Skip to main content ↑ ↓

Notice

Please note that most of the software linked on this forum is likely to be safe to use. If you are unsure, feel free to ask in the relevant topics, or send a private message to an administrator or moderator. To help curb the problems of false positives, or in the event that you do find actual malware, you can contribute through the article linked here.
Topic: Amp power and current (Read 52281 times) previous topic - next topic
0 Members and 2 Guests are viewing this topic.

Amp power and current

Reply #25 –
Quote from: Rich B on
So then that means that distortion is lower at close to full rated power too? Or is efficiency a different thing?


Efficiency and distortion are entirely unrelated.

Amp power and current

Reply #27 –
Quote from: Rich B on
Quote from: DVDdoug on
Quote
hen what I don't get is that if the current requirements are determined by the load, not the amplifier, that a 'low' current amp could be supplying enough current at a given voltage, then wouldn't it be just as high current as any other high current amp?
I think you do get it.  There's a reason you almost never see an output voltage or current spec for a power amplifier...  The important thing is how much power it can drive into a given load.


Okay, so this is a rather extreme example to test my assumption with.



McIntosh 1.2KW monoblocks. That's 1200W into 8 ohms and probably 2KW into 4 ohms, thereabouts.

So if I take this amp, compare it to an AVR, the big 1.2KW monoblocks will not be supplying any more current to a speaker than the AVR can, at any volume, until the point where the AVR runs out of current and becomes voltage-limited?


Pretty much so, except that when the AVR runs out of current it becomes current limited, and when it runs out of voltage it becomes voltage limited. I presume that was a typo in your post.

Quote
That's quite amazing, actually,


Not amazing at all. It is a result of a common sense application of some pretty basic laws of physics, and it is also what one observes on the test bench or in the lab  or  listening room.

For example if I hook a cheap AVR (like this Yamaha RXV 371) up to some speakers that go down below 4 ohms (like these Infinty Primus P363) and crank them up to a peak voltage of say 20 volts which is clearly within the linear capabilities of the RXV 371 (and the speakers), then I will observe a highly undistorted waveform across the speakers. If I turn around and hook up some high wattage pro-audio amps that can deliver far more current and voltage to the same speakers, but only turn things up to the point where the peak voltage across the speakers  is again 20 volts, I will observe the same highly undistorted waveform across the speakers. 

The amps will sound the same and will also sound pretty good, all other things being the same and suitable.

Quote
If that's true. Many people probably think that a big amp like the above can supply high current by default. I used to be one of those people.


In the above example, the AVR will start clipping when the voltage across the speakers is more like 33 volts.  This is really pretty loud, probably > 100 dB SPL with those particular speakers.

The high power, high voltage high current amp will not clip at 33 volts, and may be able to deliver > 100 volts to the speakers without the amp clipping. Of course, the speakers will shortly be damaged quite severely.

Amp power and current

Reply #28 –
Quote from: Rich B on
I think, and I stand corrected on this one, the bigger the caps on the power stage, the better it can handle the demands? High current amps often draw lots of current and having bigger caps can help a bit?


The purpose of the large capacitors in the power supply is to fill in the output of the power supply, at those times when the power line voltage is less than the desired power supply output voltage.

The most common situation (in the US) is a traditional linear power supply with a full wave rectifier powered by a 60 Hz power line.



As you can see, the full wave rectifier creates a wave that varies between 0 volts and some far higher voltage 120 times a second.  The higher voltage is usually several volts higher than that required of the power supply by full power operation of the amplifier. So if the power amp circuit needs 60 volts to deliver an undistorted signal, the power supply may be designed to top out at more like 70 volts.

The critical problem is that the wave from the full wave rectifier drops down to 0 volts 120 times a second. The purpose of the capacitor is to fill in for the rectifier as is shown by the red line below:



Once the filter caps are large enough to perform this function, further increases in their size have no technical benefits. Usually something between 2,000 uF and 10,0000 uF suffices but that varies quite a bit with the power capabilities of the power amp.

Amp power and current

Reply #29 –
"Highly undistorted" sounds weird.
"I hear it when I see it."

Amp power and current

Reply #31 –
If you plug your table lamp into a 15 amp circuit, or plug it into a 20 amp circuit, it will be exactly the same brightness in either case. Only when you try to plug 20 100W lamps into the same circuit will it make a difference.

It's the same with the AVR vs. the 1200W amp. You won't hear a difference until you turn it up REALLY LOUD!

