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Watts and Current

post #1 of 227
Thread Starter 
Hi guys,

Would like to know from those more experienced, what the nitty gritty is with current and wattage. I've been told that some amplifiers have lots of wattage but not much current, others have low wattage and high current. What is important in an amplifier : High wattage or high current? Thanks.
post #2 of 227
There one and the same sort of.
The current is what goes down the cable and is measured in amps; watts are a measurement of energy over a period of time.
post #3 of 227
Power (measured in Watts) is the product of current and potential difference (measured in Volts).

think about it.
post #4 of 227
Quote:
Originally Posted by Heinrich S View Post

Hi guys,

Would like to know from those more experienced, what the nitty gritty is with current and wattage. I've been told that some amplifiers have lots of wattage but not much current, others have low wattage and high current. What is important in an amplifier : High wattage or high current? Thanks.

That doesn't make sense. Wattage is (current * voltage). For example a 100 watt light bulb uses 0.83 amps (100w / 120v) = 0.83a. Now say we live in Europe where the line voltage is 240v. There (100w / 240v)= 0.41a. Half the current but still 100 watts as the voltage is doubled. 100w car headlight would need 8.3amps. (100w / 12v).

So you can have more voltage and less current or more current and less voltage to make the same watts.

For an audio amplifier, RMS specified watts is watts. And since the speaker is a known resistance (well sort of), it's pretty simple.

To develop 50 watts into an 8ohm speaker you will need 20 volts of audio at 2.5 amps.

Look up Ohm's Law.
post #5 of 227
Thread Starter 
So an amplifier cannot have lots of current without lots of wattage?
post #6 of 227
Quote:
Originally Posted by Heinrich S View Post

So an amplifier cannot have lots of current without lots of wattage?

Generally speaking for 4-8ohm speakers that statement is true.

But consider this, I run tube amps. The current on the output tubes is less than 200ma yet the amp produces 70 watts. The voltage however is over 500 volts. However this ratio is useless for a 4-8ohm speaker. So tube amplifiers have an output transformer than converts the high voltage / low current audio signal to a high current / low voltage signal. But the wattage is the same, minus some minor loses through the output transformer. Solid state amps are inherently high current low voltage devices so they don't need output transformers. (although there were and are SS amps made with output transformers).
post #7 of 227
A 100 Watt , 2 Ohm speaker needs: 7.1 Amps and 14.1 Volts
A 100 Watt, 16 Ohm speaker needs: 2.5 Amps and 40 Volts

Not all amplifiers are capable of sourcing the large amount of current that low impedance speakers need.
post #8 of 227
Quote:
Originally Posted by Speedskater View Post

A 100 Watt , 2 Ohm speaker needs: 7.1 Amps and 14.1 Volts
A 100 Watt, 16 Ohm speaker needs: 2.5 Amps and 40 Volts

Not all amplifiers are capable of sourcing the large amount of current that low impedance speakers need.

But then, using your example, they can't say the amp produces 100w at 2ohms. Because to do that it must source the current you stated.
post #9 of 227
Thread Starter 
So Glimmie, if someone says that wattage isn't everything, current is -- that that is really nonsense?
post #10 of 227
They are related.

Impedance magnitude in Ohms = R
Power in Watts = P
Voltage in Volts= V
Current in Amperes = I

Note there is no such thing as "RMS Watts", only RMS (root-mean-square) voltage and current.

Ohm's Law: V = I * R

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

You must have the appropriate voltage and current into a given load impedance to obtain the stated wattage. You cannot specify any parameter without involving the others.

Speaker impedances change over frequency so the value provided in data sheets is an average or estimate of the nominal impedance. Some specify min and max values, some include plots.

Wikipedia has articles about all this.
post #11 of 227
Quote:
Originally Posted by Heinrich S View Post

So Glimmie, if someone says that wattage isn't everything, current is -- that that is really nonsense?

It means they don't understand Ohms law
post #12 of 227
Thread Starter 
I'll look up Ohms Law. It's just that I see some amplifiers have a claimed wattage, like an Onkyo or a Yamaha, and other amplifiers like a NAD are considered to be higher current amplifiers, so I try to understand why some people maintain the belief that current is so important, not just the watttage. How some receivers have a high wattage but a low current and vice-versa. That's what I don't understand.

