Amplifier current vrs watts; why is current more important?


Lately when talking to knowledgeable people in the audio industry I’ve been hearing how current delivery is more important than watts in determining weather an amplifier will drive a speaker.
So what exactly is current and how does it effect speaker performance? How can a amplifier rated at 150 watts into 8 ohms vs one rated at 400 watts into 8 ohms be a better match for a hard to drive speaker?
hiendmmoe
hiendmmoe
It's not more important. Power, Voltage, Current and Impedance are all related.

P=I^2*Z = V^2/Z

So depending on the impedance of your speakers I or V could be the limiting factor.
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The late Charles Hansen used to say that an amplifier is basically a power supply design in which the supply is being modulated by an external musical signal.  

Wattage measurement is a static number that has no "time" variable, but in music, timing is everything.  Current is also a static variable without "time".  For example, if you say I want 7Watts but it could mean it will take 2 secs or 10 secs to get 7 watts.  In an orchestra crescendo, you need 7 watts right away.  A good amp will supply that 7 watts almost instantaneously whereas an inferior amp will not, therefore reducing the impact of the music.

A better metric for measuring an amp transient response is called "slew rate", that is how fast it responses to a change in input.

For any given amp, the slew rate will respond proportionally to the load (impedance) of the speaker.  That is the higher the load (higher impedance), the faster the slew rate.  That is why speakers with low impedance, you need higher power amp so that the slew rate will be adequate otherwise the music won't have any dynamic.  


As an analogy using cars, watt is like a car speed, whereas slew rate is car acceleration.  

Let's say you have two car: a good old American muscle car and a Ferrari.  Both cars may be able to do 150mph, but the Ferrari will accelerate to 150mph much faster than the muscle car which is what most people want.
Andy, are you addressing the Q of the OP or changing the topic? You say that both wattage and current are both static variables. How have you advanced his understanding about his Q?
Andy, are you addressing the Q of the OP or changing the topic?

I think the OP wants to know why current is more important than "watt", but I think what he really wants to ask is why two amplifiers may have the same "wattage" rating but one amp may appear to have more "power" than the other.  He is thinking maybe because one amp has more current therefore more power.  It is only partly true.

My response was meant to say the "current" itself may actually not it, but a better characterization of how "dynamic" the amp is by using slew rate measurement.  

So in that sense, I was responding to his "inquiry" but may be in a different way. 
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Because amplifiers don’t deliver watts. (How many watts of light does a lamp deliver? Depends on the bulb, i.e., the load).

What they mean is that amplifier wattage is rated at load of 8 ohms and any change in that load will demand more current from the amplifier.

Example:

100 watts into 8 ohms ---> Voltage is 28V, current is 3.5 amps, transformer 100 VA (~watts)

When the speaker dips to 4 ohms, the voltage can only remain constant if the power supply can deliver 7 amps. To do this, the transformer has to be at least 200 VA. Twice as big.

At 2 ohm load, the PS has to supply 14 amps to maintain voltage, meaning the transformer has to be 400 VA minimum.

If the voltage cannot remain constant -- the transformer/filter caps too small -- then the amp has a hard time driving the speaker through its varying load. This is an oversimplification but by Ohm’s law shows why current is dependent on the power supply and not the nameplate rating.
Also a read from Pass Labs First Watt will help.  


Current is only of import when speakers dip to low impedance or have challenging phase angles, or both. Otherwise it is of little consequence.

There are so many wrong's in this statement.
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Your typical amplifier outputs volts
It may be a bit oversimplification.  Two power amplifiers that have the same power supply voltage may not have the same output current.  Ultimately it's neither the voltage or current, but the overall design that determines the quality of the sound.

