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A question about current

post #1 of 8
Thread Starter 

I'm wondering if there's any way to calculate the available current output of a portable (i.e. battery powered) headphone amplifier / DAP?

 

I'm guessing that the power is limited by the battery (unlike in mains powered devices where it's limited by fuses, etc. to my understanding).

 

Is it a simple question of the ratings of the battery? I'm guessing it's not quite that simple.

 

If anyone can share ways to calculate or measure the current capabilities I'd be very appreciative!

post #2 of 8

The battery is certainly a limiting factor,  but not a big one as fully charged batteries can supply enough current to damage most headphones.  Depends on the battery, of course.  9V batteries can supply far less current than AA, for example, and discharged batteries will of course supply less than fully charged ones.   But the real limit is in the amp design.  Some amps that use only an opamp as the output stage. Many of these cannot supply a lot of current (really power, which is voltage times current) to the load (I'm thinking of the CMOY here).  It's also a little more complex, as some amps have current limits that prevent the maximum power to be realized at certain output voltages, but suffice it to say that the limits are built in, so the choice of opamp really is where the limit begins.   Other amps that use higher power output designs have enough power to drive a small speaker.  And of course, there's everything in between.  

 

There's no way to calculate the maximum power output of a completely unknown amp.  You need at least a little information on whats inside.  If you knew what the output device was, you could make a guess from component specs.  Or, with the right test equipment, you can measure it.  

 

But the maximum power isn't usually the thing to be most concerned with.  The interesting figure is output impedance, which has an actual sonic impact on some headphones.  The general idea is for the amp to have a much lower output impedance than the headphones impedance.  For example, if you had 30 ohm headphones, you'd want your amp to be around 3 to 5 ohms.  Next, you want an amp with enough output capability to drive your headphones given their sensitivity.  Higher sensitivity phones need less voltage to be loud, and of course the inverse is true.  If you get those things right, it mostly works fine.  Most of that can be found in specifications or review data.

post #3 of 8

This is a total guesstimate, but I'd say most DAPs can or should be able to handle 16 ohm loads with (almost) full output.

 

Let's take 1 V (approximate ipod output voltage) into 16 ohms:

 

I = V / R = 0.0625 ampere.

 

if it can handle 8 ohms: 0.125 A


Edited by xnor - 4/18/13 at 2:05pm
post #4 of 8
Quote:
Originally Posted by xnor View Post

This is a total guesstimate, but I'd say most DAPs can or should be able to handle 16 ohm loads with (almost) full output.

 

Let's take 1 V (approximate ipod output voltage) into 16 ohms:

 

I = V / R = 0.0625 ampere.

 

if it can handle 8 ohms: 0.125 A

Probably true of most DAPs, not true of all amps.  Some CMOY versions in particular wimp on output current, yet inexplicably remain popular.  Go figure. 

post #5 of 8
Thread Starter 

Great answers - thank you!

 

I am trying to figure out the inexplicable choice of 22ohm output impedance on the iRiver AK100. The only provided specs are 1.5Vrms output and 22ohm output impedance.

 

There are no figures of power delivered per load and I don't have the gear to measure it so I can't see if/where it's limited.

 

I do know that it uses a FET design instead of opamp design, but not sure what impact that has on current.

 

iRiver have given some rather confusing responses about the choice of 22ohm output, suggesting that it has something to do with optimising the output for high-impedance cans. There's a lot of discussion, but I can't see that anyone understands how the 22ohm output really improves performance at any load. (That is, I'm not sure it does improve performance in any way)

post #6 of 8
Quote:
Originally Posted by jaddie View Post

Some CMOY versions in particular wimp on output current, yet inexplicably remain popular.  Go figure. 

Bu.. bu.. but that is euphonic distortion. You know, the kind of distortion that improves everything!

 

Quote:
Originally Posted by Loquah View Post

 

I am trying to figure out the inexplicable choice of 22ohm output impedance on the iRiver AK100. The only provided specs are 1.5Vrms output and 22ohm output impedance.

 

There are no figures of power delivered per load and I don't have the gear to measure it so I can't see if/where it's limited.

 

I do know that it uses a FET design instead of opamp design, but not sure what impact that has on current.

 

iRiver have given some rather confusing responses about the choice of 22ohm output, suggesting that it has something to do with optimising the output for high-impedance cans. There's a lot of discussion, but I can't see that anyone understands how the 22ohm output really improves performance at any load. (That is, I'm not sure it does improve performance in any way)

So the minimum load is about 22+8 = 30 ohms. With 1.5V output (which is most likely only achieved with 30+ ohm loads) you get about 50 mA across the load max. Most likely quite a bit lower.

 

Output resistors have some "uses". It's a simple trick to increase stability of picky opamps. Output resistors also make low impedance headphones receive less voltage (see wikipedia: voltage divider), so that you can switch from a 600 ohm headphone to 16 ohm earphones without instantly ruining your hearing. ;) For many dynamic headphones they also cause a bass boost similar to a peaking EQ filter. ;(


Edited by xnor - 4/18/13 at 3:41pm
post #7 of 8
Thread Starter 
Quote:
Originally Posted by xnor View Post

Bu.. bu.. but that is euphonic distortion. You know, the kind of distortion that improves everything!

 

So the minimum load is about 22+8 = 30 ohms. With 1.5V output (which is most likely only achieved with 30+ ohm loads) you get about 50 mA across the load max. Most likely quite a bit lower.

 

Output resistors have some "uses". It's a simple trick to increase stability of picky opamps. Output resistors also make low impedance headphones receive less voltage (see wikipedia: voltage divider), so that you can switch from a 600 ohm headphone to 16 ohm earphones without instantly ruining your hearing. ;) For many dynamic headphones they also cause a bass boost similar to a peaking EQ filter. ;(

 

OK. So maybe they did it to allow enough power for high impedance cans while also having the ability to drive (and not destroy) lower impedance IEMs?

post #8 of 8

In terms of power, it doesn't work that way. Max output power is at about 22 ohms. According to a measurement I found 20 mW (0.66 V, 30 mA) into 22 ohms. The voltage output rises a bit with higher impedance loads, but not enough to result in higher output power. 1.3 V into 250 ohms = 7 mW and 5 mA. Max output voltage is only reached when it is virtually unloaded.

 

But depending on the headphones and music you're listening to that could still be enough to damage your hearing. wink.gif


Edited by xnor - 4/19/13 at 5:51am
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