Amp power and current

Reply #32 –
Quote from: pdq on
If you plug your table lamp into a 15 amp circuit, or plug it into a 20 amp circuit, it will be exactly the same brightness in either case. Only when you try to plug 20 100W lamps into the same circuit will it make a difference.

It's the same with the AVR vs. the 1200W amp. You won't hear a difference until you turn it up REALLY LOUD!



Great example. I find it interesting that most everybody knows that light bulbs work as you state, but many obviously believe the audiophile myths about how high current or high powered amplifiers with vast untapped reserves improve sound quality.

I should add that back in the days of tubes, particularly the early-mid 1950s,  amp ratings were often flaky and distortion could rise well below rated power.  Amps also lost a lot of power capabilities as the tubed aged.

A Dyna 70 with a year on the tubes was more like a Dyna 50, while a Mac 275 could still be a Mac 275 4-5 years later, based on the measurements I saw at Mac clinics of the day and my own measurements.

Amp power and current

Reply #33 –
Quote from: julf on
Quote from: Rich B on
So then that means that distortion is lower at close to full rated power too? Or is efficiency a different thing?


Efficiency and distortion are entirely unrelated.


So the amp operating more efficiently will perhaps result in a longer lifespan? But like you say, distortion has no relation to efficiency or lack thereof.

When audiophiles say that a big amp driving a speaker at moderate levels, vs a smaller amp running closer towards its limits, to get to the same SPL, will sound better because the larger amp isn't being taxed as much as the smaller amp.

Isn't that a valid argument? Suppose I run a 200W amp, and I require 80W from the power amp. And I use a 100W amp, and require 80W from it. Wouldn't the larger amp offer lower distortion if asked to handle the same power?

Amp power and current

Reply #34 –
To return to my analogy, if a 120 volt circuit is only rated at 1 amp (120W) won't the 100W light bulb look exactly the same?

Amp power and current

Reply #35 –
Quote from: pdq on
To return to my analogy, if a 120 volt circuit is only rated at 1 amp (120W) won't the 100W light bulb look exactly the same?



Generally no, but it also depends on what you mean by "120 volt circuit is only rated at 1 amp (120W)"

If the rating is limited by the wiring gauge then the wire will probably measurably, but not visibly limit the power reaching the bulb.

If you actually run a 120 watt incandescent bulb so it receives only 100 watts the filament will be running a bit cold and the color temperature as well as the brightness will IME probably be visibly wrong.

The brigghtness loss is usually more than you'd get with a 100w bulb running with 100 watts, and of course the color temp of the 100 w bulb will be closer to spec.

If the bulb is a CFL or LED, IME there will be likely no visible results.  Most of them seem to have built in solid state regulation, and cutting back 20% will likely be pretty moot.

I've seen LEDs running off 25 VAC in a brown out, and they were amazingly bright and the color temp seemed about perfect.

Amp power and current

Reply #36 –
Lower efficiency basically just means that you have to put in more power to get the same output, which usually means larger heatsinks because of the extra heat. Lifespan would only suffer if you cannot conduct the heat away properly.

The way you stated it, the argument is not valid. A 100W amp could output 80W cleaner than a 200W amp, but as you approach the 100W THD will rise. At that point it is unlikely that the 200W amp would have higher THD.
The argument is valid if you need, let's say, 10W on average but 200W peaks.
"I hear it when I see it."

Amp power and current

Reply #38 –
Quote from: Rich B on
When audiophiles say that a big amp driving a speaker at moderate levels, vs a smaller amp running closer towards its limits, to get to the same SPL, will sound better because the larger amp isn't being taxed as much as the smaller amp.

Isn't that a valid argument? Suppose I run a 200W amp, and I require 80W from the power amp. And I use a 100W amp, and require 80W from it. Wouldn't the larger amp offer lower distortion if asked to handle the same power?


If you really only require 80 W, then neither amp is clipping, and the amp with the lower distortion in general will be the one with lower distortion. Amps don't "work hard" or get "taxed", they clip at a certain, amp-dependent level, and they can or can not sustain the power level needed.

As xnor pointed out, even if you think you only need 80 W, you might actuallly have peaks in the music that require more, so it all depends on how you define your "80 W".



Amp power and current

Reply #39 –
Quote from: Rich B on
Quote from: stephan_g on
How much the volume changes at various points in turning a knob usually isn't an indicator of much more than the linearity of the potentiometer the knob is actuating.

And/or amplifier gain, of course.

So then correct me if I'm wrong, but the position of the volume knob does not reflect the percentage of power supplied? So different amplifiers have different gain structures? So if the volume pots on both amplifiers are probably completely different with different gain, then that would mean that at the same relative position one amp may sound stronger even if it has less rated power?