You may have a massive 800W power amplifier with massive output current but it may not provide quality music reproduction compared to a more modest 250W amp.
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'High current' is a marketing thing. As others have pointed out, you don't need that much current to drive high power into low impedances.
If you are looking closely and doing the math, it becomes apparent that the current 'rating' of many solid state amps isn't real. Here's more on that topic:http://www.atma-sphere.com/Resources/Common_Amplifier_Myths.php
Because those high current ratings you often see aren't about the output of the amp, there are tube amps with similar current ratings, such as our own MA-2, which has a 'current rating' of about 50 amps.
Your typical amplifier outputs volts.

not quite right. Voltage (more correctly known as EMF, electro motive force) is the force that pushes energy out of the amp, down the cable, and into the speaker. Therefore, saying that an amp "outputs" volts is incorrect. I know it is confusing because you often see the term "output voltage", but that means the EMF measured at the output terminals, not that it is putting out volts.

Most amplifiers today are designed to act as much as possible like perfect voltage sources i.e. no matter how much energy you try to get out of the amp the voltage will remain constant. This is of course impossible, but what the original OP is referring to is how well a particular amp achieves this goal. There are also amplifiers designed to act as much as possible like perfect current sources including some of the offerings by Pass at First Watt

If an amp can maintain its voltage as the impedance drops then it has to deliver more current to do so. The better it is at doing this the "stiffer" it is. Conventional wisdom is that the stiffer an amp, all else being equal, the better it is. Of course, nothing is ever equal so whether or not this correlates to better performance is debatable and very dependent on the load the speaker presents to the amp.

Two power amplifiers that have the same power supply voltage may not have the same output current.

also not quite right. If 2 amps have identical output voltage into the same load they will have identical output current. It should say that 2 amps with the same power supply voltage may not be able to deliver the same amount of current. I know I am being picky but the lack of succinctness is what causes a lot of confusion among those with a limited knowledge of electronics. Amps do not output volts, they output current. Amps do not "have current", they output current.


Massive current gain stages are usually less linear than more modest stages and require more feedback to keep behaved

this is also incorrect. Typically an amp consists of a voltage amplifier or stages of voltage amplification followed by current gain from the final stage. This current stage is usually some variation on an emitter follower which is extremely linear. It is the previous voltage stages which tend to be non-linear where negative feedback is often employed to make them more linear. It is not unusual to have an amp with less unloaded voltage gain that is capable of delivering a lot more current into a low impedance load than one with a much higher unloaded voltage gain.


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Surely you need to throw damping factor into the overall consideration too.
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As someone who imports and sells a line focused on current vs. wattage, I have a POV. Art Audio amps are biased toward current. That is, our amps are high current and deliver lower wattage vs. many of our competitors. This means that with any given tube, we could deliver a much higher wattage number but opt against it.

This absolutely does allow for better handling of lower impedance speakers. But that is not the only reason and it is not just marketing.

The extra current delivers better frequency extremes. You get lighter & airier treble with better nuance and detail. Bass response is tighter and more refined. Maximize wattage in SET or Push Pull triode and treble gets rolled off and bass gets flabby.

You can find 35w SETs with 845 tubes. Why would Art Audio choose to produce an 18w amp when we could produce a 35w amp? The 35w amp will have flabby bass and rolled off treble while the 18w Carissa will deliver frequency extremes you woudn’t think are possible from a tube amp. But it certainly won’t drive every speaker.

That being said, the comment above where a kosst_amojan says ’it depends" really is true. It comes down to your speakers. I have seen a speaker with a high SPL of 98dB but has a minimum impedance of 3.2 ohms. Way harder to drive than you would think and even though 5w of power is enough to drive them in normal circumstances. You need 5 really good, current heavy watts.

Alternatively, I will be introducing a speaker (Verdant Blackthorn 1) with an 84dB SPL and a minimum impedance of 7.2 ohms. Not a difficult speaker to drive at all and current is of minimal concern. You just need enough power into 8 ohms to hit the volume you are looking for. 5w would deliver a whisper, 30w a bare minimum to have 10dB in headroom above 85dB.

I can say that my 18w Carissa could drive Focal Sopra Floor Standers mentioned by kosst_amojan with 15dB of headroom above 85dB. That same amp can’t drive my Blackthorn 1.

Alternatively, I have a 140w SS Class A/B amp that would do a wonderful job with my Blackthorn 1 but would blow a fuse if I tried to drive Sopras with it because it is barely stable at 4 ohms.
Two power amplifiers that have the same power supply voltage may not have the same output current.

