The suggestion of getting an amplifier with higher “current” vs just the high output power has been brought up many times. However, this is not an easy measurement one can tell from the product specifications alone. Can someone advise a good way to spot if an amplifier offers “high current”? Also, between tube amp, class a, a/b, and d, if there is a consistent approach to evaluate the current number? And if “current” is so important, why this is not a more easily marketable measure on the box of the product? Another one is the damping factor. Similarly, it’s very frequently brought up when recommending speaker match. Just trying to be more scientific and objective here.

Just trying to be more scientific and objective here.

Good luck with that. With solid state, if an amp doubles its power at 4 ohms and can handle low impedances, no worries. On the tube amp side, tube amps usually have current reserves to run the vacuum tubes but not always.

The idea is that a high current amp will perform consistently well even as your speaker impedance changes. If your speaker impedance is stable, or high enough this becomes pretty irrelevant. Sometimes you might even like the combination of a low current amp and low impedance speaker if the dips happen where you like or need them to.

The general first thing to look for is that the current doubles as impedance halves:

8 Ohms: 100 Watts

4 Ohms: 200 Watts

2 Ohms: 400 Watts

Another thing to look for is being rated to drive 2 Ohm loads.

But unless you have unusually difficult to drive speakers this isn't usually a problem for most good solid state amplifiers. Electrostatic speakers are a notable "always a problem" speakers. :)

I get the case where you have double the wattage with half the impedance, but such case only show the device is qualified as high current, but now by how high as a quantifiable measurement. It’s only a relative measurement. So there is no one measurement available to indicate the “current” capability? I don’t have electrical technical back, and I feel this concept is being mystified…

As eric said, the rating into different impedances tells you more about the amp’s capability, and you should couple that to your room size and speaker requirements for selecting a power rating.

Keep in mind amplifiers do not put out power, the speaker (or the load) consumes the power. The amplifier has to be capable of feeding the speaker as a constant voltage source and that capability is measured in watts, which is the voltage multiplied by the current carrying capability of the power supply transformer and filtering caps . The confusion about power and current can be cleared up with an example:

Suppose there are two amplifiers, each rated at 100 watts into 8 ohms. To get that rating, the amps must gain up the input signal to 28 volts and the power supply keeps at least a constant 28 volts with an 8-ohm load. Attach an 8-ohm resistor load to each amp, and crank up the volume until the signal maxes out at 28 volts. Both amps show a clean sine wave and that means each amp is at 100 watts. At 100 watts over 8-ohms, that is 3.5 amps of current.

Now replace the load with 4-ohm resistors. The first amp shows a clean sine wave to 28 volts. The other amp shows the sinewave clipping at 20 volts. It reached max output. The first amp is now putting out 200 watts and into 4-ohms that is 7-amps. The second amp is putting out 100 watts (power rating didn’t change!) and into 4-ohms the current is 5-amps. The second amp is putting out less current at 4-ohms.

Replace the load with a 2-ohm resistor. If the first amp still shows a clean sine wave at 28 volts, it is putting out 400 watts and 14 amps. If the second amp clips at 14 volts, it is still putting out 100 watts but only 7-amps of current.

Obviously, the first amp is more powerful and will feed difficult speaker loads better than the first, even though the second amp is "putting out" the same power. However, the second amp may work only as good as the first amp when the speaker load is fairly constant or maybe when playing at low volumes.

Tube amps are a different animal altogether. The power rating depends only on the output transformer and power tube because the load is reflected from the speaker to the tube. A changing speaker impedance on the xfmr secondary is reflected back to the xfmr primary, which presents a changing impedance to the tube. The power is pretty much constant no matter what the speaker impedance does, but in order to get the rated power the output xfmr tap (4,8 or 16 ohm) impedance has to closely match the speaker impedance.

High current amplifiers will usually put the specifications of the maximum current output. The others will not. @erik_squires is correct the doubling is a good proxy.

My real interaction with this issue was in 1980, I bought a 200 watt/channel (?) 75 pound, highly regarded Yamaha amp… it was a complete whimp… I bought a 250 watt / challenge 125Pound Threshold s500 that completely and utterly blew away the Yamaha… the Yamaha sounded like a cheap transistor radio… in comparison to the Threshold. The Threshold had at least 10 times the current. Took hold of the speakers and told them it was in charge.

Dragoncave. There is no one answer that is going to give you the magic number. Welcome to audio. It's a complex topic. Experience helps. There are guys that think a 200 buck amp is going to deliver the same performance as a high end amp at multiple dollars more. That's just pure fantasy, just like everything else you buy, quality costs money.

I get the case where you have double the wattage with half the impedance, but such case only show the device is qualified as high current, but now by how high as a quantifiable measurement. It’s only a relative measurement. So there is no one measurement available to indicate the “current” capability? I don’t have electrical technical back, and I feel this concept is being mystified…

Measure the voltage.
The equation V=IR can be refactored to be I = V/R.