So if one amp is rated at 60W, and another at 100W, the 60W amp may appear to be more powerful because it has more gain, but the 100W amp may have less gain, so the increase in power as the volume control is increased is more gradual? I'm just thinking out a loud here, please tell me if I'm right. I'm trying to understand how a lower spec amp might sound more dynamic than a higher powered model if the gain settings were a little tweaked.


But for a given input level/signal and speaker load the position of the gain control at any point in time would determine the maximum output? And as the input signal varies say in a song the maximum output at the the selected gain position would also therefore change?

Amp power and current

Reply #40 –
Quote
But for a given input level/signal and speaker load the position of the gain control at any point in time would determine the maximum output? And as the input signal varies say in a song the maximum output at the the selected gain position would also therefore change?
What???

Of course, the volume knob and the musical signal can change the output.   

Amp power and current

Reply #41 –
Let's break this down into a simple diagram:

Input -> Volume Control -> Voltage Gain -> Output Stage -> Output

A 2V signal is applied at the input, gets attenuated by the volume control to let's say 0.2V (1/10th or -20 dB), then the gain stage amplifies this voltage by 15x (about 24dB) to 3V. Finally the output stage provides the current to drive 4 ohm speakers.
The resulting power into 4 ohms is 3*3/4 = 2.25W.

You can adjust the input level at your source (volume control of your CD player, for example), and the volume control at the amplifier.

2V would be a standard level for some CD players when playing a full-scale sine tone. The average RMS voltage of real music would be lower ... roughly 0.8V to 0.2V.
"I hear it when I see it."

Amp power and current

Reply #42 –
Quote from: xnor on
Let's break this down into a simple diagram:

Input -> Volume Control -> Voltage Gain -> Output Stage -> Output

A 2V signal is applied at the input, gets attenuated by the volume control to let's say 0.2V (1/10th or -20 dB), then the gain stage amplifies this voltage by 15x (about 24dB) to 3V. Finally the output stage provides the current to drive 4 ohm speakers.
The resulting power into 4 ohms is 3*3/4 = 2.25W.

You can adjust the input level at your source (volume control of your CD player, for example), and the volume control at the amplifier.

2V would be a standard level for some CD players when playing a full-scale sine tone. The average RMS voltage of real music would be lower ... roughly 0.8V to 0.2V.


Okay, but what is the above telling me? How would I reach full power from the amp? Half the volume? All the way?

Amp power and current

Reply #43 –
Quote from: Rich B on
Quote from: xnor on
Let's break this down into a simple diagram:

Input -> Volume Control -> Voltage Gain -> Output Stage -> Output

A 2V signal is applied at the input, gets attenuated by the volume control to let's say 0.2V (1/10th or -20 dB), then the gain stage amplifies this voltage by 15x (about 24dB) to 3V. Finally the output stage provides the current to drive 4 ohm speakers.
The resulting power into 4 ohms is 3*3/4 = 2.25W.

You can adjust the input level at your source (volume control of your CD player, for example), and the volume control at the amplifier.

2V would be a standard level for some CD players when playing a full-scale sine tone. The average RMS voltage of real music would be lower ... roughly 0.8V to 0.2V.


Okay, but what is the above telling me?


This is a bad way to ask questions because its impossible for someone to give you a useful answer.  Instead, you should explain what you think you know and then ask questions about what you don't understand.

Amp power and current

Reply #44 –
Quote from: saratoga on
Quote from: Rich B on
Quote from: xnor on
Let's break this down into a simple diagram:

Input -> Volume Control -> Voltage Gain -> Output Stage -> Output

A 2V signal is applied at the input, gets attenuated by the volume control to let's say 0.2V (1/10th or -20 dB), then the gain stage amplifies this voltage by 15x (about 24dB) to 3V. Finally the output stage provides the current to drive 4 ohm speakers.
The resulting power into 4 ohms is 3*3/4 = 2.25W.

You can adjust the input level at your source (volume control of your CD player, for example), and the volume control at the amplifier.

2V would be a standard level for some CD players when playing a full-scale sine tone. The average RMS voltage of real music would be lower ... roughly 0.8V to 0.2V.


Okay, but what is the above telling me?


This is a bad way to ask questions because its impossible for someone to give you a useful answer.  Instead, you should explain what you think you know and then ask questions about what you don't understand.


What I would like to know is how do I know when the amp is supplying maximum power into the load? At what point on the volume control? Can the maximum power be supplied at half the volume control? At 1/4?

