Jazzdogg
Less barking, more wagging!
AMPLIFIER LOADS
Excerpted from TalkBass
http://www.talkbass.com/forum/showthread.php?t=683677
KJung:
Tube amps… … 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.
Dmusic148:
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.
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.
__________________
SactoBass:
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.
__________________
RBonner:
Sorry Sacto, incorrect.
The output transformer is an impedance matching device. It matches a high impedance the tube out load impedance to the low impedance of the speakers.
Putting the wrong speaker on a toob amp reflects the incorrect image to the tubes, creating a mismatch, which dissipates too much power in the tube plates and also creates a situation of too high load, a high voltage situation on the primary toob side and can arc the transformer. That’s why you never disconnect the speaker on a toob amp.
Excerpted from TalkBass
http://www.talkbass.com/forum/showthread.php?t=683677
KJung:
Tube amps… … 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.
Dmusic148:
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.
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.
__________________
SactoBass:
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.
__________________
RBonner:
Sorry Sacto, incorrect.
The output transformer is an impedance matching device. It matches a high impedance the tube out load impedance to the low impedance of the speakers.
Putting the wrong speaker on a toob amp reflects the incorrect image to the tubes, creating a mismatch, which dissipates too much power in the tube plates and also creates a situation of too high load, a high voltage situation on the primary toob side and can arc the transformer. That’s why you never disconnect the speaker on a toob amp.
