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Topic: Amp power and current (Read 52839 times) previous topic - next topic
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Amp power and current

Reply #100 –
On the thread subject: I am not an engineer, but I find things electrical harder to understand than mechanical because except on instruments and via effects, nothing electrical is visible. So, perhaps a dumb question:
If two amps are both equally honestly stated to be 100 wpc rms, and are faced with a speaker that has impedances that dip to 2 ohms at times, when that happens will both amps deliver the same amount of current to the speakers? If not, what is it about the amp that delivers more current than the other? Is that something found in usually published amp specs?

Amp power and current

Reply #102 –
Quote
If two amps are both equally honestly stated to be 100 wpc rms, and are faced with a speaker that has impedances that dip to 2 ohms at times, when that happens will both amps deliver the same amount of current to the speakers? If not, what is it about the amp that delivers more current than the other? Is that something found in usually published amp specs?


100W into 2 Ohms is 14 Volts at 7 Amps  (approximately).    If these two amps can honestly drive a 2 Ohm load at 100W, then there will be no difference in current.    If the amplifier can't supply the current, the voltage and current will be less (see Ohm's Law) and you'll get clipping.

Power = Voltage x Current

From the above and Ohm's Law, the following can be derived:

Power = (Voltage squared)/Resistance
Power = (Current squared) x Resistance

Amp power and current

Reply #103 –
Quote
If these two amps can honestly drive a 2 Ohm load at 100W, then there will be no difference in current. If the amplifier can't supply the current, the voltage and current will be less (see Ohm's Law) and you'll get clipping.


I like to look at amp power measurements at SounandVision. You can compare a lot of receivers and power amps on their site as they all on the same bench.

But the more interesting thing is that power that is not used, can't improve the sound right? Since it's unused, it means it's unused.

So a 300W amp that supplies 50W to a speaker and a 100W that supplies 50W to the same speaker will be producing the same voltage and the same current, at 50W. Correct me if I'm wrong DVDdoug. So then if 50W from a 300W amp at 4 ohms, and 50W from a 100W at 4 ohms, both amps will be supplying the same current into the load.

Got it right?

Amp power and current

Reply #104 –
Quote
If you think about the question, the answer is reasonably obvious. The amp does not have "current available" it has potential, or reserve, that isn't being called upon. Playing at low volume means you're further from the limit. A low impedance load demands more current for any given voltage.


I think what was/is tripping me was how much potential power the amp can supply, and how much power the speaker needs, at some voltage, which I understand now to be two very different things. The amp has reserve current, and larger amps have more reserve current available at higher voltages. Hope I'm on the right track.

Amp power and current

Reply #105 –
Quote
There is no "required". You are not "asking" or "demanding" current. If you put more pressure in a water pipe, more water will flow through it per unit of time. You don't "ask" or "demand" more water.


But if you turn the volume down, the amount of current that the speaker needs at whatever impedance will be less, surely? Therefore the amount of current needed goes down at that voltage. If you turn the volume up higher, more current is needed, assuming the same impedance.

Correct me if I'm wrong, but if a speaker has a benign impedance like 6 ohms minimum, and above that over the frequency range, then even if the voltages are high, the current that the speaker needs won't be high, because the impedance isn't dipping low enough?

Amp power and current

Reply #106 –
Quote from: DVDdoug on
If these two amps can honestly drive a 2 Ohm load at 100W, then there will be no difference in current.    If the amplifier can't supply the current, the voltage and current will be less (see Ohm's Law) and you'll get clipping.

Thanks, but the question remains: If two amp are honestly specified at 100 wpc rms, will both be able to honestly drive the 2 ohm load as stated above? Or is there something other than the 100 wpc measurement that is relevant?
I ask because often the power part of the amp spec is just the wpc number.

Amp power and current

Reply #107 –
Quote from: Rich B on
Quote
There is no "required". You are not "asking" or "demanding" current. If you put more pressure in a water pipe, more water will flow through it per unit of time. You don't "ask" or "demand" more water.


But if you turn the volume down, the amount of current that the speaker needs at whatever impedance will be less, surely?


That's true and that is a point that people have been trying to make.

The classic audiophile myth is often stated: "I have power hungry speakers".

This mystifies people with a technical background because these speakers usually have average (90 dB/W) efficiency. The speakers may be capable of handling a lot of power, but that doesn't mean that they demand a lot of power as long as you use them as most people do.

Quote
Therefore the amount of current needed goes down at that voltage. If you turn the volume up higher, more current is needed, assuming the same impedance.