Amp power and current

Reply #45 –
It is telling you that if you turn up the volume control (no attenuation, -0 dB) the 2V at the input will be amplified by 15x.
2V*15 = 30V
P = 30*30/4 = 225W

The amplifier may only be rated for 100W into 4 ohms though. With the same 2V source it would clip at a volume control position of -3.5 dB, because:
20*20/4 = 100W
So we know 20V clip the amp, but above we had 30.
20*log10(20V/30V) = -3.5 dB

On a volume control with an analog audio potentiometer, this would be roughly 3 o'clock. Above that: clipping.


If you look at the specs of some amplifiers you can often find a spec called "Input Sensitivity". In the example above it would be roughly 1.3V, but it can be as low as 0.15V in other amps!
"I hear it when I see it."

Amp power and current

Reply #46 –
Quote from: Rich B on
What I would like to know is how do I know when the amp is supplying maximum power into the load? At what point on the volume control?


Power output increases with volume, at least until you saturate the amplifier, in which case it may or may not keep increasing (or even short out and break!). 

Amp power and current

Reply #47 –
Quote
Okay, but what is the above telling me? How would I reach full power from the amp? Half the volume? All the way?
Unless your amp has a peak-indicating power meter, or you have some test equipment, you'll probably never know (unless you over-drive your amp into audible distortion). 

That McIntosh amplifier apparently has a peak-reading meter
Quote
WATTMETER

Swift, accurate response reveals program peaks so staying within the power limits of your loudspeakers is easy.


An Oscilloscope would do it.    A multimeter usually won't work because they are averaging (or RMS) devices and they won't give you the peaks.



P.S.
A oscilloscope measures voltage, so you'd have to calculate the wattage from the impedance.    You can also see clipping on an oscilloscope (if the waveform is distorted badly enough to see it visually).  Wikipedia has a nice picture of clipping on a 'scope. 

That's a clipped continuous sine wave...  With music, you need a digital storage 'scope to capture a "clean" image of the waveform similar to that...  Music (and regular audio) in real-time pretty-much looks like a jumbled-mess on an oscilloscope, but you can usually pick-out the peaks to get a reasonable peak-voltage measurement.

Amp power and current

Reply #48 –
Quote from: DVDdoug on
Quote
Okay, but what is the above telling me? How would I reach full power from the amp? Half the volume? All the way?
Unless your amp has a peak-indicating power meter, or you have some test equipment, you'll probably never know (unless you over-drive your amp into audible distortion). 

That McIntosh amplifier apparently has a peak-reading meter
Quote
WATTMETER

Swift, accurate response reveals program peaks so staying within the power limits of your loudspeakers is easy.


An Oscilloscope would do it.    A multimeter usually won't work because they are averaging (or RMS) devices and they won't give you the peaks.



P.S.
A oscilloscope measures voltage, so you'd have to calculate the wattage from the impedance.    You can also see clipping on an oscilloscope (if the waveform is distorted badly enough to see it visually).  Wikipedia has a nice picture of clipping on a 'scope. 

That's a clipped continuous sine wave...  With music, you need a digital storage 'scope to capture a "clean" image of the waveform similar to that...  Music (and regular audio) in real-time pretty-much looks like a jumbled-mess on an oscilloscope, but you can usually pick-out the peaks to get a reasonable peak-voltage measurement.


Doug, so let me ask you a question. If the gain structure of two amps are different then it explains why one can't casually compare amplifiers and get valid results.

But here are some questions for you, and please let me know if this is possible. You can have a less powerful amplifier that has a higher gain over a given range of the volume pot, giving one the perception of improved dynamics, "faster" sound, more "bass slam" etc.

A stronger amp may have lower gain over the same relative position on the volume pot, so at the same relative level, the sound may appear to be "laid back", or "sterile", or "flat sounding" etc, until the volume is pushed sufficiently.

I just want to know if it is possible for a lower powered amp to have a higher gain, so that more power is spread when the volume control is increased vs a more powerful amp that has a lower gain, but a more gradual one - ie not as jumpy.

Amp power and current

Reply #49 –
No, both attenuation by the volume control and amplification (gain) by the amp are linear effects. Neither discriminate between, for example, 0.01V and 10V.
Only when you hit the limits of the amplifier, the peaks of the waveform will start getting compressed/clipped.

Different amps have different volume controls, so same positions can give you different attenuation.
Different amps also have different gains. So even if everything else is equal, you will get different output levels.

The real trouble is matching by ear. At first it might seem that you matched two amps properly, but after some listening one amp will sound <insert audiophile terms here>. That's usually because the matching is off by a dB.
"I hear it when I see it."