Also, is it true that less powerful amps driven into clipping have more chance of destroying the speakers than driving the speaker with an amp that delivers more clean power than the speakers can handle?
post #13 of 227
Quote:
Originally Posted by Heinrich S View Post

.....It's just that I see some amplifiers have a claimed wattage, like an Onkyo or a Yamaha, and other amplifiers like a NAD are considered to be higher current amplifiers....
Marketing hype. Unless it means the amp is good down to two ohms like described above. But that's the same thing as saying the amp will produce X watts at 2 ohms.
Quote:
Also, is it true that less powerful amps driven into clipping have more chance of destroying the speakers than driving the speaker with an amp that delivers more clean power than the speakers can handle?
Yes that can happen. An amplifier that is clipping is outputting DC when clipping. Too much clipping can destroy a speaker voice coil. Personally though, I don't see how anyone could listen to this clipping long enough to damage a voice coil.
post #14 of 227
The problem is that the consumer is given only a rating for two channels, with a resistive load of 8 ohms used for testing, and a sine wave of only one frequency as a signal. This is as realistic as testing a vehicle at 40 MPH on a dynamometer and basing its desirability and performance entirely on that one very limited test! On such a test a Porsche and a Yugo will probably score the same. Are they in fact the same??

This very simple very limited test is what the Federal Trade Commission decided on 50 years ago, when only 2 channels existed (to prevent excessive power claims). This is a very limited test that is not meant to prove amplifier capabilities or sound quality; just one very specific and very limited thing.

First of all, a power RATING determined this way is now next to useless, because there are VERY FEW speakers today that are truly 8 ohm speakers. The fact that the manufacturer may CALL the speaker an "8 ohm" speaker is Bullsnort (and they know it). They don't exist.

Almost every speaker has an impedance of 5 ohms or less at some frequencies, and a test run with an 8 ohm RESISTOR loading the amplifier does not require the amplifier to put out the maximum CURRENT that the speaker will draw in the real world.

In other words, these ratings are next to useless and completely outdated!. They are not a significant test of the amplifier's capabilities in the real world with a speaker for a load. The FTC needs to come out of the dark ages and design some more meaningful test criteria and procedures.

The other thing that a test with a RESISTOR for a load ignores is the fact that speakers have inductance and capacitance, which vary as frequency varies, and this makes the job of the amplifier immeasurably more difficult, as it has to put out varying currents at varying Phases as well as different magnitudes when it is amplifying MUSIC instead of a single sine wave.

More expensive amplifiers SOUND BETTER because they have larger more robust MORE EXPENSIVE power supplies that can handle these highly variable current demands without loading down and causing the amplifier circuits to distort.

These are all well-known BASIC CONSIDERATIONS that every amplifier design engineer deals with daily. He has the extensive technical education and experience to deal with these issues intelligently and effectively.

Obviously, most non-technical people have a very imperfect understanding of these issues. The confusion and lack of technical knowledge inherent in the comments of many of the previous posts is exactly what is to be expected.

The bottom line is that the design of a good amplifier involves a lot of tradeoffs in the cost of the power supply components as well as the amplifier circuits themselves. This makes for very large differences in the cost of the design and production of the amplifier.

That is why sound professionals will willingly spend several thousand dollars for a good power amplifier, while cheap amplifiers with similar "power ratings" can be had for far less.

They know that the cheaper amplifier does not have the "balls" to drive a REAL speaker with low distortion and high reliability. They know it from EXPERIENCE with each type, and they know which one will DELIVER what they need.

People who say that any two amplifers with the same power rating should sound the same do not fully understand how limited and imperfect the test is that determines the rating. They also do not understand the REAL demands on an amplifier or how incredibly hard they are to emulate in a simple lab test procedure.
Edited by commsysman - 6/24/13 at 6:09pm
post #15 of 227
Quote:
Originally Posted by commsysman View Post

More expensive amplifiers SOUND BETTER because they have larger more robust MORE EXPENSIVE power supplies that can handle these highly variable current demands without loading down and causing the amplifier circuits to distort.
Do you have any evidence of this? You've been asked this before but you have yet to present one so I'm asking.
Quote:
Obviously, most non-technical people have a very imperfect understanding of these issues. The confusion and lack of technical knowledge inherent in the comments of many of the previous posts is exactly what is to be expected.
Lack of technical knowledge?:
Quote:
Originally Posted by commsysman View Post

A 3 db increase in SPL is perceived as "twice as loud". You are claiming it takes a 10db increase, which is nonsense.


