Laptop headphones socket - voltage/current?

[breakshift]

Hey there, I need to know roughly how much power my laptop is delivering to the headphones socket (full volume). Do laptop manufacturers make this kind of information available to customers?

By the way, I assume it's not line level... it must be amplified in order to power headphones, right?

Thanks.

Mike.

 

[Nigel Goodwin]

Hey there, I need to know roughly how much power my laptop is delivering to the headphones socket (full volume). Do laptop manufacturers make this kind of information available to customers?

By the way, I assume it's not line level... it must be amplified in order to power headphones, right?

Basically it's line level - and the socket is intended to feed either headphones or a line level input.

 

[breakshift]

Okay so that's roughly 0.316V right?

Thanks.

 

[MikeMl]

Okay so that's roughly 0.316V right?

Thanks.

No. Think of it this way: The amplifier that drives the "headphone/line out" jack is an audio amplifier chip. That chip is powered on an internal system voltage, likely on the order of ~6V. This means that the chip can drive the audio output to about 5V peak-to-peak (which is ~1.7V rms) into a high impedance. Since the audio amp chip has a lot of negative feedback, it has effectively a very low output impedance (<1 Ohm, i.e. it acts like a voltage source), but it may have an output current sourcing/sinking limit which means that the output may go into clipping if you ask it to drive a low impedance like a 4 Ohms speaker.

Most ear bud and lightweight headphones used with Mp3 players have an impedance of >30 Ohms. You will find that <1V p-p will sound loud in these headphones.

 

[breakshift]

Okay, thanks for the reply.

I guess I may as well explain what I'm trying to do: my old laptop broke so I took it apart (no reason really), and thought I could do something cool with the two speakers (as they're very small). Now, I have a new laptop and I want to split the audio output between my normal PC speakers and these two new ones.

I've bought a splitter, and a pot to control the volume on the old laptop speakers. The input impedance of these speakers is 4.3Ω. Do I need to increase their impedance in order for this to work? If the audio amplifier chip inside a laptop doesn't like powering low impedances, why are these speakers only ~4Ω?

Thanks a lot.

 

[MikeMl]

If the original audio amp drove internal speakers, then it obviously will work with those. I was assuming that you were using an output jack like the line-level output from a sound-card, which will drive headphones, but not speakers.

 

[breakshift]

Yes, you're right. I want to use the headphone jack on the laptop to drive the speakers.

I was just wondering why the audio amp chip works best with a high impedance connected to it's headphone jack, but when nothing is connected (i.e driving the internal speakers) it works fine with the ~4Ohm internal speakers. How does it switch between the two?

Thanks a lot.

 

[crutschow]

The internal speakers may have their own separate amplifier that can drive 4 ohm speakers.

 

[Nigel Goodwin]

Yes, the internal speakers will be fed from an amplifier - the OP needs such an amplifier to feed his speakers.

 

[tcmtech]

This sounds like a job for the good old LM386 amplifier IC!