Agreed.

Quote
Correct me if I'm wrong, but if a speaker has a benign impedance like 6 ohms minimum, and above that over the frequency range, then even if the voltages are high, the current that the speaker needs won't be high, because the impedance isn't dipping low enough?


That depends on how efficient the speaker is. Some speakers mostly planar speakers or speakers that are physically small have relatively low efficiency (80 dB/W)  10 dB less efficiency means 10 times the power for the same loudness.

There are three major factors in how hard the amp has to work to drive the speakers:

(1) Efficiency
(2) Impedance magnitude
(3) Actual  need for the speaker to generate SPL, which are based primarily on (a) listener preference for loudness, (b) listening distance, and © speaker placement.

For example, the audiophile rule that speakers have to be widely separated from walls can increase the need for amplifier power by 3-10 dB.  Room size doesn't matter nearly as much as listening distance, but of course large rooms tend to foster larger listening distances.  The largest uncontrolled variable is how loud people want to listen at, which can vary all over the map.

And, effective use of subwoofers can dramatically reduce the power that needs to the main speakers to make them play loud.

Amp power and current

Reply #108 –
Quote from: KumarK on
Quote from: DVDdoug on
If these two amps can honestly drive a 2 Ohm load at 100W, then there will be no difference in current.    If the amplifier can't supply the current, the voltage and current will be less (see Ohm's Law) and you'll get clipping.

Thanks, but the question remains: If two amp are honestly specified at 100 wpc rms, will both be able to honestly drive the 2 ohm load as stated above? Or is there something other than the 100 wpc measurement that is relevant?
I ask because often the power part of the amp spec is just the wpc number.



If an amplifier power output specification is does not have impedance attached to it, it is not a proper specification. 

The question is confusing because DVDdoug went out of his way to give a proper power specification with an impedance attached to it. Then we get this off-the wall question based on the specification being improper and lacking impedance information.  Guess what. If something is improper and has critical pieces missing, it is improper and has critical pieces missing. What is so hard about that?  Why these senseless digressions?

Amp power and current

Reply #109 –
Quote from: Arnold B. Krueger on
Why these senseless digressions?

If it is senseless to you, don't answer it.
For anyone else who may want to/be able to:
A typical Marantz spec for power reads - 45W (8 ohms), 60W (4 ohms)
If another amp has the same spec and assuming both are being honest, will both deliver the same current to a given speaker during music play for a specific sound output level that is within the capability of one?

Amp power and current

Reply #110 –
Quote
A typical Marantz spec for power reads - 45W (8 ohms), 60W (4 ohms)
If another amp has the same spec and assuming both are being honest, will both deliver the same current to a given speaker during music play for a specific sound output level that is within the capability of one?


Please re-read the formulas I posted above.  With a little algebra, you can plug-in the wattage & impedance and calculate the current yourself.  ...The physics and associated math don't care if it's a Marantz or some other brand.

If the speaker impedance "dips" below the nominal value, probably nothing bad will happen with real music and real-world listening conditions.    But as Arny says, if you are operating outside of the amplifier's specs the amplifier may perform in unspecified ways.  If you run constant full-power test-tones with out-of-spec impedance, the amplifier may overheat, or something else unspecified may happen...

If you are paranoid, get an amp specified down to 2 Ohms.    (High-power car-stereo amps are often spec'd down to 2 Ohms.  It's rare for a home hi-fi amp, since most home speakers are 8 Ohms.    I think you can pro PA amps spec'd to 2 Ohms.)

Amp power and current

Reply #111 –
Quote from: KumarK on
If another amp has the same spec and assuming both are being honest, will both deliver the same current to a given speaker during music play for a specific sound output level that is within the capability of one?

If you haven't changed the speaker and you drive the amp to the exact same SPL and the amp is kept within its capability, logic suggests that the outcome has to be identical currents and voltages present. To suggest otherwise implies either a) the laws of physics vary or b) one amp is or both amps are flawed. The latter might be true to a minor degree but I don't think the laws of physics in this regard have been shown to vary.

A lot hinges on "both are being honest". Loudspeakers can be complex loads and amplifiers aren't quite perfect. Add in a manufacturer's desire to make their design stand out, so the specs might not quite be apples compared to apples and there could be differences, whether audible or not. With a modern, competent design however, you might struggle to measure differences, let alone hear any.

Amp power and current

Reply #112 –
Quote from: DVDdoug on
Power = Voltage x Current


If you are paranoid, get an amp specified down to 2 Ohms.