A 10db increase in SPL is DEFINED as 10 times as loud!

Quote:
Originally Posted by commsysman View Post

People who say that any two amplifers with the same power rating should sound the same do not fully understand how limited and imperfect the test is that determines the rating. They also do not understand the REAL demands on an amplifier or how hard they are to emulate in a simple lab test.
That depends on how imprecisely the amps are compared:
Quote:
Originally Posted by commsysman View Post

Using a SPL meter and repeatable test tones is an excellent method for matching levels, as any qualified audio engineer will tell you. They have been doing it that way for many years, and everyone except you is satisfied that it does the job very well.


In any case, I am perfectly satisfied that it does the job well, and is an accepted and proven method among those who are highly qualified in the field.
post #16 of 227
Dio-

As long as you try to substitute sophistry for logical reasoning, your statements are without force or effect.

Have a nice day.
post #17 of 227
Quote:
Originally Posted by commsysman View Post

.......The confusion and lack of technical knowledge inherent in the comments of many of the previous posts is exactly what is to be expected.
The above posts deal with Ohms Law. The 2ohm problem was accurately outlined. The fact that speaker impedance is nominal was also acknowledged.

So where is this "lack of knowledge"?

OTOH, your post reads like a Best Buy ad!
post #18 of 227
Quote:
Originally Posted by Heinrich S View Post

I'll look up Ohms Law. It's just that I see some amplifiers have a claimed wattage, like an Onkyo or a Yamaha, and other amplifiers like a NAD are considered to be higher current amplifiers, so I try to understand why some people maintain the belief that current is so important, not just the watttage.

I feel your pain but this here engineering degree they let me earn some time ago helps me bail water. ;-)

The first thing to remember that after you add a reasonable safety margin (say, 100%) to what you actually need, additional power capability serves no logical purpose. After studying the matter and also living with a real world audio system, it looks to me like 35-50 wpc will drive a typical system to standard reference level with a reasonable allowance for peaks, explosions, and etc.

The second thing is that others who are critical of how amplifiers are rated are correct to be critical. The traditional means for testing amplifiers on the test bench with resistive loads and steady tones is unrealistic. I became acutely aware of this after I developed a loudspeaker simulator that closely matched the kind of a load that a fairly tough speaker load put on an amplifier. My simulator went down to 3 ohms which as others have pointed out is characteristic of some 8 ohm speakers.

The first thing I noticed is that when loaded by my speaker simulator, amplifiers did not get hot or strain like they did with standard load resistors, not even close. And this was with swept full power test tones and full power pink noise signals.! The second thing I noticed was that real world music, even that which was chosen to be extra demanding, stressed the amplifiers even less.
Quote:
How some receivers have a high wattage but a low current and vice-versa. That's what I don't understand.

They don't. In order to have high power they have to have high current. However, in order to create good advertising copy some people have built super-current amplifiers that go way into overkill.

With modern solid state devices it is relatively inexpensive to build amplifiers that can put out tremendous amounts of current, especially for short periods of time. Construction costs for power amplifiers have been historically dominated by power transformers and heat sinks. However, you don't need huge heat sinks and power transformers in order to build an amplifier that amplifies music well. What you actually need is power transistors that won't burn out when there are short term power demands.

Quote:
Also, is it true that less powerful amps driven into clipping have more chance of destroying the speakers than driving the speaker with an amp that delivers more clean power than the speakers can handle?

That's another audiophile myth.
post #19 of 227
Thread Starter 
Thank you Arnyk. But this :
Quote:
That's another audiophile myth.

Glimmie said it can happen and you say it's an audiophile myth. I am confused. confused.gif
post #20 of 227
Thread Starter 
Quote:
They don't. In order to have high power they have to have high current

So due to Ohms Law, an amp that claims high current but has a lower wattage is just BS? Like NAD amps compared to Yamaha amps in the same price range. They tend to produce more power in all channels than Yamaha do according to test bench results. I figure that has something to do with the additional current in the NAD, but maybe NAD are just fudging the numbers on the spec sheets so that when they get tested the results look better than they are?
post #21 of 227
Quote:
Originally Posted by Heinrich S View Post

Quote:
They don't. In order to have high power they have to have high current

So due to Ohms Law, an amp that claims high current but has a lower wattage is just BS?