Once you have V and the impedance plot, then you know R at any, and all, frequencies.

If it is a constant 8 ohms, then I=V/8.
If it is 2 ohms, the I=V/2
If it varies with frequency, then it gets harder.

There is not a lot more to it than ^that^.

If the amp it trying to hit 50V, but only have rails at 28V, then you run out of voltage.
If the impedance that you are trying to drive is low, and the power supply has little capacitance and undersized in the transformer, then you cannot maintain the “umph” for very long.

If you are concerned with current, then you likely have a speaker than is very low impedance, or maybe have been told that you should be looking for current specs as a way to determine which amp to use.

I will just add that amplifiers with FET's typically have a higher current rating than bi-polar devices. This also yields to a high slew-rate which helps with transient peaks. I've not seen tube amplifiers that can match current capabilities of SS FET amplifiers.

Most amps in the “Hi-Fi” realm have info associated with them discussing current, watts etc. Unless you are trying to drive very inefficient speakers and/or a speaker that dips into painful loads under 2 ohms, and listen very loud, it’s not a problem. The knowledge of the doubling down factor of watts/ohms is something that is known and easy to find out. If an amp doubles down from 100/8 to 200/4 but craps out down to 2, then you know the amp will struggle if put into a hard to drive speaker. This info is often listed, or a quick phone call to ascertain if the amp is stable at two ohms gets the job done.

I was thinking of trying Peachtree’s new 1000w GAN digital amp. But their power rating numbers didn’t look great past 4ohms. I called them and they recommended I not use it as it wasn’t really a good mix for 2ohm loads, which is fairly typical of digital amps. Other than those factors, the actual current doesn’t really matter much. You just need to be sure to get an appropriate amp for the speakers you intend to use.

Thank you all for the comprehensive science lesson. Really learned a lot here. I guess my point is the "lineality" (not sure if the right word) of co-relation between volts and impedance can be measured, and should be clearly marketed when a consumer is looking for a "current capable" amplifier, rather than using indirect formula to figure it out by themselves. 100/8 and 200/4 is just an indirect measurement, and i am sure there are ones perform 100/8 and 180/4, or more, or less. Why not just use one measurement to tell clearly, instead of leaving the consumer scratching the head and guestimate the math himself.

So there is no one measurement available to indicate the “current” capability? I don’t have electrical technical back, and I feel this concept is being mystified…

The problem you’re up against is marketing. Often solid state amps are advertised as having a certain amount of ’current’. That can be quite different from the current required to drive the speaker load properly when the amp is at full power. You might want to read the article at this link:

As an additional hint for good sound: avoid loudspeakers that are excessively low impedance (less than 4 Ohms) or low efficiency. The reason is that all amplifiers made more distortion driving lower impedances, and low efficiency loudspeakers (less than 87dB) tend to suffer thermal compression caused by heating in the voice coil. You might think that additional distortion is inaudible (that its 'negligible') but that isn’t the case. The ear interprets all forms of distortion as a tonality and it is keenly sensitive to the higher ordered harmonics since it uses them to sense how loud a sound is (and the ear has over 120dB range)- so more sensitive to those harmonics than anything else! So when your amp has to work hard for a living, the additional distortion it makes will be mostly the higher orders. This contributes to harshness and brightness (the tonality assigned by the ear) as well as a loss of detail, which affects things like sound stage depth and width, as well as nuances.

So the difference in distortion is not only audible its also measurable.

Thank you all for the comprehensive science lesson. Really learned a lot here. I guess my point is the "lineality" (not sure if the right word) of co-relation between volts and impedance can be measured, and should be clearly marketed when a consumer is looking for a "current capable" amplifier, rather than using indirect formula to figure it out by themselves. 100/8 and 200/4 is just an indirect measurement, and i am sure there are ones perform 100/8 and 180/4, or more, or less. Why not just use one measurement to tell clearly, instead of leaving the consumer scratching the head and guestimate the math himself.

The education system’s lack of science is also reflected in a lack of critical thinking skills, that make the populous more prone to conspiracy theories.

Then add in marketing and prose, and it is like a shark feeding frenzy for a salesman.

I’ve mentioned the speaker’s impedance curve a few times.
If you were serious about making it easy, then I would have suspected you to have posted that? Or at least the name of the speakers in question.

You mentioned:

And if “current” is so important, why this is not a more easily marketable measure on the box of the product?

Are any speaker’s marketed as requiring specific amounts of current?

The reason why current is not easily marked on the specs, is that it is not really important except in some odd corner cases.

Current is not important.
- If the amp makes its voltage, then it also makes the needed current.

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