> too, adding about 2.5 volts DC to the incoming audio so the
> entire signal
> is a positive voltage.  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

It will depend on the application, as ever, but a reasonable first step is
to put a dc bias network on the 'ac' end of the capacitor.  The cap doesn't
mind if it's two ends are at different dc values.  To ac, the bias network
will look like the two resistors in parallel to 'ground'.

For example, if you have a 1uF cap, and 10K to +5V, 10K to 0V on the input
to the filter, it will form a highpass filter with 1uF, 5kohms, so 3dB
point is at 1/(2*pi*R*C) = 32Hz

Nigel
--
Nigel Orr, Design Engineer                 nigel@axoninstruments.co.uk
Axon Instruments Ltd., Wardes Road,Inverurie,Aberdeenshire,UK,AB51 3TT
               Tel:+44 1467 622332 Fax:+44 1467 625235
                   http://www.axoninstruments.co.uk

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