I don't know what you mean by that, because words like "high" and "low" are relative, not absolute.

It takes a certain amount of voltage and amperage to have a certain amount of power.

The "high current" amplifiers that I am aware of claimed to be able to provide more current than was required. They may have been able to do that. However, excess current capacity is just like any other excess - its well, excess! ;-)
post #22 of 227
Quote:
As long as you try to substitute sophistry for logical reasoning, your statements are without force or effect.
Quote:
Originally Posted by commsysman View Post

A 3 db increase in SPL is perceived as "twice as loud". You are claiming it takes a 10db increase, which is nonsense.


A 10db increase in SPL is DEFINED as 10 times as loud!

Looks like you have a severe lack of technical knowledge.
post #23 of 227
+3 dB requires twice the power. (Force)
+10 dB is required for a perceived doubling of "volume". (Effect).
post #24 of 227
Quote:
Originally Posted by Heinrich S View Post

So due to Ohms Law, an amp that claims high current but has a lower wattage is just BS? Like NAD amps compared to Yamaha amps in the same price range. They tend to produce more power in all channels than Yamaha do according to test bench results. I figure that has something to do with the additional current in the NAD, but maybe NAD are just fudging the numbers on the spec sheets so that when they get tested the results look better than they are?
What you say here actually leads to the answer you are looking for but is not obvious smile.gif. As noted before, you can use Ohm's law to arrive at Power = V*V/R. It seems therefore that current does not enter the equation and all that matter is the voltage. Indeed, when designing a power amplifier, the first thing you do is decide on the voltage as that sets the upper limit on how much power you can have. A car audio amplifier for example that uses the car's battery voltage of say, 13 volts as is, will only be able to produce 13 * 13 / 8 = 21 watts with an 8 ohm load. This is why typical car stereos have power in this range. High power car audio amps got around this limit by having dc-to-dc converters that raise this voltage up substantially in order to create say, hundreds of watts. The car battery can produce huge amount of current but that was of no value at its low standard voltage without the dc-to-dc convert to raise it.

So what about the current? As I said, it seems to not matter as i just computed the power using just voltage. The answer to that lies in the example you give. A home theater AVR may rate two channels driven at 100 watts each, but with all 5 channels driven, the output may drop to 40 watts/channel. Ohm's law and the computation above clearly fail to explain this. After all, if the amp had sufficient voltage to produce 100 watts, and the load is kept constant, how did it all of a sudden fail to produce that same power when all channels were driven? That is your question, right? The answer is that voltage did not say constant. It actually dropped due to lack of current. The power supply had insufficient current to drive all five channels to 100 watts. When it ran out of current, its voltage sags and lowers. Once that voltage drops, the above formula then predicts that our power output drops just the same. In other words, the voltage is dependent on current draw due to design limitations in practice.

There are actually two solutions to above. One is to boost the power supply capacity to power all the channels. Or alternatively as Pioneer does, use more efficient class-D amplification. A ton of current/power is wasted in traditional amplifiers due to heat. That is why those giant heat sinks are there to keep the output transistors from getting too hit and destroying themselves. Here is a quick graph showing the difference between two classes of amps:

pwm-f1.gif

I know NAD has some class D designs but have not looked at their AVRs to know if this is how they get additional power or simple brute force of more beefy power supply.

As I think was noted, "audiophile" products are commonly over-built in this department. Their market is not price sensitive and is easy to simply beef up the various subsystems to have ample current and extra headroom. This reduces the cross-talk you may get between channels where pushing one causes the output of the other channels to potentially droop.

In AVRs, i am a huge fan of these higher efficiency amps. People often put these things in cabinets and such, with reduced air flow. The result is poor reliability. My pioneer elite runs cool in a totally enclosed cabinet. I have a fan but never turn it on. It actually produces less heat than my DSL modem! The prior Onkyo I had would heat up this large cabinet that the granite top would get too hot to touch above it. We are talking about an 8 foot by 3 foot slab of stone getting that hot! The fact that the pioneer maintains its output across multiple channels is just a bonus.
post #25 of 227
Thread Starter 
Quote:
Originally Posted by arnyk 
The first thing to remember that after you add a reasonable safety margin (say, 100%) to what you actually need, additional power capability serves no logical purpose. After studying the matter and also living with a real world audio system, it looks to me like 35-50 wpc will drive a typical system to standard reference level with a reasonable allowance for peaks, explosions, and etc.

