Differences between high current amps and normal ones

[TLS Guy]

Coming from you (an obvious experts in this field) I am a little disappointed with the math presented. It is not that the math itself is wrong, but your example implies the loads (8, 4 ohms) are purely resistive. The fact is, an 8 ohm reactive load (say that happens at 1 kHz) will demand more current than an 8 ohm purely resistive load for the amp to deliver the same wattage. The VA will be the same but the Watts will be different, higher in the case of a reactive load. This is why I prefer the more complete formula that include the power factor.

I know I am splitting hair, your math is correct.

You are absolutely correct, but if we get into that we may engender confusion. However the principles are correct, because every speaker will have an impedance at a given frequency. So you can use those formulae to determine the power a given amp, will deliver to a specific speaker, at a given frequency.

Now it gets a little more complicated than that as current leads voltage into a capacitative load, and voltage leads current into an inductive load. Voltage and current are 90 degrees apart with either load, but the phase angles between the two above situations are different. In a capacitative load current is 90 degrees ahead of voltage, and in an inductive load 90 degrees behind voltage. However the members should get put in the right direction from my original post.

 

[PENG]

I know I was nit picking, just couldn't resist it....

 

[TLS Guy]

This subject has caught my interest as I have been told by some dealers that with certain speakers it's not the amount of wattage you give them that's important, but the amount of current.

My best experience has been with the B&W CM1. I heard them with both a Yamaha receiver and a Rotel RB-1070 high current amp. While at high volumes I would have a hard time telling them apart, it's when I listened to them at low to moderate levels that they really differentiated themselves. They were just 2 completely different speakers, which is what got me thinking that there actually might be something there in regards to high current amps. What do you guys think about this? Is there really something there between high current amps and normal (class D?) ones?

I should have explained the likely cause of the poor performance of the Yamaha compared to the Rotel at low to moderate volume. You will see from my previous posts in this thread it can have have nothing to do with peak current, but everything to do with quiescent current and efficiency. In other words the Yamaha is likely biased too far to class B, to reduce power consumption and heat production. Obviously this is tempting when everything is in one case. The result is more crossover distortion. The ear finds this type of distortion most unpleasant at low and moderate listening levels. So in the Yamaha the balance was too far to class B bias. The Rotel was likely in class A on the linear part of the power curve at the listening levels of your evaluation. The A/B balance can be very different between designs.

As I have stated before, I have never owned a receiver of any stripe. However I have made an educated guess that with five or seven amps plus switching and processing in one case, these issues are more likely to surface than not

 

[fmw]

Possibly what you encountered is a difference in volume levels. If you play two otherwise identical sonic presentations with a single db difference, people will prefer the louder one every time, without even sensing the difference in level. It is part of the way our brains interprest what we hear. At low levels it doesn't take much of a volume difference to put this effect into effect.

Another possibility is that you preferred the sound from the power amp simply because you expected to or somebody told you you would. That is another way in which our brains interpret what our ears hear.

It is also possible, but very unlikely, that one or both amplifiers produce a coloration to the sound and you preferred one coloration to another.

Either way, current delivery at low volumes would not have been the difference. Current delivery is the difference when you run out of it.

 

[PENG]

Good to see that now we are talking sense. As I said before, difference in SQ at low and moderate volume wouldn't be due only (if at all) to high current capability, especially in this case gus said it was difficult to tell them apart at loud volume. Thanks to fmw and TLS we now have other possible reasons for the perceived sound difference.

 

[PENG]

Another thing to think about are the bass peaks.

Have you ever seen those old styled power amps with analog meters? I had one, and it was quite educational to observe how much power was being used during music playback.

During most playback, the meter would just hover around 1 or 2 watts. But when there are bass heavy passages, the meter would swing all the way to the other end. I guess the larger power supply in an external amp deals better with these bass peaks than the smaller power supply in a mid range receiver.

