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Topic: Power: definition, watts/voltage/current, wrt audio equipment (Read 23781 times) previous topic - next topic
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Power: definition, watts/voltage/current, wrt audio equipment

I had someone earlier this week tell me that when it comes to power, it's not about how many watts you have, it's how much current you have.

But I thought power is the volts times the current. So watts therefore matter as far as current goes, doesn't it?

Power: definition, watts/voltage/current, wrt audio equipment

Reply #1 –
I believe the context where you were told that is rated power versus power delivered.

The formula as you wrote is correct, but the formula obtains a result based on other aspects, like resistance (ohms) and nonlinearities.

So, in the end, the current that the amplifier can deliver is determined, and the power delivered is different depending on those factors. Determine the factors, and you have a power value ( like Watts RMS at a determined frequency).

Power: definition, watts/voltage/current, wrt audio equipment

Reply #2 –
To my way of thinking, the power an amp can supply is not that relevant if the load does not require it. Since that's the most important point, whether the speaker requires the current or not.

And I keep being told by people that it's not the wattage that matters but the current that matters. But an amp that has a high wattage also means it can supply more current, or at least that's what I think. Not an EE or anything, so correct me if I'm wrong.

The current the speaker requires is dependent on, correct me if I'm off, the voltage applied and on the impedance of the speaker, but if I have a low powered amp that can deliver the current to the speaker, at a specified voltage, then what benefit would a larger amp have over that speaker, one that could supply even more current?

My way of thinking is that the speaker 'sees' a voltage, and a current is drawn, and that is dependent on the volume you're playing at, and on the source material you're using.



Power: definition, watts/voltage/current, wrt audio equipment

Reply #3 –
I don't see any contradiction in there.

Just a few comments on semantics, which don't change the global meaning:

A speaker does not require a current. A current goes out of the amp and into the speaker (and out and back to the amp), and this current will be in relation with the voltage seen and the resistance of the circuit to that current. Since audio is an AC signal, the correct term to use is impedance, as you wrote.

Now, IIRC, an amplifier modulates the current, not the voltage, but the maximum voltage that it can provide between the two endpoints is limited. The voltage that the speaker sees is a consequence of the impedance of the whole circuit, including it's own "Ohms".


So, if one amplifier can supply enough current to the speaker, to generate the dB SPL that the listening conditions require, a higher Wattage amplifier will not make any difference in this regard.


If you really require more in-depth knowledge, I'm afraid someone else will need to explain it.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #4 –
Then I think I'm understanding things correctly. I just don't understand why some people claim it's not the wattage but the current that matters, when the two are linked.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #5 –
Quote from: Yahzi on
Then I think I'm understanding things correctly. I just don't understand why some people claim it's not the wattage but the current that matters, when the two are linked.


Was this on an internet forum?
 

Power: definition, watts/voltage/current, wrt audio equipment

Reply #6 –
Quote from: julf on
Quote from: Yahzi on
Then I think I'm understanding things correctly. I just don't understand why some people claim it's not the wattage but the current that matters, when the two are linked.


Was this on an internet forum?


Yep. I hear comments like "you don't need lots of watts, just lots of current", or "its not the watts that counts, but how high current the amp is" etc.

I don't understand that. Higher wattage means the amp can also deliver more current, so I can't make heads or tails out of the above.

At a lower level of power dissipation, I would think it wouldn't matter as the current and voltage would remain the same between one amp and another at a given output. Correct me if I'm wrong here.

Between a Krell monster amp and an AVR, at a lower level of power dissipation, not at rated power, the current demands would be the same between both, or am I smoking something?

Power: definition, watts/voltage/current, wrt audio equipment

Reply #7 –
Quote from: Yahzi on
Yep. I hear comments like "you don't need lots of watts, just lots of current", or "its not the watts that counts, but how high current the amp is" etc.


In that case I would not lose sleep over it - can't believe all the msiguided BS that gets spewed on some forums.

Quote
At a lower level of power dissipation, I would think it wouldn't matter as the current and voltage would remain the same between one amp and another at a given output. Correct me if I'm wrong here.