It should say that 2 amps with the same power supply voltage may not be able to deliver the same amount of current

Hm ... are these two statements not the same? 
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As someone who imports and sells a line focused on current vs. wattage, I have a POV. Art Audio amps are biased toward current. That is, our amps are high current and deliver lower wattage vs. many of our competitors. This means that with any given tube, we could deliver a much higher wattage number but opt against it.

This absolutely does allow for better handling of lower impedance speakers. But that is not the only reason and it is not just marketing.

The extra current delivers better frequency extremes. You get lighter & airier treble with better nuance and detail. Bass response is tighter and more refined. Maximize wattage in SET or Push Pull triode and treble gets rolled off and bass gets flabby.

Most of this is false. Current does not exist without voltage; further, wattage does not exist without voltage and current. IOW you can't make one (current) without the others (voltage and wattage).

Similarly, extra current (if it were somehow to exist) does not affect bandwidth. What **does** affect bandwidth is output transformers; generally speaking the bigger you make them the less bandwidth you get. In voltage amplifiers if you can't put enough current through the tube you might lose some high frequency bandwidth; that is why 12AX7s are not a good input tube to use in a power amplifier unless bandwidth isn't important.



Similarly, extra current (if it were somehow to exist) does not affect bandwidth. What **does** affect bandwidth is output transformers; generally speaking the bigger you make them the less bandwidth you get.
This seems to be limited to tube amp which requires an output transformer.  Interestingly, atmasphere specializes in OTL tube amp which does not need an output transformer.  But this seems to be getting away from the OP original question.  
Andy2-you were a little off on your car comparison. Wattage/current is kind of like a car horsepower/torque. Torque is your bottom end power. Torque will get you off the line much faster or when you want to pull away. Some cars could have decent horsepower with low torque and that means its gets off the line slower, no power to pass, and all the horsepower is at a higher rpm. Your Sony/pioneer amps might have a decent wattage rating but no current to drive the lower end or react instantly to high peaks
rbstehno,

I guess no analogy is perfect!  It's up to one's choosing.  
Amplifier current vrs watts; why is current more important?

KEEPING IT SIMPLE.
Amps with big watts and little current alone don’t drive speakers with low impedances and stay flat in frequency response, amps with big current do.

EG: One of the worst low impedance speakers ever made, the Wilson Alexia v1, they go down to 0.9ohm in the bass.
A 500w stereo P.A. amp or a 500w Class-D will work into them to a point only, but will sound like a tone control.
 
But a monoblock amp like the "Legendary" old Mark Levinson ML2 monoblocks that are only 25w!! into 8ohms, will sound better into them, because it’s said they double their wattage for each halving of impedance all the way down to 1ohm with the current it can generate, so they will stay flat in frequency response, where the 500w’ers will sound like a tone control.

Cheers George

Here is proof to what I posted above, and the speaker load in both instances, is a very easy to drive Kantor simulated speaker load, a bad load will look far worse.

Look at the black wavy line.

This is the well known Prima Luna Prologue Premium 25w tube amp with very questionable current.

8ohm tap + - 7db!! frequency response, "a tone control"
https://www.stereophile.com/images/1116PLPPfig01.jpg

4ohm tap+ - 5db!! frequency response "still a tone control"
https://www.stereophile.com/images/612PPPfig01.jpg

Here is the Pass XA25 25watter with far more current that the PL above, it’s +- 0.1db frequency response. Virtually flat as it should be!!!
https://www.stereophile.com/images/218PXA25fig1.jpg

Cheers George


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viridian
So I do it backwards and choose the amp first.
That’s fine, so long as the speaker then chosen doesn’t cause that amp to become a "tone control"
To me I would think in that easy "Kantor Simulated Speaker Load" graphs I posted above, that Stereophile uses, that +- 1 or 2db deviation from flat would be acceptable.

That means easier loads than the "Kantor" horns, JBL ect the amp would be almost flat and on harder to drive than the "Kator load" the deviation would still be just acceptable at +-3db deviation from flat.
.
Cheers George
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A speaker is an INDUCTIVE load.  What that means is that it proportionally resists the rise of voltage across it.  In Calculus, the voltage across an inductor is given as:

V = L x di / dt

L: inductance in henries and di / dt is the rate of change of current thru it.