Analog meters are great for indicating, but if you are talking about those on some old power amps, the indicators are mechanical devices. They do swing all the way quite a bit but that does not mean those base peaks are in fact drawing that much power. The mechanical pointing device may just be over travelling because of the momemtum. I have done experiments with digital ammeter that has peak holding capability and digital SPL meter. Heavy bass passages in most CDs don't cause power demand to jump from 1 to 2 watts to, like 50 watts. If you are listening to music and the power draw is 2W, it would take a peak of 15 dB to demad 64W.

 

[furrycute]

I like your explanation. I think that might be what is going on. The receiver's amp is biased more towards class B, while the external amplifier is biased more towards class A.

Maybe that is the reason why there is a perceived difference in sound quality. When I had both my external amp and integrated amp, the external amp did run hot than the integrated amp.

In other words the Yamaha is likely biased too far to class B, to reduce power consumption and heat production. Obviously this is tempting when everything is in one case. The result is more crossover distortion. The ear finds this type of distortion most unpleasant at low and moderate listening levels. So in the Yamaha the balance was too far to class B bias. The Rotel was likely in class A on the linear part of the power curve at the listening levels of your evaluation. The A/B balance can be very different between designs.

 

[furrycute]

Could be. Again my physics is kind of rusty.

Analog meters are great for indicating, but if you are talking about those on some old power amps, the indicators are mechanical devices. They do swing all the way quite a bit but that does not mean those base peaks are in fact drawing that much power. The mechanical pointing device may just be over travelling because of the momemtum. I have done experiments with digital ammeter that has peak holding capability and digital SPL meter. Heavy bass passages in most CDs don't cause power demand to jump from 1 to 2 watts to, like 50 watts. If you are listening to music and the power draw is 2W, it would take a peak of 15 dB to demad 64W.

 

[furrycute]

Well, class A does kind of sound more, full bodied, if that's the proper way to describe the sound.

Yep, people say a lot of things and most are incorrect audio is no exception when people speak

 

[fmw]

Well, class A does kind of sound more, full bodied, if that's the proper way to describe the sound.

That might have been true in the old days. Don't forget that the purpose of amplifier biasing is the reduction of distortion. Historically, the more efficiency you incorporated into the design through biasing, the more distortion you produced. So class A amps were the the lowest in distortion.

These days, however, D and even G and H amps have inaudible distortion. Personally, I don't think biasing or efficiency is an issue at all in modern amplifiers - at least in terms of audibility.

There shouldn't be anything audible in a modern amplifier. No coloration. You should get the input with more gain to the output and nothing else. This occurs in virtually all modern hi fi amplifiers.

 

[furrycute]

Thanks. I am kind of lost in all this physics talk.

So in your opinion, what kind of receivers would have a larger power supply that can adequately drive 4 ohm speakers?

And are there any "digital" amps that you recommend for driving 4 ohm speakers? Most of these so called "digital" amps are really expensive, which I really don't understand, since these class D amps are so inexpensive to make.

That might have been true in the old days. Don't forget that the purpose of amplifier biasing is the reduction of distortion. Historically, the more efficiency you incorporated into the design through biasing, the more distortion you produced. So class A amps were the the lowest in distortion.

These days, however, D and even G and H amps have inaudible distortion. Personally, I don't think biasing or efficiency is an issue at all in modern amplifiers - at least in terms of audibility.

There shouldn't be anything audible in a modern amplifier. No coloration. You should get the input with more gain to the output and nothing else. This occurs in virtually all modern hi fi amplifiers.

 

[fmw]

There aren't a lot of receivers with amps rated to drive 4ohm nominal loads. There are some. There are also some that aren't rated for 4 ohms but will drive them just fine. You want to look for the term 4 ohm rated or 4 ohm stable. My own receiver, a Pioneer 92, drives 4 ohm speakers without getting warm. I features these new fangled, efficient amps.

In general, the receivers that can handle 4 ohm loads are higher up in the pecking order.