Forgive my ignorance, but when you say safety margins, what do you mean? Are you suggesting eg that is a speaker A has a power handling of 100 watts, that I should double that to have extra headroom? Also how would you know if you have enough power, or if you have too little power? I read another comment from you way back where you said that you have not seen reliable evidence that people know how much power they really need. Which I find interesting because that is a good point. Then people are just upgrading their power because either other people suggest it, not because they necessarily need it, or because they read something in a magazine.

One guy I spoke to today said that you need an amp that has high current, not high wattage. So as you imagine, it confuses the hell out of me. biggrin.gif And it's always the audiophile that makes these suggestions. But that confuses me, because surely a high current amp depends on the situation and the circumstances? Maybe you don't need a high current amp with your speakers, in your room at your listening levels and at your seated distance listening to your own music/movies?
Edited by Heinrich S - 6/25/13 at 9:14am
post #26 of 227
Quote:
Originally Posted by Heinrich S View Post

Thank you Arnyk. But this :
Glimmie said it can happen and you say it's an audiophile myth. I am confused. confused.gif

Well it can happen in theory. That doesn't mean ANY underpowered amp can damage ANY speaker. And I will admit I have never seen or heard about it first hand so a lot of credible people like Arny don't believe it. And like I said, I don't see how anyone, including a teenager, could stand that level of distortion long enough for damage to occur.

So it's safe to say don't worry about it.
post #27 of 227
Quote:
Originally Posted by Glimmie View Post

Well it can happen in theory. That doesn't mean ANY underpowered amp can damage ANY speaker. And I will admit I have never seen or heard about it first hand so a lot of credible people like Arny don't believe it. And like I said, I don't see how anyone, including a teenager, could stand that level of distortion long enough for damage to occur.
When I used to repair stereos on the side, we routinely had blown speakers and fuses on Mondays after the weekend parties. Same equipment had worked fine for a long time but during the party someone had cranked up the volume leading to these failures. With all the noise in the party, and folks who just want to dance to a tune, increased distortion and clipping can easily be ignored.

I am not sure why Arny says it is a myth that clipping damages speakers. Here is the last discussion we had with him on this topic: http://www.avsforum.com/t/1413903/new-article-debunking-some-speaker-amp-myths. There, Arny cites this article: http://sound.westhost.com/articles/speaker-failure.html

"3.2 - Amplifier Clipping is the Culprit (??)
Power output ratings given by the makers of high power audio amplifiers are almost without exception quite genuine. Amplifiers are rated to produce continuous, sine wave power in real watts. Also, most power amplifiers can produce a lot more watts than the figures show if allowed to operate into clipping distortion. This is simply because a square wave shape has twice the power of a sine wave of the same amplitude.

The hazard with allowing an amplifier to clip is NOT that there is anything inherently evil about a clipped (or square) audio frequency wave but simply that the effective power output has increased - often dramatically!

With music programme, the average power delivered to the speaker can increase by up to 10 times when an amplifier is clipping compared with the non-clipping situation. The reason behind this is that clipping is the simplest form of audio compression and compression INCREASES the average level of a signal. It's not the fact that the amp is clipping that causes failures - it's the extra power that's delivered to the speaker."



The argument and perhaps the "myth" I thought was around why clipping damages speakers, not that it does at all. Clearly if clipping creates 10X the power, then that could damage a speaker that could not handle such power increase. Anyway, whatever you wanted to know about that topic, is in the other thread smile.gif.
post #28 of 227
post #29 of 227
Thread Starter 
amirm, I'm not technically adept enough to follow that thread. I have no idea if Arny or Jneutron was winning the debate or who knew more about the subject matter. What little I've read of Jneutron makes my head spin, so I definitely can't follow him. Way out of my depth.
post #30 of 227
Look at an amp/receiver and some speakers. Ask for yeas or nays.
Don't sweat the technical details, especially since some folks tend to get overly technical and will fog your brain.

It's good to try to get a grasp on the details. But beware, too much interaction here can hurt/confuse you. Also, starts yet another "contest" to see who can appear smarter.
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