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Topic: Amp power and current (Read 52853 times) previous topic - next topic
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Amp power and current

Reply #200 –
Sorry, about the voltage-limited and current-limited thing. Could you explain how this would work? If you could please give me an example of an amp being voltage limited and current limited, it would be most appreciated.

Amp power and current

Reply #201 –
Quote from: Rich B on
About the power handling, so if you approach the power handling limits of any speaker, distortion both linear and nonlinear tends to increase. What about mechanical non-linearities?


Where do you think the distortion comes from?

Amp power and current

Reply #202 –
Quote from: Rich B on
Sorry, about the voltage-limited and current-limited thing. Could you explain how this would work? If you could please give me an example of an amp being voltage limited and current limited, it would be most appreciated.


OK, let's take the Hypex nc400 class D module. The rated output power is specified as 200 W into 8 ohm, 400 W into 4 ohm and 580 W into 2 ohm. The fact that the power doubles when going from 8 to 4 ohm shows that the maximum output voltage is 40 V (square root of 200 * 8 or 400 * 4), with current being 5 A into 8 ohm and 10 A into 4 ohm. For 2 ohm, the voltage starts to drop (to 34 V) because of the high current required (17 A).

Amp power and current

Reply #203 –
Quote from: Rich B on
I'm just trying to understand what would be required in order for a given amp that has a given rated power to exploit that rated power. And you are saying that is load dependent?


If you want to get the most power out of an amp, listen far louder than is comfortable for you  using as few as possible low efficiency, low impedance speakers, no subwoofer, speakers  sitting in the middle of a large well-padded room while you sit as far away as possible.

Amp power and current

Reply #204 –
And to grossly simplify the power requirements, crest factor and other factors:

Let's say your 8 ohm speaker produces 87 dB SPL with 1W input.
You want to reach 105 dB SPL. 105-87 = +18 dB, which is a factor of 63.
So we need 63W into 8 ohms, that's 22.5 V and 2.8 A.

If your amp can deliver that you could produce a sustained SPL of 105 dB with a full-scale sine wave. Real music is not even close to full-scale sine waves, so both the average SPL and output power will be much lower.

If you amp can't output that voltage, it will clip. If it can't output this sustained current, it might still be able to output enough current for the short peaks in real music.
"I hear it when I see it."

Amp power and current

Reply #205 –
Quote from: julf on
Quote from: Rich B on
About the power handling, so if you approach the power handling limits of any speaker, distortion both linear and nonlinear tends to increase. What about mechanical non-linearities?


Where do you think the distortion comes from?


I thought there was a difference between distortion caused by thermal means vs mechanical means.

Amp power and current

Reply #206 –
Quote from: Rich B on
I thought there was a difference between distortion caused by thermal means vs mechanical means.


The effect of the loudspeaker (especially magnet) warming up is lower efficiency, causing a bit of compression. Actual distortion is more the result of mechanical factors.

Amp power and current

Reply #207 –
Quote from: Rich B on
I asked in plain English :

"So then it is not possible for a weaker amp to have a higher gain than a stronger amp, but with a stronger volume control


Sorry, but that's not "plain English" at all.  What the hell does "weaker amp" even mean?  What in heaven's name do you mean by a "stronger volume control"? 

You really can't expect good answers until you learn to ask good questions.  A question as ill formed as the one above doesn't have an answer because as far as I can tell it has no real meaning.  You might as well ask "Is green wind better than red wind?".
Ed Seedhouse
VA7SDH

Amp power and current

Reply #209 –
Would it not be correct to say that speakers are fed voltages, not just a voltage, when the volume is increased on the amp? Because music signals are constantly changing, doesn't that mean the voltage is constantly changing too? So there are lots of voltages taking place all the time. Correct?

Amp power and current

Reply #210 –
Quote from: Ed Seedhouse on
Quote from: Rich B on
I asked in plain English :

"So then it is not possible for a weaker amp to have a higher gain than a stronger amp, but with a stronger volume control


Sorry, but that's not "plain English" at all.  What the hell does "weaker amp" even mean?  What in heaven's name do you mean by a "stronger volume control"? 