 

[gus6464]

I think I am beginning to understand all the many differences in regards to amps. So when one looks at amp reviews and they state how an amp sounds better than the other, it's not really that the amp sounds different, it's more how the amp interacts with the speakers? So for example if you take very high efficient speakers (>95dB), no matter what amp type you use it will most likely sound exactly the same.

But then let's take for example a pair of Mark and Daniel Ruby with the following specs.

Nominal Impedance: 3-6 Ohms
Average Efficiency: 82.5dB/2.83V/1m
Power Handling: ≧ 80 Watts per channel

As you can see they dip down very low and are not sensitive at all. So when you power them with a non class A amp, you are more likely to hear distortion from the amp as it does not have enough current to properly drive the speaker when it dips below 4ohm?

 

[TLS Guy]

I think I am beginning to understand all the many differences in regards to amps. So when one looks at amp reviews and they state how an amp sounds better than the other, it's not really that the amp sounds different, it's more how the amp interacts with the speakers? So for example if you take very high efficient speakers (>95dB), no matter what amp type you use it will most likely sound exactly the same.

But then let's take for example a pair of Mark and Daniel Ruby with the following specs.

Nominal Impedance: 3-6 Ohms
Average Efficiency: 82.5dB/2.83V/1m
Power Handling: ≧ 80 Watts per channel

As you can see they dip down very low and are not sensitive at all. So when you power them with a non class A amp, you are more likely to hear distortion from the amp as it does not have enough current to properly drive the speaker when it dips below 4ohm?

Unfortunately you still have not quite got it. I know this is difficult and may be I'm a poor teacher.

Class per se, does not have anything to do with an amps ability to deliver current into low impedance loads. Class just is a way of classifying the operating principles of amps, and to some degree their circuit topology.

I touched on some of the advantages and disadvantages of the classes of audio amplifiers.

An amps ability to deliver large amounts of current is determined largely by two factors.

The ability of the power supply to supply enough current to the output devices.

The characteristics, specification and number of output devices.

Please look at my math again in my first post in this thread last night. The big point is how, as the impedance drops, the amp has to maintain voltage at the output. The math will show you how much more current has to provided to maintain output voltage as the current drops. It also shows what happens if the power the amp can not maintain voltage in the face of falling impedance. You will see how this limits the amps ability to provide the power called for.

For Peng's benefit I used the term impedance rather than resistance. But most people are familiar wit Ohm's law and I hope I have not added another confounding factor. Just substitute I for R in the Ohm's law equation.

Please get out your calculator and try some examples for yourself.

I do hope this helps. I have noted so much confusion and misunderstanding on these issues in these forums. It is one of those topics we revisit again and again.

This forum needs some sort of search trigger that, before new threads are started on these difficult topics, what has gone before comes up. I just hope we don't have to keep rehashing this and other topics so many times. But may be I'm impatient and frequent revision is part of the solution.

 

[fmw]

I think I am beginning to understand all the many differences in regards to amps. So when one looks at amp reviews and they state how an amp sounds better than the other, it's not really that the amp sounds different, it's more how the amp interacts with the speakers? So for example if you take very high efficient speakers (>95dB), no matter what amp type you use it will most likely sound exactly the same.

But then let's take for example a pair of Mark and Daniel Ruby with the following specs.

Nominal Impedance: 3-6 Ohms
Average Efficiency: 82.5dB/2.83V/1m
Power Handling: ≧ 80 Watts per channel

As you can see they dip down very low and are not sensitive at all. So when you power them with a non class A amp, you are more likely to hear distortion from the amp as it does not have enough current to properly drive the speaker when it dips below 4ohm?

For the most part, when magazine reviewers talk about how an amplifier sounds, they are expressing a subjective opinion which, unfortunately, isn't very meaningful. They need to do it in order to keep readers. Their readers don't want to hear that amps sound the same. As I've said, amplifiers shouldn't have a sound and most of them don't. They should take the signal and make it louder without changing anything else about it. For the most part they do.

Distortion would result from overdriving the amp. That means pushing the volume level to a point where distortion would occur. There are so many variables in that that it would be speculative to suggest any numbers.