When transients occur, the amp has to overcome the resistance of the speaker to increasing the voltage across it.  That is why it is difficult to drive inductive loads.  To be able to do that, amp has to be able to deliver loads of current.

Electronics 101: inductive loads.

PS: Music is not a straight sinusoid signal.  It is a MESS of transient signals.  A piano for example is almost like a steep 90 degree rise in signal.  To faithfully reproduce it, an amp must be able to supply LOTS of transient current, hence the need for an ENORMOUS power supply.

Having said that electrostatic speakers a different ball game.  They are capacitive loads.  Combine different frequencies and it is a miracle that modern amps actually work :-)




The issue is a little complex when discussing audio frequency modulated currents. Voice coil speakers use alternating electric current in copper coils in close proximity to the speaker magnets to create the mechanical motion of the speaker cone. Audio modulated AC current in any coil produces frequency dependent reactance and impedance not simple 8 ohm DC resistance that might be inferred by the speaker specification. The accuracy of overall sound reproduction is the ability of the amplifier to overcome the reactance and impedance across the audio spectrum otherwise sound distortion is introduced. There is also the issue of electrical resonance and the amplifier needs to be able to dampen such. In general all modern amplifier designs effectively deal with these issues unless you’re an electronics hobbyist building your own amp. 
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But a monoblock amp like the "Legendary" old Mark Levinson ML2 monoblocks that are only 25w!! into 8ohms, will sound better into them, because it’s said they double their wattage for each halving of impedance all the way down to 1ohm with the current it can generate, so they will stay flat in frequency response, where the 500w’ers will sound like a tone control.
What is important is that the amp behave as a voltage source, not whether it can double full power as the load is cut in half. The latter is a subset of the former.

Of course, a good number of loudspeakers in high end audio don't even expect the amp to behave as a voltage source. That is why there are so many successful  zero feedback tube designs, which behave more like a power source.

Doesn't matter what "spin" is put on it, when you have good current, the amp stays flatter in frequency response, instead of looking like a tone control.
An amp needs current along with the watts, as these frequency response graphs show, of the same "wattage" amps with and without good current ability into the same easy speaker load.
https://forum.audiogon.com/posts/1730185

Cheers George


 
Hi atmasphere. I love your amps by the way. If I had reasonably normal loudspeakers I'd have a pair for sure. I have my eye on a pair of Soundlabs which I understand do wonderfully with your MA 2s.
I am not sure I understand your last statement. Are you saying that zero feedback amps can supply more current and would do better driving low impedance loudspeakers?
I am not sure I understand your last statement. Are you saying that zero feedback amps can supply more current and would do better driving low impedance loudspeakers?
No. I am saying that some loudspeakers (like Sound Labs) don't need a very low output impedance in order for the amp to sound right on them. Other examples are most horns, most full-range-driver loudspeakers (like Lowthers) and other speakers where the designer found that he preferred the smoother sound of tubes.


Unless you have a very benign flat impedance speaker (rare) that doesn't wander to much from being a flat impedance between 20hz to 20khz, only then is an amp with watts but little current OK.

You need an amp with both "watts" to suit the dB efficiency to get the loudness you want, and "good current" to keep the "loudness" you want during the "low impedance" dips (that nearly all speakers have).
This stops the amp becoming a tone control, and modifying the flat frequency response. Just as in the graphs I put up.   https://forum.audiogon.com/posts/1730185

Cheers George
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And for me, that’s OK and perfectly fine. A little response tailoring often suits me. It’s not your place to decide what’s OK for folks.
It’s just that we don’t all want that.
  
No need to get your knickers in a knot.

That fine, if you're willing to take the chance, as the "response tailoring" as you so quaintly call it, will be different for every speaker you put it on.

 So you never know if your going to like the "tailoring" it's doing until you bought it.
BTW: It's still a tone control that doing EQ, which ever way you look at it, which will be a different EQ for every different speaker.