Almost all amps are voltage amplifiers. For a given signal level, they output a given voltage. The resulting current (and power) depends on speaker impedance, not the amp.

Current drive amps (very rare) are another story altogether.

Quote
Between a Krell monster amp and an AVR, at a lower level of power dissipation, not at rated power, the current demands would be the same between both, or am I smoking something?


Yes, assuming they are both set for the same gain, they would both produce the same voltage, current and power.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #8 –
Quote from: [JAZ] on

A speaker does not require a current.


In some sense it does, since its current that generates the magnetic field to move a driver.

Quote from: [JAZ] on

Now, IIRC, an amplifier modulates the current, not the voltage, but the maximum voltage that it can provide between the two endpoints is limited. The voltage that the speaker sees is a consequence of the impedance of the whole circuit, including it's own "Ohms".


V= I*Z

so if I is I(t) and Z is time invariant then:

V(t)=I(t)*Z

So really you can think of it as modulating either, they are coupled.


Quote from: Yahzi on
I just don't understand why some people claim it's not the wattage but the current that matters, when the two are linked.


The term that couples voltage and current is Z.  Z varies from device to device.  Any given amp has a maximum voltage and current.  If Z is small, then the ratio of V to I will be stacked in favor of I, and an amp will be more likely to run into limits on I.  If Z is large, then V will be relatively greater, and an amp is more likely to run out of V.  That doesn't mean it will, you can make amps that can generate huge voltages but very little current, but for a typical amp this is less likely.

If someone claims that current matters, probably they are referring to a low impedance device like a loud speaker.  For example, on a 4 ohm speaker, you are much more likely to be current limited than on a 600 ohm pair of headphones.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #9 –
Quote from: saratoga on
Quote from: [JAZ] on

A speaker does not require a current.


In some sense it does, since its current that generates the magnetic field to move a driver.

Quote from: [JAZ] on

Now, IIRC, an amplifier modulates the current, not the voltage, but the maximum voltage that it can provide between the two endpoints is limited. The voltage that the speaker sees is a consequence of the impedance of the whole circuit, including it's own "Ohms".


V= I*Z

so if I is I(t) and Z is time invariant then:

V(t)=I(t)*Z

So really you can think of it as modulating either, they are coupled.


Quote from: Yahzi on
I just don't understand why some people claim it's not the wattage but the current that matters, when the two are linked.


The term that couples voltage and current is Z.  Z varies from device to device.  Any given amp has a maximum voltage and current.  If Z is small, then the ratio of V to I will be stacked in favor of I, and an amp will be more likely to run into limits on I.  If Z is large, then V will be relatively greater, and an amp is more likely to run out of V.  That doesn't mean it will, you can make amps that can generate huge voltages but very little current, but for a typical amp this is less likely.

If someone claims that current matters, probably they are referring to a low impedance device like a loud speaker.  For example, on a 4 ohm speaker, you are much more likely to be current limited than on a 600 ohm pair of headphones.


Yeah, but the load determines whether the amp must deliver more current at a specific voltage, not the amp. The amp must just supply the required current, which is why I don't get these types of comments. The load sets the limits. The limits will be the same for any amp at a given output, correct me if I'm wrong here. How much current the amp can dish out has nothing at all to do with what the load requires at a given voltage.

A high current amp can't deliver more current than a low current amp at a given voltage. I believe that is law, Ohms Law. The current demands will be the same for both amps, at the same voltage for any speaker load.

What can a higher current amp offer over an amp that can supply less overall current, if the load can't detect the additional reserves? I don't see much benefit unless the power dissipated in the weaker amp was more than it could deliver at a specific voltage. If it's below that, then (as I see it) there can be no advantage. The amp could have unlimited current reserves but how much is needed is determined by the load, not the amp.

Am I understanding things correctly? What is 'high current' anyway? Current isn't high unless the load, source and volume require it to be high, so that's another thing which has confused me on internet forums.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #10 –
Quote from: Yahzi on
I don't see much benefit unless the power dissipated was more than it could deliver at a specific voltage.


This is the benefit people are referring to. 

