Power from headphone socket?

  • Thread starter ojsimon
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  • #1
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Hi
I was wondering if there is power from the headphone socket on appliances? if so how much? also is there power from a line in 3.5mm port? again how much?

Thanks for your help
 

Answers and Replies

  • #2
uart
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It very much depends on the device, but typical values would be only few hundred mW or less for a headphone output.

No "line-in" does not provide output power.
 
  • #3
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i was thinking along the lines of an ipod also how much p.d. would it supply? also any idea about a line in on a stereo system?

Thanks
 
  • #4
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anyone?
 
  • #5
f95toli
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Inputs will -by definition- never SUPPLY power unless there is something seriously wrong with the device. An input is after all meant to RECEIVE signals, not to send them out.

An iPod will probably give you at most 30-40 mW (depends on the load); it varies a bit from model to model.
 
  • #6
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thanks but if inputs never supply power how do microphones work then?
 
  • #7
f95toli
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A basic microphone is essentially a device that converts the energy in the soundwave to an electrical signal; so the power is coming from the sound itself.

Some types of microphones DO require external power but they don't work with normal microphone inputs (they can only be used with microphone amplifiers that can supply what is known as "phantom power").
 
  • #8
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ok thanks
 
  • #9
negitron
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Microphones make their own (weak) voltage when sound impinges on the transducer. The input of the amplifier is AC-coupled through a capacitor (or less commonly a transformer) so you don't see the DC bias of the input transistor stage. The output of the mic is an AC waveform which passes through the input capacitor to amplifier front end stage.
 
  • #10
vk6kro
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Computer sound cards supply DC power on the microphone input.

They have a stereo socket and only a mono microphone input. The other side of the stereo socket supplies 5 volts in series with about 4700 ohms. So, it will only supply about 1 mA, even into a short circuit.

It is meant to power electret microphones which require DC power to work.

Apple Ipods use a docking connector which does have DC power on it, but it is an Apple plug so it might be expensive.
http://nuxx.net/wiki/iPod_Dock_Connector

Other devices may supply DC power for the microphone, but you would have to check the specs.
 
  • #11
uart
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And for future refenence for the OP, a "line-in" and a "microphone-in" port are two different things.
 
  • #12
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thanks oh really what is the difference then?
 
  • #13
uart
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thanks oh really what is the difference then?
Hi ojsimon, there are lots of differences. The first big difference is that the microphone input is usually only mono. Computer microphone inputs usually use a 3.5mm stereo jack, but as previously explained one of the wires is actually used to provide a small bias voltage for electret microphones.

The other big difference is the gain of the microphone input will typically be somewhat larger than that of a line-in level (and this is usually associated with a higher noise level). The bottom line is that the microphone input (on a computer) should only be used to connect a microphone, if you need to connect anything else like a radio or tape player etc then you always get a better result going in through the line-in connector.
 
  • #14
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thank you very much for your expanation.
 
  • #15
vk6kro
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It is important to point out that there is DC power as available from a battery and there is signal power which is always an AC waveform.

Microphone inputs should never be greater than, say, 20 mV (0.02 volts) peak to peak or they will get distorted, regardless of volume control settings.
Line input signals should never exceed, say, 2 volts peak to peak or they will get distorted regardless of volume control settings.

Some recent computers and laptops only have microphone inputs, so larger signals have to be reduced to microphone levels using resistors.

DC power is available from a computer on the USB sockets and, on older computers, on the "game" socket. This is usually unused and makes a handy source of 5 volts.
 

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