> I need to process an audio input before it runs into a modem chip.
>
> ...
>
> The problem is that the
> first component in the active filter is a capacitor, which, of course,
> removes the DC bias.  There must be some common way of handling
> this problem

What problem?  You said this was an *audio* signal.  "High fidelity" audio
is considered to be 20Hz to 20KHz because that exceeds most people's
hearing range.  Note that 0Hz, or the DC bias, isn't in this range.  You
can't hear it, and the sound system couldn't reproduce it anyway.

Audio circuits commonly make use of this by adding DC bias in various
places for convenience of the circuit.  In fact, you wouldn't want an
audio system to pass DC since that is essentially a noise signal, as is
any other signal outside the 20Hz to 20KHz range.  By the way, voice is
reproduced quite well with only a 4KHz upper frequency limit, and is
understandable lower than that.

Your circuit also adds a DC bias so that the signal is centered within the
available voltage range, a common thing to do.  So, what exactly is the
problem?  Most audio systems will block DC at the input just like your
circuit does, so you can probably connect its output directly to the next
input.  In case the next stage doesn't was a DC bias, you could add an
appropriate blocking capacitor on the output.  Note that you might need a
resistor or two to guarantee that the capacitor won't effect the frequency
response in the pass region.


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