science behind amps?
Aug 5, 2005 at 9:36 AM Thread Starter Post #1 of 12

onikiller

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I've long been curious; reading through the forums, it is pretty much the concensus that if you have good headphones, you should not be listening to some low mW output source.

What I would like to know is:
-how does increasing the mW benefit the sound (not: it makes the sound better...) I mean a real explaination for why its better
-how do mW and volume relate, if i have a 5mW per channel output on one device vs a 50 mW per channel output on another device, will the maximum volume increase? Tied into the first bullet, is the volume the only thing to increase.
-when i set my 5 mW device to a certain perceived volume, and i match that perceived volume with my 50 mW device, is there going to be a difference in the sound?

I suppose the three bullets are different ways of asking the same question, but it should give you an idea of what i'm trying to get at. I know that if you us a good amp, the components are better than say a portable audio device, so you don't have to explain that.

A second related question is that some people who use an external amp for their portable audio devices admit to using the stock headphone jack. It seems to me that the weaknesses of the built in amp on the portable device would carry through to the external amp and then on to the headphones, in which case is the only benefit in allowing those high impedance headphones to use the portable device as a source.

Thanks for you help
 
Aug 6, 2005 at 12:06 AM Post #2 of 12
I will answer none of your questions, not because I am
evil_smiley.gif
but because the main issue is not in mW.

The benefit of a good amp is its ability to provide power on demand.
That is why good amps have better separation and detail, and better bass than the headphone jacks.

As far as amplifying the headphone jack.. this reduces the load on the headphone out significantly and puts the load onto the amp. True, you are limited by the quality of the output from the player but it is no longer under the burden of driving power hungry electromagnetic transducers. All it has to do is drive the input stage of the amp.
 
Aug 6, 2005 at 1:26 AM Post #3 of 12
Quote:

Originally Posted by recephasan
I will answer none of your questions, not because I am
evil_smiley.gif
but because the main issue is not in mW.

The benefit of a good amp is its ability to provide power on demand.



watt [for James Watt], abbr. W, unit of power, or work done per unit time, equal to 1 joule per second. It is used as a measure of electrical and mechanical power.

maybe, then, you could be a little more clear as to what you mean by "power" here?
 
Aug 6, 2005 at 3:27 AM Post #4 of 12
try cruising headwize/library of articles and technical papers for more info

http://headwize.com/articles/hearing_art.htm

can show you the difference between average loudness wihich most amplifiers can easily overdo vs peak headroom required to reproduce dynamic peaks of uncompressed music in a lifelike manner - for which you need lots more mW in reserve
 
Aug 6, 2005 at 5:17 AM Post #5 of 12
Quote:

Originally Posted by onikiller
(...) -how does increasing the mW benefit the sound (not: it makes the sound better...) I mean a real explaination for why its better


Well, for one thing, the more powerful device in general should produce less distortion. Distribution of harmonic distortion over output power and frequency will look different anyway - even if you compare identically powered amps with equal power, but different circuitry and components.

Quote:

Originally Posted by onikiller
-how do mW and volume relate, if i have a 5mW per channel output on one device vs a 50 mW per channel output on another device, will the maximum volume increase? Tied into the first bullet, is the volume the only thing to increase.


A couple of other conditions (like identical input sensitivity and gain) provided, yes, the maximum volume will increase - and together with it, snr will increase, too.

Quote:

Originally Posted by onikiller
-when i set my 5 mW device to a certain perceived volume, and i match that perceived volume with my 50 mW device, is there going to be a difference in the sound?


Yes, there will be a difference - see reasons above!
smily_headphones1.gif


Quote:

Originally Posted by onikiller
A second related question is that some people who use an external amp for their portable audio devices admit to using the stock headphone jack. It seems to me that the weaknesses of the built in amp on the portable device would carry through to the external amp and then on to the headphones, in which case is the only benefit in allowing those high impedance headphones to use the portable device as a source.


Well, of course, an amp won't cure every weakness, but as you can conclude from the reasons above, with an amp being a less demanding load than a headphone for the portable, your chances to reduce distortion overall with an external amp are fairly good - even if only connected to the headphone out.

Quote:

Originally Posted by onikiller
Thanks for you help


You're welcome.

Greetings from Hannover!

Manfred / lini
 
Aug 6, 2005 at 5:03 PM Post #6 of 12
-how does increasing the mW benefit the sound (not: it makes the sound better...) I mean a real explaination for why its better

Higher power capability from the headphone output of an audio player or the headphone or speaker outputs of an amplifier enable you to listen at louder acoustic levels before the onset of clipping and audible distortion.

-how do mW and volume relate, if i have a 5mW per channel output on one device vs a 50 mW per channel output on another device, will the maximum volume increase? Tied into the first bullet, is the volume the only thing to increase.

Differences in power, measured in watts or milliwatts, only relates to absolute acoustic decibel levels (dbA) if you are using the same speaker or headphone for comparison. The sensitivity of the headphones will determine the acoustic output for a given power level. A 50mw amplifier is capable of delivering 10X the power (before the onset of clipping and excessive distortion) as the 5mw output from the audio player's headphone jack. This will equate to a 10X increase in the acoustic output (dbA), however, due to the way the human ear and brain interpret sound levels this will be perceived as being 2X as loud not 10X as loud. Depending on the sensitivity of the headphones being used, 50mw may provide deafeningly loud listening levels or only moderately loud levels.