Power: definition, watts/voltage/current, wrt audio equipment

Reply #11 –
Quote
Almost all amps are voltage amplifiers. For a given signal level, they output a given voltage. The resulting current (and power) depends on speaker impedance, not the amp.


Sorry to pick your brain here, but when some people say watts don't mean anything, it seems to be like they are shooting themselves in the foot, because power (watts) is voltage times current, so more watts would mean more current, correct?

Power: definition, watts/voltage/current, wrt audio equipment

Reply #12 –
Quote from: saratoga on
Quote from: Yahzi on
I don't see much benefit unless the power dissipated was more than it could deliver at a specific voltage.


This is the benefit people are referring to.


I can understand in some situations where more power is beneficial, and hence more current, but I'm getting the feeling that some people think that high current amps just supply higher current than weaker amps at any given voltage. As if the load had no say in the matter. I'm sure many people go out and buy power amps based on that assumption.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #13 –
Quote from: Yahzi on
Quote from: saratoga on
Quote from: Yahzi on
I don't see much benefit unless the power dissipated was more than it could deliver at a specific voltage.


This is the benefit people are referring to.


I can understand in some situations where more power is beneficial, and hence more current, but I'm getting the feeling that some people think that high current amps just supply higher current than weaker amps at any given voltage.


Its not a binary thing.  Performance degrades gradually as one approaches the limits of an amp.  The available current is often expressed in terms of an amplifier's output impedance.  A lower output impedance will in fact drive a load better, which leads to the recommendation that an amp's output impedance be less than 1/8th of its load impedance to ensure proper driving.  Below that there is generally no improvement.  Above that a load may be underdamped. 

Quote from: Yahzi on
As if the load had no say in the matter. I'm sure many people go out and buy power amps based on that assumption.


If you buy an amp without considering the application for it, you are being foolish.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #14 –
Quote from: Yahzi on
A high current amp can't deliver more current than a low current amp at a given voltage. I believe that is law, Ohms Law.


But they only fine you if you break it in urban zones.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #15 –
Quote from: Yahzi on
I had someone earlier this week tell me that when it comes to power, it's not about how many watts you have, it's how much current you have.

But I thought power is the volts times the current. So watts therefore matter as far as current goes, doesn't it?


What you describe is correct - for a load resistor but your speakers aren't a resistor. If you plot the impedance vs frequency you'll find it to be all over the place. The amplifier is typically a constant Voltage source (meaning the output Voltage is a constant multiple of the signal input). The current into the load will always follow Ohm's law but the current may be higher or lower than you predict based on a resistor. Just like you want to stay under Voltage clipping (distortion), you also want to stay under current clipping which is why you want higher currents available. Some low end amplifiers will be OK into the load resistor and meet specs but behave poorly into a real world load. The better amplifiers won't suffer from it.
G²

Power: definition, watts/voltage/current, wrt audio equipment

Reply #16 –
Quote from: Glenn Gundlach on
Quote from: Yahzi on
I had someone earlier this week tell me that when it comes to power, it's not about how many watts you have, it's how much current you have.

But I thought power is the volts times the current. So watts therefore matter as far as current goes, doesn't it?


What you describe is correct - for a load resistor but your speakers aren't a resistor. If you plot the impedance vs frequency you'll find it to be all over the place. The amplifier is typically a constant Voltage source (meaning the output Voltage is a constant multiple of the signal input). The current into the load will always follow Ohm's law but the current may be higher or lower than you predict based on a resistor. Just like you want to stay under Voltage clipping (distortion), you also want to stay under current clipping which is why you want higher currents available. Some low end amplifiers will be OK into the load resistor and meet specs but behave poorly into a real world load. The better amplifiers won't suffer from it.
G²


So power equals voltage times current only applies to load resistors? I thought it was a universal law.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #17 –
Just to confuse things, speakers generally don't have a constant impedance over the whole frequency range. While a speaker may claim it's 8 ohm, that can change. A speaker is not just a single resistor.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #18 –
I'm talking about the amplifier now. An amplifier that can deliver more watts (power) can deliver more voltage and current. So while the current may be fluctuating depending on the speakers impedance, the more powerful amplifier should have more current available, because it was more wattage available. Right?