Quote from: antz on
logic suggests that the outcome has to be identical currents and voltages present. To suggest otherwise implies either a) the laws of physics vary or b) one amp is or both amps are flawed. The latter might be true to a minor degree but I don't think the laws of physics in this regard have been shown to vary.

I read the formula above to understand that the Watts - used in the wpc numbers - are voltage x current, which means that one can, for example, double the voltage and halve the current and still get the same watts and the watts determine the speaker output levels, so both combinations will have the same effect on sound output/quality.
Does that mean that two amps of the same specified watts at a given ohm resistance may do so by delivering different combinations of volts and amps?
On the question of an amp specified down to 2 ohms, using the example of 45w into 8 ohms and 60 into 4, could two amps of this power delivery spec put out different levels of watts at 2 ohms? If yes, what is different in construction about the amp that delivers more power than the other at 2 ohms?
The reason for these questions isn't to delve into Electricity 101 - it is because at times I read about amp power referred to in terms of the current it can deliver, higher current levels deemed to be desirable, so I am trying to understand why the same cannot be stated with reference to its capability in terms of watts/ohms, this being what is usually stated in amp specs, and not current in amperes.

Amp power and current

Reply #113 –
Quote from: KumarK on
Does that mean that two amps of the same specified watts at a given ohm resistance may do so by delivering different combinations of volts and amps?

You are forgetting Ohm's law - ohms equals volts divided by amps. For a given watts and ohms, the volts and amps must also be given: volts equals the square root of watts times ohms, and amps equals the square root of watts divided by ohms.

Amp power and current

Reply #114 –
Quote from: pdq on
For a given watts and ohms, the volts and amps must also be given

If the first three are given, the amps can be calculated? And the amps will vary inversely as the volts to solve the equation for the same watt/ohm numbers? Does it matter then for audio if the volts/amps vary in this manner so long as for a given ohm number, the watts remain the same?

Amp power and current

Reply #115 –
Quote from: KumarK on
I read the formula above to understand that the Watts - used in the wpc numbers - are voltage x current, which means that one can, for example, double the voltage and halve the current and still get the same watts and the watts determine the speaker output levels, so both combinations will have the same effect on sound output/quality.


When you double the voltage and half the current you vastly change the implied load impedance.  While Power = E * I, it is also true that  Resistance = E / I  If you change either E or I in one equation, you change it in the other!

Quote
Does that mean that two amps of the same specified watts at a given ohm resistance may do so by delivering different combinations of volts and amps?


Absolutely not! Please see above.

Quote
On the question of an amp specified down to 2 ohms, using the example of 45w into 8 ohms and 60 into 4, could two amps of this power delivery spec put out different levels of watts at 2 ohms?


Yes. 2 ohms is an independent load impedance and using your example, we could have two amps that produce 45w into 8 ohms and 60 into 4, but one would immediately burst into flames as soon as you hook up the 2 ohm load and apply power, while the other could be as happy as a claim and provide 70 clean watts into a 2 ohm load.

Quote
If yes, what is different in construction about the amp that delivers more power than the other at 2 ohms?


Any number of things. It could be a simple matter of a protective circuit that is more conservatively designed.  It could be a fuse. It could be any other part, but usually major ones like power transformers and heat sinks. But some amps are unstable with low impedance loads and that could be a simple matter of a small part like a resistor or capacitor.

Quote
The reason for these questions isn't to delve into Electricity 101 - it is because at times I read about amp power referred to in terms of the current it can deliver, higher current levels deemed to be desirable,


Higher current levels are desirable in some cases but far from every case. To some degree there are two poles - current and voltage.  Go too far in either direction and the results are wasteful.

Quote
so I am trying to understand why the same cannot be stated with reference to its capability in terms of watts/ohms, this being what is usually stated in amp specs, and not current in amperes.


There is no reason. As I showed above any counter example breaks some electrical law of physics.

Amp power and current

Reply #116 –
Quote from: KumarK on
Quote from: pdq on
For a given watts and ohms, the volts and amps must also be given

If the first three are given, the amps can be calculated? And the amps will vary inversely as the volts to solve the equation for the same watt/ohm numbers? Does it matter then for audio if the volts/amps vary in this manner so long as for a given ohm number, the watts remain the same?

I'm not convinced you've grasped the consequences of Ohm's Law. For a given resistance and power, both the current and voltage are fixed and related. You cannot achieve (say) 100W with an 8-ohm speaker by using 100V and 1A or 10V and 10A simply because you want to choose those values. 100W into 8-ohms can only be achieved by having 28.28V and 3.53A, no other figures are possible. Bear in mind that assumes the simple case of a resistive load, which speakers are not. Nevertheless, if you don't change the speakers the values must be the same for any given power.