-when i set my 5 mW device to a certain perceived volume, and i match that perceived volume with my 50 mW device, is there going to be a difference in the sound?

If all you did was listen to constant amplitude sine waves, there would not be any difference as long as the power demand did not exceed the 5mW output capability of the lower powered device. But music consists of signals of constantly changing frequencies and amplitude. The change in amplitude (audio level) will determine the instantaneous power demand placed on an amplifier. Assuming a dynamic range of 10db between the average signal level and the peak level, the power demand will vary by 10:1. So it is very likely that while the average listening level may be only 2 or 3 mw of power, peak levels will require 20 or 30mw of power. Raise the average listening level by just 3db (perceived as being audibly louder by most people, but not much louder) and the average level will be perhaps 5mw while the peak power demand will increase to 50mw or greater. Extra headroom in an amplifier is a good thing. It ensures that you will not drive the amplifier into clipping at any normal listening level likely to be encountered.

Having 100mw more of clean undistorted power available means that you will never drive the amplifier to the point where distortion will be audible under normal listening. Depending on the sensitivity of the headphones being used, anything over a few watts of power is probably overkill. The AKG K1000 would be one notable exception. Due to the low sensitivity of these phones, having as much as 8~10 watts of power available at the rated impedance of these phones would not be too much power.
 
Aug 6, 2005 at 5:23 PM Post #7 of 12
There is a lot consider when thinking about how an amplifier sounds, or what it does to the sound.

Detailed measurements reveal this. A simple, single value of distortion @ a power rating is a small part of the story.

It also depends on what the distortion products are, and where in the frequency spectrum they occur, etc.

For example, stereophile do detailed measurements like so:

http://stereophile.com/amplification...ev/index4.html

Every amplifier will have it's own signature...
 
Aug 6, 2005 at 9:33 PM Post #8 of 12
Quote:

Originally Posted by ossicle
watt [for James Watt], abbr. W, unit of power, or work done per unit time, equal to 1 joule per second. It is used as a measure of electrical and mechanical power.


Smart a** comments like the one above notwithstanding, I will tell you what I mean.

"The benefit of a good amp is its ability to provide power on demand." means it can deliver the required power when it goes from 0 to X mW in Y milliseconds.

The equivalent of slew rate in opamps, under load.

If there is a fast drumkick, a weak output feeding power hungry headphones will not give the same feeling of speed and kick as an amp that can increase its power output when the input is likewise increased.
Same goes for decay, which is the decrease in output, as in sounds dying out.

Also, power increase in a certain frequency decreases power output in others in poor amplifiers. Good amps do not have a problem with detail in frequency-rich passages.

Forget the specs, thd etc. They tell you nothing whatsoever. The specs of a $10k CD player is not that much different from a $700 one, but people buy the expensive one because it sounds better.
What cannot be expressed with a single number, and thus is not mentioned in the specs is harmonics, and they make a big difference in the way an amp sounds.

So listening before buying is the best option.
 
Aug 7, 2005 at 12:07 AM Post #9 of 12
wow everyone, thanks for the help!

so if i may clarify, if the device i'm using can easily drive my headphones without cranking it up, then the benefit of buying an amp would be mostly in better components?
 
Aug 7, 2005 at 2:20 AM Post #10 of 12
Quote:

Originally Posted by onikiller
wow everyone, thanks for the help!

so if i may clarify, if the device i'm using can easily drive my headphones without cranking it up, then the benefit of buying an amp would be mostly in better components?



Yes, superior circuit designs, and parts will bring refinement to the overall sound. A good amplifier can make a mediocre pair of phones sound better, and high-end pair of phones sound phenomenal.
 
Aug 7, 2005 at 3:06 AM Post #11 of 12
And yet again, some amps really don't do much for some phones. So there is no one rule that says an amp will surely make your cans sound better.
You can also not tell whether it will match well with your source. A shrill sounding source with a bright amp is hell for many ears.
 
Aug 7, 2005 at 8:18 AM Post #12 of 12
Unfortunately, power into headphones is a complicated issue.
Amps don't just deliver xxx mW into a headphone.
How much power can be delivered into the headphone depends on output impedance of the amp and impedance (a function of frequency) of headphone. A poor match in impedance can generate bad results. The amp's maximum voltage, current or power output may all depend on the load. There is just no convenient way to specify everything.

If the amp can drive the headphone to 105 dB SPL to your ear, it should be enough and extra power is useless. But one amp's 5 mW is not equivalent to another amp's 5 mW becuase they all have their own distortion. Generally when an amp is working at its maximum power, it has higher distortion. This is why some smaller amps don't sound good with insensitive headphones even though it is loud enough. Distortion in an amplifier, again is a complicated issue and not easily measurable. THD or S/N are commonly measure but they tell you almost nothing about sound quality.
 

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