[ilovesocks]

People often favor low-impedence headphones (<60 ohms) for portable applications because portable players often lack "power."

I've been pairing up my Shuffle with the 600-ohm AKG k240M, which has worked out surprisingly well. The Shuffle seems to have plenty of "power," supplying more than enough volume for me to listen during car rides (maxing out the volume is way too loud). Plus, the super-high impedence should prolong the battery life because the k240M's require less current, right?

Is my reasoning correct? What is it that allows the Shuffle to supply ample volume to these headphones? It is independent of current?

Has anyone else tried using higher-impedence headphones with portables?

 

[Brent Hutto]

The amount of sound used per unit of power consumed is the sensitivity of a headphone. A natural unit would be decibels of sound output per milliwatt of power (if it's unhelpfully expressed in decibels per millivolt then you have to do a calculation including the impedence to get to decibels per milliwatt).

What the impedence tells you is how much voltage is needed to transfer a given amount of power to the headphone. High impedence headphones use a relatively large voltage and relatively small current for any given power output (maybe 10mW typically for a high sensitivity headphone).

Since portable music players are battery powered, there is a fundamental limit on how much voltage they can produce at the output jack. The ones that use a single AA battery will only be able to produce (best case) maybe 800mV or so. So a not particularly sensitive headphone of high impedence (like 600ohms) on a player with limited voltage output may not be able to produce high sound volumes.

OTOH, low impedence headphones draw a lot of current and therefore tend to accentuate certain weaknesses of the output stage. For instance a coupling capacitor that's too small will be much more obvious with an output current of 30mA to a low-impedence load than it would driving a high-impedence load with 3mA to achieve the same power output.

So in voltage-limited circumstances you want a low enough impedence to produce the peak required sound output and with marginal output stage designs you want high enough impedence to minimize the shortcomings of the circuitry. High sensitivity helps in either circumstance by making it possible to get higher sound output with less voltage and current at any given impedence. The only downside to high sensitivity is that it will accentuate any residual noise (i.e. hiss or hum) present at the output during quiet moments.

 

[GreatDane]

I just tried my K240M straight out of my ZV:M and at full volume I measured around 78 dB peaks @ C weighted/slow. If that's all I had to listen with I could live with it but it sounded mighty thin.

 

[ilovesocks]

Ah, so a high-impedence headphone may be a good solution for a player that has a relatively high voltage output, as it won't require much current. On the other hand, if a player doesn't put out much voltage, a lower impedence is needed?

Another question: Portable players' amps (and indeed, most amps) use a system where the signal is amplified a constant amount, and then the signal is reduced with a resistor whose strength is decided by the volume control, right? So the load on the amp (and the battery, in this case) doesn't change as you raise or lower the volume? And this means that there are no sonic disadvantages to maxing out the volume on your player in order to get the music loud enough?

Thanks for the replies.

 

[Brent Hutto]

Quote:

Originally Posted by ilovesocks Portable players' amps (and indeed, most amps) use a system where the signal is amplified a constant amount, and then the signal is reduced with a resistor whose strength is decided by the volume control, right?

Almost never. That's one of many possible ways to implement a volume control and it is seldom used.