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On the portable power rating 5mW vs 10mW

post #1 of 7
Thread Starter 
We all complain about the low power amps on todays portables. It is always on these pages that a portable with a 10 mW amp is regarded as more suitable in driving some full size cans.

Recently I could compare two Sharp portables, a friend's MT88 which is rated at 2x10 mW @ 32 ohms and MT866 (which I own) rated at 2x5 mW @ 32 ohms. These are manufacturer specs. I wanted to see how much louder the 88 goes with its 10 mW amp.
Guess what... I could hear no difference in SPL at any volume setting with both easy and a bit harder to drive cans. I used Sony 888, EX70 and a friend's Beyer DT250/80, which can still be powered by portables but need more juice to work good.
At 2x the power the MT88 should give me about 3dB more SPL at the same volume setting and this should be pretty audiable. I changed back and forth many times and could hear no difference.
Just to check my ears, I connected one of the portables to the optical out of a PCDP and set the recording level to +3dB. Now the difference was obvious, i.e. these 3 dB are audiable to me.
Now, either Sharp is not completely honest with their specs or my MT866 got a more powerful amp that the manual says. I remember, comparing it to my previous MDP, Sony R70 and the MT866 was juicier.
In any case to all of you who consider the cheaper line of Sharp portables mainly for their 10 mW amps, audition carefully-this may not be the case.
post #2 of 7
Uh, no. Comparing which is louder at "any volume setting" or "the same volume setting" reveals nothing. There is no fixed scale for volume settings; the numbers on digital volume controls are completely arbitrary. If anything, a manufacturer would adjust the scale so the same volume settings DO sound the same among units irrespective of how much output power each has.

What you need to do is compare which will play louder without distorting, whatever the digital volume number may be. And 3db is just not a lot of difference in any event (which is why some high-end folks spend thousands on low power amps - cleaner power is more important than higher power).

More important, but far more difficult and subjective, is to set both at moderately loud levels and see which sounds better. The higher powered amp should because it is not compressing or distorting transients in the music, has better control over the bass notes, etc. (Caution, if one is playing even slightly louder than the other, psychoaccoustically it will be perceived as sounding better, so this is a tough comparison to set up without measuring equipment).

I'm not defending the accuracy manufacturers' specs, but you can't figure it out the way you tried.
post #3 of 7
Thanks zowie
post #4 of 7
Thread Starter 
True that the volume scales may be different. Both units however use the same 30 step digital volume scale, so at least at max volume there should be a difference.
About distortion, neighter of the units distorted at any volume with the phones I used and the sound quality was also undistinguishable.
3 dB may not sound as very much but as I said I did a control experiment setting one of the units 3 dB louder and the difference in loudness was clear.
All this make me conclude that there is perhaps something wrong with the ratings on these units.
Anyway if you don't trust my ears i'll try to get an SPL and a voltmetre and repeat the measurements.
What is more more important however from purely practical point of view is that getting a portable with a 10 mW amp (on paper) does not guarantee that you'll get a lot more power.
post #5 of 7
3 db is a doubling of power, not SPL. 3db power difference is the generally accepted minimun to produce a "perceptible" difference in SPL. It is also generally accepted that it will require a difference of 10db in power (which is a 10 fold increase) to produce a doubling of SPL.

The key as stated by zowie is how clean is that power? My JVC receiver is rated at 110 watts per channel. It sounds very good. However My Harmon Kardon AVR300 receiver is rated at 55 watts per channel. That's 3db. The H-K sounds a hell of a lot better than the JVC though. High current and very clean. More than loud enough, as that 3db is "The lowest perceptible change." Trying to measure or compare as you're doing is very difficult or almost impossible without a full audio electronics lab.
post #6 of 7
To add to what gainso wrote,

If neither units distorts at max volume (or at least compresses dynamics, which is really a form of distortion) then that would only indicate that neither unit is accessing its full continuous output power due to insufficent gain, so you can't tell how much more reserve power one has over the other. In other words, if both are only playing at 3mw, you can't judge whether having 10mw makes a difference over 5mw. (Although it would make your point that the difference doesn't matter.) Incidentally, on my 10 mw Sharp MD-SR60, I get some modest distortion at or near full volume on my Grados.

Also, I didn't quite understand the description of the experiment you did to hear what +3db sounds like, but if what you did was increase record level by +3 units on the level meter, that would not necessarily correlate to a +3 db in playback volume.
post #7 of 7
Thread Starter 
What I did was to set the record level to +3 dB not watching the meter at all and playing some music via the optical connection (rec-pause mode) and then playing the same music but already recorded at 0 db on the other machine. The problem here was to avoid overloading. Most of the new CD are quite compressed and mastered to peak exactly at 0. +3dB is quite a lot for a digital recording and the distortion increases.
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