You really can't expect good answers until you learn to ask good questions.  A question as ill formed as the one above doesn't have an answer because as far as I can tell it has no real meaning.  You might as well ask "Is green wind better than red wind?".


I did not articulate myself well. I said that I didn't know what to call it. I was talking about the volume control. Some volume controls spread the power over a certain range in a smooth way. Others will have the power increasing very quickly over a small range of the knob. So I wanted to know if it was possible that a less powerful amp could have a volume control that over a smaller rotation, allows more power to be delivered compared to a stronger amp, with a given volume control, over the same relative range of rotation.

Amp power and current

Reply #211 –
At any instant in time a speaker receives just one voltage at its terminals, but yes you're correct.

However, it can be argued that silence is important in music, which is what you'll get when voltage is held constant.

Amp power and current

Reply #212 –
Quote from: xnor on
And to grossly simplify the power requirements, crest factor and other factors:

Let's say your 8 ohm speaker produces 87 dB SPL with 1W input.
You want to reach 105 dB SPL. 105-87 = +18 dB, which is a factor of 63.
So we need 63W into 8 ohms, that's 22.5 V and 2.8 A.

If your amp can deliver that you could produce a sustained SPL of 105 dB with a full-scale sine wave. Real music is not even close to full-scale sine waves, so both the average SPL and output power will be much lower.

If you amp can't output that voltage, it will clip. If it can't output this sustained current, it might still be able to output enough current for the short peaks in real music.


Ok thanks for the explanation.

If say you had three amplifiers, and each amp was playing at the same SPL (90 dB for example), with the same speaker (so same impedance), then the power into the load would remain the same between all three amps, correct? Unless one of them clipped, which is probably unlikely at a 90 dB SPL unless you were listening in a football stadium. 

Amp power and current

Reply #213 –
Quote from: Rich B on
About the power handling, so if you approach the power handling limits of any speaker, distortion both linear and nonlinear tends to increase. What about mechanical non-linearities?



Speaker nonlinear distortion comes primarily from the diaphragm running out of  linear travel, a parameter called Xmax.  Xmax is most important at the low end of the speaker's response curve. It can afflict woofers, midranges and tweeters. This is sometimes called Mechanical Limiting. It is most important for woofers and subwoofers.  It can range from less than a mm for tweeters to over and inch  for subwoofer drivers.

A secondary source of nonlinear distortion is that the voice coils of the drivers heat up and their resistance increases. This can be pretty dramatic - several 100 degrees F which s can like double the resistance of the voice coil.  This is called Thermal limiting and in the most extreme cases, the speaker voice coil is permanently damaged. 

Most modern drivers can handle temperature increases  of a 100 degrees or more.

Some midranges and tweeters have a thermal/magnetic fluid in their voice coil gap that damps the driver and helps dissipate heat. It's not used in woofers because of the damping.

Amp power and current

Reply #214 –
Quote from: Rich B on
Would it not be correct to say that speakers are fed voltages, not just a voltage, when the volume is increased on the amp?


To start with, increasing the volume has nothing to do with it (as long as it isn't at "0") - the speakers get fed a voltage even if you don't keep changing to volume setting.

If you feed a speaker several voltages, the speaker will still only see one voltage - the sum of all the separate voltages you have fed it.

If you feed the speaker an actual music signal, as opposed to DC, the voltage will change continuously.

Quote
Because music signals are constantly changing, doesn't that mean the voltage is constantly changing too?


Yes.

Quote
So there are lots of voltages taking place all the time. Correct?


Yes. No. Maybe.

If you feed a speaker several voltages, the speaker will still only see one voltage - the sum of all the separate voltages you have fed it.

Again, don't take this the wrong way, but are you at all familiar with basic electrical theory and mathematics (high school level)? It is an honest question.

