DAC output to audio amplifier

[andy257]

hi all,

i have a problem with the interface of one of my projects. Basically i have a 12 bit DAc that is a unipolar device. I wish for the output to be bipolar.

my project is based on audio signal conversion. The DAc is connected to a audio amplifier.
However from the looks of things its also a unipolar device.

Now somewhere along the line i have to convert this unipolar signal into a bipolar signal before i pass it into a speaker.

My confussion comes from the audio ic, from my interpretation the ic is ran from a single supply. (Link to datasheet provided below.) Now if this ic is described as an audio amplifier how can it perform as described if there is no negative supply. As far as i am aware audio signals have a negative cycle and therefore will require a negative voltage swing????

Can anyone comment on this.

Thanks

Andy

www.ortodoxism.ro/datasheets/nationalsemiconductor/LM4838.pdf

 

[Roff]

Andy, if you look at Fig. 2 in the datasheet, you will see that the audio is coupled into and out of the amplifier through DC blocking capacitors. Do you understand this concept?

 

[andy257]

Hi Roff

Thanks for the quick reply. I never noticed them at all. In truth i didnt pay much attension to the schematic and i was looking at the electrical spec instead. Yes i believe i have a grasp on dc blocking capacitors, essentially ac coupling the input and output. The hard part i cannot grasp is if the circuit is powered from a single dc supply how can the negative part of the waveform be amplified if the dc input voltabge has been blocked.

perhaps i havnt grasped this concept at all.

I can now rest easy knowing this chip will work with my application.

Thanks

Andy

 

[Roff]

I just noticed that the output AC coupling is only for headphones. The speaker outputs are bridged, meaning neither of the speaker terminals is grounded. The speakers are driven push-pull. There will be no DC across the speakers.
The inputs are internally biased, so on the IC side of the input coupling (DC blocking) capacitors, the signal will swing plus or minus around some DC voltage determined by the internal circuitry.

 

[audioguru]

The amplifier's input has a DC bias voltage. The capacitor-coupled AC input modulates the amplifier's input above (positive-going) and below (negative-going) the bias voltage. Then the input is AC.

The output of the amplifier is actually two amplifiers per speaker. One amplifier drives its speaker wire in direction that is the opposite to the other amplifier's direction that drives the other speaker wire. So the speaker also gets an AC signal with the DC cancelled since it is the same on each speaker wire. The headphones are capacitor-coupled to the outputs so they get the DC blocked but have the AC signal fed to them.