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> An audio jack provides no power. The signal is created by electromagnetic induction. The previous version of Square used a battery to amplify that signal. Hu
by devicenull 13y ago
> An audio jack provides no power. The signal is created by electromagnetic induction. The previous version of Square used a battery to amplify that signal.
Huh? I thought that you could get a limited amount of power from the audio jack by playing a tone through it while something's connected. http://stackoverflow.com/questions/5560867/how-to-get-electric-power-from-head-phone-jack http://stackoverflow.com/questions/5560867/how-to-get-electr... seems to agree.
- riobard 13y agoI have similar doubts too. If an audio jack doesn't provide any power, how does it drive headphones?
- ChuckMcM 13y agoTechnically the 'audio out' signal isn't power. However as folks have discovered you can send an audio signal, rectify it, and lightly load the audio amplifier (which will think it is driving some really crappy headphones :-) This is like using a side effect in software to initialize a variable. Clever but not durably so.
- sliverstorm 13y agoTechnically the 'audio out' signal isn't power. Explain? Even if we ignore losses, the 'audio out' signal does work when it drives headphones. I understand that an AC signal does not perform work when the current and voltage are perfectly out of phase, but that does not describe audio or else there would be no power transfer to drive the transducer.
- ChuckMcM 13y agoThe audio amplifier is designed with certain assumptions "baked in" which are different than those of a power supplying signal pin. In the case of the headphone amplifier, there is the impedance of the headphones and the duty cycle, or power over time component. So lets say you have a "1 Watt" headphone amplifier. Generally that specifies the peak power output when reproducing a signal. However, and many folks who have sent random tones through their stereo amplifiers have experienced this, if you send a 'peak power' tone through the amplifier continuously, the output stage may not (and often does not) have the power dissipation to keep that going. Instead it overheats and shuts off. You can of course design the power amplifier to be able to dissipate its peak power output continuously, but that is not generally necessary when listening to audio from it (even if you are a severe heavy metal listener). Something designed to supply power will deliver its rated power continuously. So the audio out signal is designed to drive headphones without distortion between a lower and upper frequency. It also has a power dissipation limit based on what the guy designing it figured you might be listening too and for how long and at what volume. If you guess wrong (as some of the imported "external speakers" for the iPhone did), you risk damaging the headphone amplifier permanently. So "Yes" there is going to be power available in the audio out signal, how much over what time period, is not part of the externally visible specification. If you damage your phone by attempting to draw too much power they don't fix it because "technically" you were not using it in an approved way. Does that help?
- sliverstorm 13y agoOk, that confirms what I suspected you meant. I can agree with what you are saying; basically that an audio driver is just about the world's worst electrical power supply and thus should generally not be treated as supply. I was just a little perturbed, fearing you were trying to claim an audio signal delivers no power.