Amp power and current

Reply #215 –


At any point in time, the electrical signal has one value. The speaker tries to follow this signal.
"I hear it when I see it."

Amp power and current

Reply #216 –
Quote
Again, don't take this the wrong way, but are you at all familiar with basic electrical theory and mathematics (high school level)? It is an honest question.


It's been a while since I was in High School, so no, I've probably forgotten most of what I learned.

But I've learned a great deal regardless on this forum. 

Amp power and current

Reply #217 –
Quote from: julf on
If you feed a speaker several voltages, the speaker will still only see one voltage - the sum of all the separate voltages you have fed it.

Again, don't take this the wrong way, but are you at all familiar with basic electrical theory and mathematics (high school level)? It is an honest question.



Pedantic Point of Order.

If you connect multiple ideal voltage sources to a speaker the one with the greatest current capacity will rule the day. If they are truely ideal the current flows among the sources will be chaotic and, it will be a mess.

If you connect multiple ideal current sources to a speaker they will sum as described above,  but what sums will be strongly dependent on the impedance curve of the speaker.

Amp power and current

Reply #218 –
Quote from: Arnold B. Krueger on
If you connect multiple ideal voltage sources to a speaker the one with the greatest current capacity will rule the day. If they are truely ideal the current flows among the sources will be chaotic and, it will be a mess.

If you connect multiple ideal current sources to a speaker they will sum as described above,  but what sums will be strongly dependent on the impedance curve of the speaker.


True. I was talking about the summing-of-signals scenario that xnor illustrated.

Amp power and current

Reply #219 –
Quote from: Rich B on
Quote from: Ed Seedhouse on

You really can't expect good answers until you learn to ask good questions.


I did not articulate myself well. I said that I didn't know what to call it.


But why don't you?  There are plenty of well written books on the subject available for you to read, and some of them will probably be available for free in your local public library.

Any technical field such as sound reproduction will develop technical terms that everybody understands and agrees with.  If you want to learn something about that field then you really have to learn the terms and what they mean.  Until you've done at least some of that you aren't equipped to even ask meaningful questions.  For instance, do you understand the difference between electrical resistance and electrical impedance?  Such an understanding, which is quite basic, is necessary to understand how audio devices work.  If you don't even grasp this fairly basic point you aren't equipped to even ask good questions, nor to understand the replies when you get them.

Gaining this basic elementary understanding is up to you, not to the others in this forum.  It isn't hard, and it shouldn't take very long.  But this is a forum, not a school.


Ed Seedhouse
VA7SDH

Amp power and current

Reply #220 –
The speaker can't sense how much power? Speakers do tell you if it senses that back emf...

Amp power and current

Reply #221 –
Quote from: Yahzi on
The speaker can't sense how much power?


Speakers are passive devices, they can't sense anything. 

Amplifiers can measure how much power they're using though if thats what you mean (although not many do this).

Amp power and current

Reply #222 –
Quote from: Yahzi on
Speakers do tell you if it senses that back emf...

I don't understand this sentence.

Counter EMF is a voltage developed by the coil and has the opposite polarity of the input voltage. The counter EMF is at a maximum at the resonant frequency of the driver.
"I hear it when I see it."

Amp power and current

Reply #223 –
Thanks for the help guys.

Amp power and current

Reply #224 –
Quote from: Yahzi on
The speaker can't sense how much power? Speakers do tell you if it senses that back emf...


Let's clear up some confusion about back EMF.  Back EMF is a way to explain the fact that a speaker's impedance rises at its fundamental bass resonance.  This differs from the fact that a speaker's impedance also rises at high frequencies because its voice coil is a big inductor.

All the amp knows is that the speaker's impedance is high at that frequency and it isn't taking a lot of current to drive it.  "Fighting the back EMF" sounds colorful and it makes it sound like something dramatic and stressful is going on, but in fact that amp is pretty much taking a break.

The amp also sees the larger picture which is that phase angle of the load is changing rapidly around resonance and a few dozen Hz up, its impedance may be as low as it gets.