EDIT: beaten to it!

Amp power and current

Reply #117 –
Quote from: antz on
Quote from: KumarK on
Quote from: pdq on
For a given watts and ohms, the volts and amps must also be given

If the first three are given, the amps can be calculated? And the amps will vary inversely as the volts to solve the equation for the same watt/ohm numbers? Does it matter then for audio if the volts/amps vary in this manner so long as for a given ohm number, the watts remain the same?

I'm not convinced you've grasped the consequences of Ohm's Law. For a given resistance and power, both the current and voltage are fixed and related. You cannot achieve (say) 100W with an 8-ohm speaker by using 100V and 1A or 10V and 10A simply because you want to choose those values. 100W into 8-ohms can only be achieved by having 28.28V and 3.53A, no other figures are possible. Bear in mind that assumes the simple case of a resistive load, which speakers are not. Nevertheless, if you don't change the speakers the values must be the same for any given power.

EDIT: beaten to it!


I don't think there was a race, However the same basic point has been explained any number of slightly different ways by now. Based on past experience we will now be accused of contradicting each other because our explanations while all correct, do differ in some details.  I consider that a plus.

Many people work at such a level of dogmatism that they can't grasp that a given question can have more than one correct answer. They will ask 2 people how far to the next town and one will say 5 miles and the other 8 kilometers and they will walk away muttering that everybody in this town is crazy.

Amp power and current

Reply #118 –
Quote from: Arnold B. Krueger on
When you double the voltage and half the current you vastly change the implied load impedance.  While Power = E * I, it is also true that  Resistance = E / I  If you change either E or I in one equation, you change it in the other!

Quote
so I am trying to understand why the same cannot be stated with reference to its capability in terms of watts/ohms, this being what is usually stated in amp specs, and not current in amperes.


There is no reason. As I showed above any counter example breaks some electrical law of physics.

Understood. Thank you very much for the detailed explanation.
Why would some amp reviews then refer to the current delivery performance of the amplifier, quoting ampere values, when most specs quote - and most people I would think can better understand relative power differences of amps under comparison via the watts/ohms numbers? Is it just to be "clever"?

Quote from: antz on
I'm not convinced you've grasped the consequences of Ohm's Law.

I agree - I hadn't understood all the inter relationships. I have now, see above, but I am not sure how long that state will last:-). In any event, these questions were to understand the answer to the question posed above in this post.

Amp power and current

Reply #119 –
Actually, on reflection, there may be another question to answer, clouding the matter. "Does it matter then for audio if the volts/amps vary in this manner so long as for a given ohm number, the watts remain the same?"

No, it does not matter and that can be the case BUT the "given ohm number" (impedance/resistance) must have been changed, i.e. it cannot be the same speaker.

Amp power and current

Reply #120 –
Quote from: KumarK on
Why would some amp reviews then refer to the current delivery performance of the amplifier, quoting ampere values, when most specs quote - and most people I would think can better understand relative power differences of amps under comparison via the watts/ohms numbers? Is it just to be "clever"?

In any event, these questions were to understand the answer to the question posed above in this post.

Because if someone has speakers that have very low impedance dips at certain frequencies, it may be more meaningful to know what peak current the amp can supply. The current will tell you what power that corresponds to at the dip-frequency's impedance. Of course, there are also "bragging rights" to a high-current design, even if their real-world benefit could be zero.

In the real world, speakers are not resistors, they are complex loads. Most domestic speakers are rated at 8-ohms nominal but the spec should also include a minimum figure and the real impedance at any given frequency can vary by quite a lot. The power figures quoted by an amplifier's manufacturer will be derived from testing into an ideal load (a resistor). A "45W amp" may not deliver 45W into a real speaker.

Amp power and current

Reply #121 –
Quote from: antz on
Because if someone has speakers that have very low impedance dips at certain frequencies, it may be more meaningful to know what peak current the amp can supply. The current will tell you what power that corresponds to at the dip-frequency's impedance.

Yes, but could the same information not be given in watts/ohms?
I ask because I am currently using a 55wpc into 8/110 wpc into 4 ohm amp. Some time ago, I had a 140wpc into 8 and 220 wpc into 4 ohm amp. And some others of different power. Through use, I know what these numbers means for distortion free sound levels from different speakers I have used. But if I were to be told this information in amperes, it would not compute. Nor is it there in most published amplifier specs. And speaker makers also usually refer to required power level ranges in watts.