Power: definition, watts/voltage/current, wrt audio equipment

Reply #19 –
The problem with just power is that its usually specified assuming maximum voltage (or at least higher voltage) which is probably not where you are going to be listening. So your stero might be rated for 50 watts but at the volume you listen only have a V*I of 5 W.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #20 –
Quote from: Yahzi on
So while the current may be fluctuating depending on the speakers impedance, the more powerful amplifier should have more current available, because it was more wattage available. Right?


Available, yes, but the capability won't be used unless required by a combination of low impedance and high voltage.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #21 –
Quote from: julf on
Quote from: Yahzi on
So while the current may be fluctuating depending on the speakers impedance, the more powerful amplifier should have more current available, because it was more wattage available. Right?


Available, yes, but the capability won't be used unless required by a combination of low impedance and high voltage.


Yes, that's what I mean. So there is more available current, but how much of it is used is dependent on the voltage applied to the load, the load impedance and the spectral content of the recorded media (music/movie etc).

Power: definition, watts/voltage/current, wrt audio equipment

Reply #22 –
Look, this is really not that complicated.

Any amplifier will be able to put out a maximum voltage before it starts to distort. It will also be able to put out a maximum current before it starts to distort. The ratio of this voltage to this current is a value in ohms.

If the load impedance is equal to this value then the amplifier will deliver its maximum undistorted watts into this load (V x I). If the load impedance is higher or lower than this value then the maximum undistorted watts will be less.

If the load impedance is higher then the amplifier will be voltage-limited. If the load impedance is lower then the amplifier will be current limited.

Edit: I should mention that none of this has anything to do with an amplifier's output impedance, which relates to its damping factor.

Power: definition, watts/voltage/current, wrt audio equipment

Reply #23 –
Quote from: Yahzi on
Then I think I'm understanding things correctly. I just don't understand why some people claim it's not the wattage but the current that matters, when the two are linked.



Once upon a time a fair proportion of SS power amps lacked sufficient current to deliver voltages corresponding their rated power into many loudspeakers.

This resulted in what I call premature clipping.

An example of this might be the Crown DC 300 (introduced in 1969) which has been widely alleged to have this problem when used with the then highly popular AR3 . 

The current deliver capabilities for the DC 300 are specified by Crown but I know of no impedance curves for the AR3, so at this point the speculation flag flies. 

In modern times premature clipping seems to be rare.  Just a little development of SS amps and the power transistors for them has happened since then!

Power: definition, watts/voltage/current, wrt audio equipment

Reply #24 –
Some people do like to sound clever without really understanding what they're talking about (and often they like to convince their friends and themselves that their pet device is really better than the numbers make it sound).  This is one of those cases.  The volume produced by an amplifier (assuming the speakers don't explode) corresponds to the power it supplies.  As discussed, this is related to current, but a given power can be produced at almost any current from milliamps to hundreds of amps.  Also as mentioned, in some cases an amplifier may be unable to supply sufficient current to a particular speaker to output all of its nominal rated power, but that is now quite rare.  There was a time when people, not always wisely, used to crave low impedance speakers because they naturally produced a louder sound (at the same volume setting), and that pursuit to extreme low impedance inevitably hit the limits of some amplifiers.

A far more important distinction you should make before worrying about such rare cases is just what that nominal rated power really is, eg. continuous, dynamic, RMS, peak, etc, and also what level of distortion is present at the quoted power level.  It is a sad fact that big numbers are tempting to the sellers of audio equipment and they are perfectly happy to quote numbers that are not achievable in practice, or that produce unacceptable audio when they are achieved.  I have a modest car audio system and an expensive home audio system that have about the same nominal power output (the car system is actually higher!), but in practice there is no comparison by an order of magnitude.  BTW, it is nothing to do with the current the amplifiers can push

One last nit: we really should talk about impedance.  Just like resistance it is measured in ohms, but it is definitely not the same thing as resistance and the resistance of a circuit is an entirely different number to its impedance.  (Almost all) Speakers have a rated impedance, often as low as 2 ohms for large loudspeakers, but hundreds of ohms for small speakers like headphones.