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Amplify simple Arduino audio output using 28v line

I am new to circuit design - especially working with audio signals - so feel free to point me toward any good resources.

PROJECT OVERVIEW: I’m using an Uno to integrate inputs from a few signal lines into 3 different outputs including 1 audio signal (the other 2 are TTL signals – I have that part covered).
I have an isolated -28v DC line (can draw up to 3A max) that I need use to amplify the audio signal before sending it to an 8ohm speaker rated at 3W (datasheet below).  Audio quality is not important (but the longevity of the speakers is) – I will just be generating sequences of tones.

1st CIRCUIT: My first solution used tone() to drive an optoisolator that allowed the 28V line to drive the speaker. Works great, but I think there are 2 problems that could eventually destroy the speaker:
1.   tone() -> Square Waves
2.   0/-28V output (should be using -14V/+14V)

NEW DESIGN: Use an 8-bit DAC composed of a resistor ladder over 8 digital pins (I can spare the pins – or I could use 6 pins ie.PORTB because audio quality isn’t important) with a filter to generate sine waves.  I’m essentially following the instructible below.

PROBLEM: I’m struggling to find the best way to amplify the filtered signal using   the -28 volt line.  I could put the signal through the optocoupler and then use an opamp to establish a virtual ground.  Will the optocoupler distort the signal?  I would try to amplify the signal directly with an opamp, but the Arduino is powered from a wall wart so the grounds between the 2 circuits are isolated.  I was also considering trying to step down the voltage from the -28V supply and trying to power the arduino off of that – which would allow a common ground for amplification.
I appreciate any suggestions.

B

Photocoupler - http://media.digikey.com/pdf/Data%20Sheets/Toshiba%20PDFs/TLP627,-2,-4.pdf
8pin DAC design - http://www.instructables.com/id/Arduino-Audio-Output/#intro
Speaker – http://www.alliedelec.com/images/products/datasheets/bm/ICC/70115850.pdf
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I could also try to split the power coming from the wall-wart to amplify the audio.
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