why does efficiency increase when ohm load increases?

[mpowered325is 10+ year member]

Im sure the answer is wrong, but I would go the the Zap DC1100.1. I have owned everything from the old Studio and Comp series to the C2K's and Reference models. I would take a Zapco over just about any of those above. Only other amp in the list I have experience with is a Sundown.

 

[vaiboy 10+ year member]

so if an amp puts out 1000 rms at 4 ohms and you ran it at 1000 watts at 1 ohm would it have the same efficiency?well I'm just trying to understand a little bit more about how amps work, I guess just ohm load vs efficiency.

because manufacturers rate amps at let's say 80% efficient at 4 ohms and 70% at 1 ohm...just an example but is the efficiency lower just due to more watts or is there something about the lower ohm load that makes the amp less efficient?

Well to begin to understand efficiency you need to understand current, it's the number of electrons that pass a point in a set amount of time. As each electron passes the point it gives up a fraction of it's energy in the form of heat. So the more electrons that pass the point the more heat that is generated in a set amount of time. So in the circuit there is useful energy that travels through the circuit and does work, and there is energy that escapes as heat.

If we look at your example of a 1000W@4Ω amplifier the current through the output would be ~15.8 amps, we will forget about the supply side of the amp for a second. If the same amplifier were to be set up to output 1000W@1Ω the current would be ~31.6 amps. That's double the current at 4Ω, more useful energy is lost to heat but the same amount of work is being done (Work = Watts). Efficiency in amplifiers is a function of the supply to output, and the supply is doing more work to overcome the loss of useful energy to heat in the output. The amplifier would need to be set to output 250W@1Ω to match the current at 1000W@4Ω, but at 250W there would be less useful energy lost to heat on the supply side of the amplifier. So technically you could find an output wattage at 1Ω where the efficiency matches 1000W@4Ω, the output will be lower than 1000W and greater than 250W, but I don't know why you would want to.

 

[quackhead 10+ year member]

Im sure the answer is wrong, but I would go the the Zap DC1100.1. I have owned everything from the old Studio and Comp series to the C2K's and Reference models. I would take a Zapco over just about any of those above. Only other amp in the list I have experience with is a Sundown.

it wasn't set up to be a right or wrong answer...personally, I would choose the Zapco as well. I just presented the questionnaire as an exercise in study beyond 1ohm rated output.

 

[Kangaroux 5,000+ posts]

@keep_hope_alive

 

[akheathen Premium Member]

well, there are basically 2questions, and i have 2 answers. as mentioned, 1k@4ohms and 1k@1ohm in an amp is set up differently. really, current is something that is allowed to pass, determined by how much resistance is present between a voltage source. (both in static/meansured, and dynamic, i'e electromagnetic, thermal, and conductor ampacity limitations.) i mention this, because it is all too often over-looked, and amperage is mistaken as, and even referred as an applied source/effect, rather than a resulting effect.

so, let's look at the 2 different output sections. first, is the 1k@1ohm. it would usually be set at 35-45volts available on a good solid amp to give you the ~32vac needed. the 4ohm, would need to put out ~64vac, and could see 80v on a solid amp. the same exact power supply section could be used for either, but the control circuit would just monitor the output rail voltage to sustain the appropriate voltage at any power use level, up to full duty cycle, or gain (depends on type of power supply used)

there are a few different factors that effect efficiency, but the biggest is in the transformer, which is usually a "torroid" (circle thinggy with wire wrapped around it.) it should, but not always, match power transfer ability at it's most efficient level at/near full power supply duty. at power levels below that, the efficiency drops, since the driving magnetic field is lower, and less of it is transferred into the core. (more amps passed, equals more e.m. field) in a 1k, it is relatively typical to see 2-5a of current being used at absolutely 0 efficiency, during idle (on, no output) you can also see efficiency in the 40-60% range between 1/4 and 2/3 power. also, there is a bit different scaling on an unregulated power supply, and consequently often quite less efficiency.

back to the 2 amps. there are different ways they are set up. what i like to see in a 1k@1 amp is that the 2ohm output is half. this tells me that it was made to run easy, and efficient at 1 ohm when you see power levels more than half at doubble resistance, it indicates that the amp is being driven beyond what the power supply runs at full cycle. this is not always a bad indicator, as the amp could be built with components that can handle more current than what full cycle passes. if it is severe, you may get "bubbly" power, rather than solid, clean power.

1k@4: if the amp is made to run most efficient at 4ohms, then the efficiency at, say 2 ohms, would be more effected by heat and ampacity of the components in the amp than at 4 ohms.

that's my bits for now. in short, it depends a great deal on the transfer efficiency of the transformer between the power supply and output section at different power levels.

 

[keep_hope_alive Premium Member]

@akheathen and @vaiboy handled the answer well.

nice job, guys.