Amplifier Power Question

[Bob Leonard]

Are the watts in amplifier outputs the same regardless of where the volume is set. In other woods does a 100 watt amplifier output the full 100 watts regardless of the gain setting on your Pre-amp.

 

[Irvrobinson]

No. An amplifier's actual power output depends on the voltage of the input, which is determined by the position of the volume control. If you're listening quietly in a small room on even moderately efficient speakers any amplifier, regardless of rated output, may only be producing, on average, less than one watt per channel. Perhaps less than half a watt per channel. Of course, amplifier output also depends on other factors, like the impedance of the speakers at a given frequency, so this is all a generalization.

 

[Bob Leonard]

Then if I have an amplifier rated at 100 watts per channel, and an amplifier rated at 200 watts per channel. Would 1 watt output from ether amplifiers be the same. If the speakers where rated at the same sensitivity?

 

[highfigh]

Then if I have an amplifier rated at 100 watts per channel, and an amplifier rated at 200 watts per channel. Would 1 watt output from ether amplifiers be the same. If the speakers where rated at the same sensitivity?

A Watt is a unit of power or 'work', just like Horsepower, torque, Btu, etc and each is calculated using existing factors. In the case of Watts used as a measure of amplifier power, it's found using voltage, current and resistance so for a given resistance, the voltage from each amplifier would need to be equal for them to be considered 'the same'. The amplifiers would require different input sensitivity in order to produce one Watt with the same input voltage and the amplifier's gain is usually shown in the specs (if the manufacturer chooses to show it).

Generally, a Watt is a Watt, unless the amplifier's design makes its performance dependent on the speaker load and that's really not a great thing- if the speakers cause the amplifier to change its output signal, it's not a good candidate when speakers are changed frequently, it makes finding the 'right' speakers much more difficult and it may not be reliable.

 

[slipperybidness]

A Watt is a unit of power or 'work', just like Horsepower, torque, Btu, etc and each is calculated using existing factors. In the case of Watts used as a measure of amplifier power, it's found using voltage, current and resistance so for a given resistance, the voltage from each amplifier would need to be equal for them to be considered 'the same'. The amplifiers would require different input sensitivity in order to produce one Watt with the same input voltage and the amplifier's gain is usually shown in the specs (if the manufacturer chooses to show it).

Generally, a Watt is a Watt, unless the amplifier's design makes its performance dependent on the speaker load and that's really not a great thing- if the speakers cause the amplifier to change its output signal, it's not a good candidate when speakers are changed frequently, it makes finding the 'right' speakers much more difficult and it may not be reliable.

Generally a good response.

One item to nitpick/clarify--Watts measure power, and power is work per time unit.

Work is the amount of energy necessary to move an object from one point to another. On the other hand, power is the rate at which the energy is spent.

 

[Bob Leonard]

Then all watts are equal. I have read that when doubling the power “watts” there is a 3db increase in volume. If this is correct. And all watts being equal. Why would you ever need an amplifier over 100 watts per channel?

 

[slipperybidness]

Then all watts are equal. I have read that when doubling the power “watts” there is a 3db increase in volume. If this is correct. And all watts being equal. Why would you ever need an amplifier over 100 watts per channel?

You need HEADROOOM--you need the power on reserve to hit ~20dB+ peaks on the dynamic range! Double the power is defined as a 3dB increase, so you need quite a bit of power to hit +20dB--I'll leave it to you to look up the math on that.

Also, some speakers are less sensitive than others, so some need more power right out of the gate to hit reasonable listening volumes

In general, a TRUE 60W of CONTINUOUS POWER CAPABILITY will be fine for 90% of all speakers. 100W Continuous power will be fine for 95% of all speakers.

NOTE--I'm not talking about shady power ratings provided by the manufacturer, I'm talking about real, measured by a 3rd party continuous power ratings here.

 

[Bob Leonard]

The purpose of my questions is. I chose to keep in my system a 100 per channel 8 ohms 20hz-20khz doubling into 4 ohms Classe amplifier preferring the sound over a Emotiva 300 watt per channel 8 ohms 20hz-20Khz. Both amplifiers sounded equally dynamic. I have the crossover set for my subwoofer at 80 hz.

 

[ski2xblack]

http://www.cordellaudio.com/he2007/show_report.shtml

Pay close attention to the section on Peak Power Demands of Well Recorded Music.

 

[Bob Leonard]

I should guess then there are many other factors involved in how an amplifier performs other than the power “watts” output number. I would have to assume a well built and designed 100-watt amplifier can sound as good and dynamic as a more powerful 300-watt amplifier.

 

[Dave Blount]

I should guess then there are many other factors involved in how an amplifier performs other than the power “watts” output number. I would have to assume a well built and designed 100-watt amplifier can sound as good and dynamic as a more powerful 300-watt amplifier.

never assume ...............

and yes you're correct, there are many other factors contributing to an amplifiers sound

 

[ski2xblack]

If both amps have linear response, low output impedance, and low distortion, and you never actually use more than 100 watts, then yes, the less powerful amp should sound just as good.