Amp power and current

Reply #122 –
Quote from: KumarK on
Quote from: antz on
Because if someone has speakers that have very low impedance dips at certain frequencies, it may be more meaningful to know what peak current the amp can supply. The current will tell you what power that corresponds to at the dip-frequency's impedance.

Yes, but could the same information not be given in watts/ohms?
I ask because I am currently using a 55wpc into 8/110 wpc into 4 ohm amp. Some time ago, I had a 140wpc into 8 and 220 wpc into 4 ohm amp. And some others of different power. Through use, I know what these numbers means for distortion free sound levels from different speakers I have used. But if I were to be told this information in amperes, it would not compute. Nor is it there in most published amplifier specs. And speaker makers also usually refer to required power level ranges in watts.


Does it matter?

As I understand things, it doesn't really matter how much current capacity the amp has because the speaker determines the current drawn from the amp, and if the impedance doesn't dip low, then the amp won't be asked to supply high current to the load.

So it depends on the load. So if the amp can deliver 140W into 8 ohms and 220W into 4 ohms, or 60W into 8 ohms and 90W into 4 ohms, the current and voltage supplied to a load that dips down to 4 ohms at any power below the rated powers of the lower powered amp will remain the same.

I guess moral of the story is - do you have enough power to cleanly power your speakers? If not, bad things tend to happen and you'll know about it,

Amp power and current

Reply #123 –
Quote from: KumarK on
Yes, but could the same information not be given in watts/ohms?

I don't make a study of amp specs but I think it's generally the case that the published figures are watts into ohms. Often, they also quote a minimum impedance and if not it's usually safe to assume the lowest figure is the lowest used in the specs. For example, my own amp is quoted as 40W into 8 ohms, 45W into 4 ohms and the speakers should be 4-16 ohm for one pair and 8-16 ohm if two pairs are connected. The inference there is that the amp cannot safely deal with loads under 4 ohms. Since the 4 ohm power is only a little above the 8 ohm figure, it also suggests the amp doesn't have vast current capability (about 3.35A, equating to 45W into 4 ohms). My speakers have a quoted minimum impedance of 6 ohms and I use only one pair, so I'm quite safe there.

Amp power and current

Reply #124 –
Quote from: Arnold B. Krueger on
Quote from: Rich B on
Quote
There is no "required". You are not "asking" or "demanding" current. If you put more pressure in a water pipe, more water will flow through it per unit of time. You don't "ask" or "demand" more water.


But if you turn the volume down, the amount of current that the speaker needs at whatever impedance will be less, surely?


That's true and that is a point that people have been trying to make.

The classic audiophile myth is often stated: "I have power hungry speakers".

This mystifies people with a technical background because these speakers usually have average (90 dB/W) efficiency. The speakers may be capable of handling a lot of power, but that doesn't mean that they demand a lot of power as long as you use them as most people do.

Quote
Therefore the amount of current needed goes down at that voltage. If you turn the volume up higher, more current is needed, assuming the same impedance.


Agreed.

Quote
Correct me if I'm wrong, but if a speaker has a benign impedance like 6 ohms minimum, and above that over the frequency range, then even if the voltages are high, the current that the speaker needs won't be high, because the impedance isn't dipping low enough?


That depends on how efficient the speaker is. Some speakers mostly planar speakers or speakers that are physically small have relatively low efficiency (80 dB/W)  10 dB less efficiency means 10 times the power for the same loudness.

There are three major factors in how hard the amp has to work to drive the speakers:

(1) Efficiency
(2) Impedance magnitude
(3) Actual  need for the speaker to generate SPL, which are based primarily on (a) listener preference for loudness, (b) listening distance, and © speaker placement.

For example, the audiophile rule that speakers have to be widely separated from walls can increase the need for amplifier power by 3-10 dB.  Room size doesn't matter nearly as much as listening distance, but of course large rooms tend to foster larger listening distances.  The largest uncontrolled variable is how loud people want to listen at, which can vary all over the map.

And, effective use of subwoofers can dramatically reduce the power that needs to the main speakers to make them play loud.


Thank you for explaining this in a way that I can understand.

Can you please explain what you said earlier or rather the post that I brought up on the last page where you said that reducing the SPL by 6 dB doubles the amps effectiveness in driving low impedance loads?

When I brought it up in conversation I got several comments from people who disagreed with that claim, which was confusing. I'm not sure why there was a disagreement.