Coupling with an audio output

**fvnktion** · 2nd February 2007 09:10 PM

I am trying to couple a signal from a microcontroller to the output of an audio source, mp3 player, stereo, computer that will create audible sound at an earphone speaker. I am simply putting a 5volt square wave 1-10khz pulse onto the audio lines of the speaker.

It works fine when directly driving the speaker from the microcontroller, but when i connect the audio lines the pulsed uC signal is severly attenutated at the speaker. I believe that this is because of the extremely low output impedence of the audio amplifier driving the audio line??? It would seem that i need to buffer the audio line with something of a higher impedence?? It appears that i will have the same problem running the signal through an opamp configuration( follower) becuase of its low output impedence??

What would you recommend doing with minimal extra components being required??

Thank for the help

**audioguru** · 2nd February 2007 11:41 PM

You are supposed to feed a signal to the input of an audio power amplifier through a mixer circuit, not to its output. You are lucky that the microcontroller and the audio amplifier survived.

The max allowed output of a microcontroller is 25mA so you exceeded it a lot.

The output of an audio power amplifier has negative feedback. If something external attempts to drive its output then the negative feedback "tells" the amplifier to give a little of the opposite polarity output to stop the external force from doing anything. So the output is like a dead short (very low impedance).

The signal mixer for the input of the amplifier can simply be a pair of resistors.

**fvnktion** · 5th February 2007 03:52 PM

Thanks for the input Guru,

A couple questions for you if you dont mind?? Am i correct in assuming that the output of any audio device, mp3 player, computer headphone output, etc, is an audio amplifier?? So when i try to add my signal to this signal directly i am essentially shorting the added signal through the output of the driver/amp??

It sounds like i need to mix the raw signal from the computer with my microcontroller signal with a simple mixer circuit. Is it standard to run the mixed signal back through an amplifier?? I guess the resistor values that are used for the mixer dictate the amount of attenuation that will result, with this in mind do you recommend using very low value resistors?? I am trying to use minimal components so would like to only have to mix the two signals and then drive the headphone speaker directly, is this advisable??

Thanks for your patience

. Not very experienced on this analog stuff.

**audioguru** · 5th February 2007 09:38 PM

Originally Posted by fvnktion

Am i correct in assuming that the output of any audio device, mp3 player, computer headphone output, etc, is an audio amplifier?? So when i try to add my signal to this signal directly i am essentially shorting the added signal through the output of the driver/amp??

You are overloading the headphones output amplifiers and the source of the added signal. Both might blow up.

It sounds like i need to mix the raw signal from the computer with my microcontroller signal with a simple mixer circuit.

Yes.

Is it standard to run the mixed signal back through an amplifier??

It must be mixed with the headphones original signal then the output of the mixer feeds the input of the headphones amplifiers.

I guess the resistor values that are used for the mixer dictate the amount of attenuation that will result, with this in mind do you recommend using very low value resistors??

If the input resistance of the headphones amplifiers are high resistance then the resistors could be 10k ohms so they don't overload the signal sources. There won't be much attenuation.

I am trying to use minimal components so would like to only have to mix the two signals and then drive the headphone speaker directly, is this advisable??

Are you going to make your own headphones amplifier?

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