I am trying to run a speaker off of my cell phone headphone jack. Currently i have the 0.4 watt speaker (8 ohm) plugged directly into my phone and I am getting audio out. However, i want the speaker to be louder (preferably as loud as my cell phone speakerphone) and want to do that without adding a battery. Is it possible to use the power from the headphone jack line out to amplify the signal in order to get the speaker to be louder? What are my options? I would really rather not have to add an external battery to the project since i am trying to keep it small as possible.

Thank you

  • \$\begingroup\$ What exactly are you trying to accomplish? And stop putting 8 ohms across your headphone out before you break something. \$\endgroup\$ – Matt Young Aug 11 '12 at 0:47
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    \$\begingroup\$ I guess that your best option is to remove the "without external battery" requirement. \$\endgroup\$ – Bruno Ferreira Aug 11 '12 at 9:42

Is it possible to use the power from the headphone jack line out to amplify the signal in order to get the speaker to be louder?

Here's the thing: The power in the headphone jack is in the audio itself. The power in a normal wall outlet is 50/60Hz AC. The power in a headphone jack is also AC and in the frequencies that you want for your audio! There is no additional power contained in a headphone jack other than the audio itself.

So, if you could somehow convert the power in the headphone jack into something useful and convert it back into a form for driving a speaker, and could do it with 100% efficiency then you would end up with exactly what came out of the headphone jack in the first place.

In short: You can't amplify the power without another source of power. But all is not lost!

What you really want is louder audio, and a way to do that is to make your speaker more efficient. Speaker efficiency is usually expressed as "sensitivity" and is specified as XX dB at 1 meter at 1 watt. A normal speaker is 75 to 90 dB (@ 1 meter @ 1 watt). It might not look like it, but 75-90 dB is a huge range! You can simply get or build a speaker with better efficiency.

Another thing that effects efficiency is the speaker enclosure itself, and the type of speaker. Look into horn speakers. There are horn speakers that have sensitivities up to 105 dB (@1m@1w). Although these will likely be too large for your application, it might be interesting anyway.

Driving an 8 ohm speaker with a headphone jack is questionable. Some devices will do this just fine. Some devices might distort but otherwise be fine. And other devices might be permanently damaged. Unfortunately there is no good way to find out without just trying it.

In theory you could make an impedance matching transformer that would show a proper load to the headphone amplifier, but who's output is good enough to drive an 8 ohm speaker. Just keep in mind that a transformer will not increase the power, only convert the form of that power so different devices can play nice together. I have not seen such a transformer for sale, but I also have not looked for one. But in theory you could make one.

Given all the different issues, if it were me I would figure out a way to make a small battery pack with an amplifier in it. Yes, it's bigger than what you want. But it will probably be the best compromise between volume, size, cost, and robustness.


There are probably three issues involved here:

  • The phone can only output so much power

  • There is a load impedance at which maximum power will be transferred, and as Matt points out, that is probably not 8 ohms. Something like 32 ohms would be more likely, but you would have to see the phone specification to be sure. It is possible that maximum power output might be achieved at a load impedance other than what it was designed for, and in theory that could eventually cause damage though low power outputs are often fairly robust.

  • It is quite likely that there is intentional limitation of the headphone volume to reduce the chance of hearing damage. If it is the same amplifier as is used for the internal speaker, it is possible that you could override this at your won risk with changes to the software. But there may be built in attenuation in the connection to that output, or it may come from its own dedicated lower-power amplifier.

Any "line-out" functionality on the headphone jack would be device-unique and so unanswerable without more details. Most phones would provide power for an external condenser microphone, but that's probably too limited a supply (in essence, supplied through a large resistor) to run an audio output amplifier off of.


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