The speakers above are fairly insensitive. That means they would require more power to drive than speakers that are more sensitive. How much power? It would require double the amplifier power for each 3 db. If the amp is delivering, say 2 watts per channel for a given job, then it would have to deliver about 8 watts to drive speakers of around 89db to the same volume level. How much total power and overhead? Who knows? Too many variables.

The low impedance challenges the amp's ability to delivery current because lower impedances draw more current from the amp than higher impedances. The amp's ability to handle low impedance loads depends primarily on the ability of its power supply to deliver current and secondarily on the amp's ability to dissipate heat.

 

[gus6464]

Ok so I tried to do calculation on how much power you would need to drive those speakers at 2 meters and 90dB but got some crazy number. Using the highest peak the speaker reaches which is 3ohm.

How do you integrate double the distance and 8 more dB into the equation?

 

[TLS Guy]

Ok so I tried to do calculation on how much power you would need to drive those speakers at 2 meters and 90dB but got some crazy number. Using the highest peak the speaker reaches which is 3ohm.

How do you integrate double the distance and 8 more dB into the equation?

Lets make it easy and go to 88.5db and 91.5 at two meters.

To provide 82.5 db with 2.83 volts at 1 meter with with a three ohm load is 2.83 squared, which is 8.009. Divide that by the impedance 3 and we have 2.67 watts.

Now the sound intensity falls off by the square of the distance, so to produce 82.5 db at 2 meters requires 10.68 watts. Now to get to 85.5 db we need 21.36 watts. To get to 88.5 watts requires 42.714 watts and to get to 91.5 db requires 85.5 watts.

 

[gus6464]

Lets make it easy and go to 88.5db and 91.5 at two meters.

To provide 82.5 db with 2.83 volts at 1 meter with with a three ohm load is 2.83 squared, which is 8.009. Divide that by the impedance 3 and we have 2.67 watts.

Now the sound intensity falls off by the square of the distance, so to produce 82.5 db at 2 meters requires 10.68 watts. Now to get to 85.5 db we need 21.36 watts. To get to 88.5 watts requires 42.714 watts and to get to 91.5 db requires 85.5 watts.

Ahh ok I get it now, hmm that wasn't so bad So then I would have to use an amplifier that is stable at <4ohms and in the power range of 100-150w to allow for some headroom. And the problem is not finding an amp that can deliver the wattage, but an amp that can stay stable at <4ohms for the peaks. And I am guessing that is where the class would come into play? So a Class A for example would be more stable than an A/B biased toward the B side. Right?

 

[TLS Guy]

Ahh ok I get it now, hmm that wasn't so bad So then I would have to use an amplifier that is stable at <4ohms and in the power range of 100-150w to allow for some headroom. And the problem is not finding an amp that can deliver the wattage, but an amp that can stay stable at <4ohms for the peaks. And I am guessing that is where the class would come into play? So a Class A for example would be more stable than an A/B biased toward the B side. Right?

Class has nothing to do with this. It is the power supply and output stage design. You can have amps in any of those classes that will fail to drive those speakers properly. Conversely you can have amps in all of those that will. I can tell you one thing, if it is class A it will be huge heavy and expensive. You will have to increase your air conditioning and need a mortgage for your electric bill. You will likely need a savings account to replace the output devices.

In simple terms a I doubt there is a receiver out there that will do a good job of driving those speakers. That will require good separates no matter what the class of the amp.

 

[gus6464]

Class has nothing to do with this. It is the power supply and output stage design. You can have amps in any of those classes that will fail to drive those speakers properly. Conversely you can have amps in all of those that will. I can tell you one thing, if it is class A it will be huge heavy and expensive. You will have to increase your air conditioning and need a mortgage for your electric bill. You will likely need a savings account to replace the output devices.

In simple terms a I doubt there is a receiver out there that will do a good job of driving those speakers. That will require good separates no matter what the class of the amp.

Yeah I am talking about separates. I know there isn't a receiver out there that can handle them. Strictly 2ch listening.