notice it... more open low end, less power amp compression, a bit more volume.

 

[mid_life_crisis]

Some formulas to keep in mind.
Wattage(power) = Current times Voltage
Voltage = Current times impedance
So another way to look at Wattage is:
Wattage = Current squared times impedance.

What all this means is that if you want to pump Power through a load (speaker in this case) the best way to do it is to reduce the impedance of the load. For example, if you have a voltage drop across a speaker of 16 volts (just a number that divides nicely) you could pump 2 amps through 8 ohms or you could pump 4 amps through 4 ohms. The difference in power comes in because the current is squared, so for the first, the power is 2 times 2 (current squared), times 8 or 32 watts. The second is 4 times 4 (current squared) times 4 or 64 watts. This all assumes that the amp is designed to provide high current.
Running a high current amp at a higher impedance will reduce the amount of current it delivers. The problem is that you are taking it out of its design range, making it work inefficiently, which is not optimum conditions.

 

[esa372]

Lot's of good info - thanks, everyone!

There's no counter-intuitive in electronics!

Too funny, mulchor!


~esa

 

[babebambi]

Think of this going the other way. To get the same power with lower impedance, you have to squeeze out more current.

Which gear does your car go faster easier? Low impedance 1st, or high impedance 5th?

There's no counter-intuitive in electronics!

Hey this got to be one of the best metaphor on speaker load impedance.

 

[David Jayne]

A very simple way to think of it: placing a piece of nice copper wire across your amp's output(also known as a Short Circuit) would be a zero Ohm load. This will allow lots and lots of current to flow, since there is zero resistance. But all that current will get your amp really hot really fast. Just like if you put a piece of wire across a 9 v battery- the battery gets hot, because it's flowing the maximum amount of current it possibly can. See? On the other hand, an impedance of, say 16Ohms across your amp, makes it work much less hard, because the current that can flow is a lot less that with the zero Ohm load. Lower impedance loads demand more current from the amp. The amp obeys the load, if you will. This is regarding SS amps. Tube amps are different.

 

[SactoBass]

Is there any benefit - to the amplifier - to run a higher or lower impedance rig?

That is, does an amp which is capable of running 4- or 8-Ohm loads run any better/cooler/more efficiently with one or the other?

It seems to me that a higher impedance rig would cause the amp to work harder, thus generating more heat, thus needing more cooling. Is this so? ....or are there counter-intuitive electronic mysteries at work that cause the amp to be happy with either?

Thanks,

~esa

Esa372:

Your amp will run cooler at 8 ohms as compared to 4 ohms (and will draw less current from the wall socket).

Your amp will have twice the damping factor (low frequency speaker control) at 8 ohms as compared to 4 ohms. This will provide "tighter" bass (assuming that's what you want of course).

As long as you don't need more power, running at 8 ohms is better than running at 4 ohms (IMO).

Running at 4 ohms will provide more power to your speaker(s), and if you need that power, then you have two choices: run a 4 ohm speaker load, or, run an 8 ohm speaker load and buy a more expensive (more powerful) amp that delivers more power at 8 ohms. The latter is what I prefer to do. I have a pair of 8 ohm cabs that are rated at 500 watts each, and I run a stereo power amp that delivers 775 watts at 8 ohms into each cab. It works like a charm. The downside to my option is, I had to shell out more money for the more powerful amp. Some folks don't like to do that, so an option is to buy a less costly amp and run a 4 ohm speaker load and get the same wattage I am getting out of the amp. It's all personal preference.

 

[timbledum]

Stupid question then:

What exactly happens when you put lead in the wrong output in a tube amp? Eg plug an 8 ohm cab into the 2 ohm output or visi versa?? And why?

 

[SactoBass]

Stupid question then:

What exactly happens when you put lead in the wrong output in a tube amp? Eg plug an 8 ohm cab into the 2 ohm output or visi versa?? And why?

If you plug an 8 ohm (higher resistance) speaker load into a 2 ohm tap on a tube amp, nothing bad will happen. You just won't "realize" (get) the full power output of the amp due to the higher resistance of the cab.

If you plug a 2 ohm (lower resistance) speaker load into an 8 ohm tap, it will overhead the transformer (that's bad).

Think of resistance as allowing power to flow. Higher impedance (higher ohm) cabs, like 8 ohms for example, won't let as much power come out of the amp. It "resists" it more. A lower impedance (lower ohm) cab, like 4 ohms for example, has less resistance, and therefore allows more power to flow out of the amp. You need SOME resistance, otherwise too much power will flow out of the amp and burn the amp up. That's why, if you run a pair of 4 ohm cabs (which results in a 2 ohm speaker load), you need to make sure your amp is rated to go down to a 2 ohm load.

So, if you want to maximize power coming out of a tube amp, match the speaker load impedance with the ohmage of the amp's tap.

 

[agedhorse]

Everybody keeps mentioning stress that a lower (say 4 ohm) impedance load places on an amp but this is CURRENT stress, and are completely neglecting the VOLTAGE stress that an equiv. powered amp into a higher impedance load (say 8 ohm).

Amps have 2 different but equally concerning stresses that must be considered.

1. Current stresses are those due to current flow and will be higher in an amp driving lower impedance loads.

2. Voltage stresses are those due to the higher voltages required to deliver the same power into a higher impedance load.

At high power, voltage issues can become quite real and require different design techniques to maintain reliability. This generally adds more to the cost of the circuitry than delivering a simplar power level into a lower impedance load. Additionally, the protection circuits must consider the greater potential for damage due to a load fault with high supply rail voltages, and the effects of "safe operating area" under such faults.

A properly designed amp will be equally reliable and perform equally well under any of these conditions regardless of load impedance provided the amp was designed for that load.

 

[rbonner]

Sorry Sacto, incorrect.

The output transformer is an imedance matching device, it matches a high impedance the tube out load impedance to the low impedance of the speakers.

By putting the worng speaker on a toob amp it reflects the incorrect image to the tubes, creating a mismatch, which dissapates too much power in the tube plates and also creats a situation if too high load, a high voltage situation on the primary toob side and can arc the transformer. Thats why you never disconnect the speaker on a toob amp.

BOB

 

[fourfinger]

Dude, I have totally been wondering the same thing. But from a slightly different angle: Can I be more comfortable cranking a head that is going into just one 8-ohm cab, knowing that it isn't approaching its limits? With all the usual caveats about EQ, using my ears, etc. I've guessed that an amp with only an 8-ohm load is only working half as hard. But, I know nothing about the engineering side of this. So, 'scribed.

--Bomb

Allow me to grossly oversimplify.

In practical terms, if you need to play loud or not makes the difference.

Needing Low Volume:

maaaaaybe a slight wear-and-tear benefit to 8-ohms/one cab, and better control over your tone.

Needing High Volume (or greater clarity/punch at medium volume):

Adding the second cab adds SO much more volume that you probably won't need to push the amp near as hard.

 

[SactoBass]

Sorry Sacto, incorrect.

The output transformer is an imedance matching device, it matches a high impedance the tube out load impedance to the low impedance of the speakers.

By putting the worng speaker on a toob amp it reflects the incorrect image to the tubes, creating a mismatch, which dissapates too much power in the tube plates and also creats a situation if too high load, a high voltage situation on the primary toob side and can arc the transformer. Thats why you never disconnect the speaker on a toob amp.

BOB

Thanks Bob. I stand corrected. (And btw, I think you have one of the coolest avatars on TB.)

 

[rbonner]

Thanks Bob. I stand corrected. (And btw, I think you have one of the coolest avatars on TB.)

Thanks, unfortunatly JimmyM has the coolest by popular vote, hahahahaha.