Cheers George
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Get over it.

Bit rude! that response for being shown the truth. After all it’s what the subject of this thread is all about.
Educating is a form of telling, and if you don’t want to learn why current is needed from an amp, and what happens when they don’t have enough for the load they’re seeing and become tone controls because of it, then don’t listen or partake in the discussion

Cheers George
arguing against viridian is like talking to a brick lols.  
Thanks for the heads up andy.

Cheers George
Unless you have a very benign flat impedance speaker (rare) that doesn't wander to much from being a flat impedance between 20hz to 20khz, only then is an amp with watts but little current OK.
This statement is false.

The impedance *curve* is not so important. The *overall* impedance is, and more importantly than that the intention of the designer of the loudspeaker.  As an example, many ESLs have a fairly low impedance at high frequencies and yet amps with a fairly high output impedance (aka 'low current') can do quite well on them. This has to do with the simple fact that an ESL is fundamentally different from box speakers in that its impedance curve is not also a map of its efficiency. In a box speaker the impedance peak in the bass is due to the resonance of the driver in the box; its much more efficient at this point. ESLs don't behave this way at all and as a result many solid state amps have troubles making enough power into the bass region of the electrostatic.


This is only one example and there are many more, but the bottom line is that you don't need a flat impedance curve, just an impedance that is high enough.


Of course:  ... SETs. They would not exist if the above quote were true. 


Of course no speaker has flat frequency response! Trying to get flat response is a Sisyphean task. 

It would seem to me that if one wanted to tailor the frequency response; it could be done more specifically and more consistently with an equalizer than with the chance of amplifiers not lining up with impedances of loudspeakers in just the right manner towards that end.

In that loudspeakers have mechanical considerations that amplifiers do not; it seems quite understandable that it would be much more difficult to build loudspeakers with truly flat impedances, than to build amplifiers that can linearly adapt to fluctuating loudspeaker impedances.

Having a linear base line is an important consideration towards the goal of "high fidelity". Even if one wanted to deviate from that goal, amplifiers that can better adjust to loudspeaker impedance fluctuations would be easier to use with the afore mentioned equalizers.


 it could be done more specifically and more consistently with an equalizer than with the chance


So true, why have an amp that can't maintain a flat frequency response, into a speakers wavering impedance load, you may as well have a graphic equalizer, at least with it you can pick where the boost and cuts of the frequencies are going to be.

  Look below at the black wavy line, that is the frequency response you will hear!!, and that's into an easy simulated speaker load, a hard load will be even worse

This is the well known Prima Luna Prologue Premium 25w tube amp with very questionable current.

8ohm tap + - 7db!! frequency response, "a tone control"
https://www.stereophile.com/images/1116PLPPfig01.jpg

4ohm tap+ - 5db!! frequency response "still a tone control"
https://www.stereophile.com/images/612PPPfig01.jpg

Here is the Pass XA25 25watter with far more current that the PL above, it’s +- 0.1db frequency response. Virtually flat as it should be!!!
https://www.stereophile.com/images/218PXA25fig1.jpg

Cheers George
With digital room control you can do whatever you want as long as you have enough power, volts and current to support the corrections. Power is everything in a SOTA system. The problem with low power high efficiency solutions is that power requirements are geometric. They go up dramatically with peaks. Low power systems can't support this unless you just listen to string quartets. The other problem, and this is my ears talking, is that horns always sound like PA systems to me.
An amplifier is a voltage source. It's job is to pump electrons stored in the power supply into the speaker. If the speaker is high impedance fewer electrons (current) are required to do the job. Lower impedance than more electrons. If the power supply runs out of electrons all the pumping power (voltage) in the world will not do any good. None of this says anything about efficiency.  
In a simple DC circuit, Amps = Volts/Resistance. Watts = Volts X AmpsUsing the water analogy, Voltage is the pressure pushing the water thru the pipe, Amperes,(current) is the volume of water & Resistance is the size of the pipe restricting flow. When you introduce AC current with coils, capacitors & such things get much more mathematically complicated, as the previous posts outline. Thank Heaven for the brilliant engineers who design our great gear.