(This is kind of the idealized generalization; the speaker/amp interaction is anything but simple, but most speakers are not difficult loads. Most audible differences between amps relate to being overdriven, unable to reproduce transient peaks cleanly. Some lesser amps, such as in AVRs, sport protection circuitry that results in rather ugly recovery from clipping conditions, too.)

And to Dave B, an amp shouldn't have any particular sound of it's own whatsoever, and fortunately very few do. The ones that do have a distinct sonic signature are invariably pretty far off the reservation of engineering orthodoxy (for example First Watt amps from Nelson Pass, and the whole tube realm).

 

[Dave Blount]

And to Dave B, an amp shouldn't have any particular sound of it's own whatsoever, and fortunately very few do. The ones that do have a distinct sonic signature are invariably pretty far off the reservation of engineering orthodoxy (for example First Watt amps from Nelson Pass, and the whole tube realm).

I'm well aware of that and also well aware that not all amps are created 'equal'. As for tubes, depending one 'tastes' in music (euphoric distortion if you will) they can do wonders to ones listening experience

 

[ski2xblack]

As for tubes, depending one 'tastes' in music (euphoric distortion if you will) they can do wonders to ones listening experience

Yeah, but so can preconceptions. Expectation bias goes a long way.

In fairly loosely controlled comparisons I've done, with amps I've both bought and built, I've found it takes something really "bad" to achieve a distinct sonic signature. I agree with you that the natural sounding compression of soft tube clipping does perform some interesting things that transistors just don't do. Musicians have been exploiting this since Muddy Waters discovered electricity. But this whole amp-as-processor digression is muddying the waters, and drifting away from the OP's specific questions.

 

[killdozzer]

I'm so happy Bob asked this question. I wanted to do so for days but kept myself from doing it as I became the boring '1000 questions' guy. I read heaps of threads going on about amps and I read all the articles I can understand here on AH. But didn't find this particular comparison. Here we talk about a good 'weaker' and a good 'stronger' amp.

What about this; you have two weak amps of 20Wpch into 8 Ohms and with negligible distortion. Can one be so much better that at medium SPL listening it never appears 'short of breath'? And what would this entice; some sort of storing ability to put out enough power to cover all the most power hungry part of the audio material?

I see many A-class, expensive and highly reviewed amps of 20 or 25W, can they do the trick if you're not going to tear down the walls?

Let's say speakers are of medium sensitivity - 90dB, they should play 102dB with 16W, could they be made in such a way to handle even a slightly prolonged power peak?

 

[Dave Blount]

and drifting away from the OP's specific questions.

no intention of doing that. So, are you related to the late Julian Hirsch ? ......... LOL !

on serious note how much snow do you have ? what elevation are you ?

 

[ski2xblack]

Killdozer, you have to account for losses due to distance, too, which in a typical room are typically ~4 db/m. That may force you to reel in the peak output numbers. If we're limiting this discussion to linear amps (we probably should, this is Audioholics after all), it's that peak output that you need to accommodate, as you want to avoid clipping altogether, at least ideally. And given your sensitivity and power in your hypothetical, I'm with Bob Cordell on this:

this [dynamic power requirements] data still suggests that many listeners may be clipping their amplifiers more often than they think. This may especially be the case for those with tube amplifiers who are not using extraordinarily efficient speakers.

Dave, my cabin is at ~9000', with probably 50-70" of snow, and the house in town ~5600 (out on the plains in the front range, sunny and 60 degrees right now). Snowpack is above normal for this time of year. Last good powder day was Tuesday. I'm still a bit sore from skiing deep, but heavy, high moisture content snow, all day long, as though I were still 19 years old.

 

[Dave Blount]

interesting, my sons place is outside of Pine Jct (8700') and he has less than a foot !

 

[highfigh]

Generally a good response.

One item to nitpick/clarify--Watts measure power, and power is work per time unit.

Work is the amount of energy necessary to move an object from one point to another. On the other hand, power is the rate at which the energy is spent.

Yeah, I know, but I didn't feel like converting Watts to foot-pounds although for anyone who cares, 746W=1Horsepower and 33000Btu.

Just some of the things I had to know when I was in school.

 

[highfigh]

You need HEADROOOM--you need the power on reserve to hit ~20dB+ peaks on the dynamic range! Double the power is defined as a 3dB increase, so you need quite a bit of power to hit +20dB--I'll leave it to you to look up the math on that.

Also, some speakers are less sensitive than others, so some need more power right out of the gate to hit reasonable listening volumes

In general, a TRUE 60W of CONTINUOUS POWER CAPABILITY will be fine for 90% of all speakers. 100W Continuous power will be fine for 95% of all speakers.

NOTE--I'm not talking about shady power ratings provided by the manufacturer, I'm talking about real, measured by a 3rd party continuous power ratings here.

20dB increase comes from a 100x increase in power.