Jon Nicoll wrote:

> I'm thinking about making an attenuator ('volume control') for a
> hifi preamp.

  The consensus seems to say use a serial attenuator set.  Two little
tricks may help you.  Firstly, if you can get relays (e.g. reed relays)
which have separate N/O and N/C contacts, then you only need one of
each per channel for a single-ended attenuator.  One contact bridges the
attenuator when switched out of circuit, the other grounds the parallel
leg when in-circuit.

  Secondly, when using your PIC to drive such a system, code it to
switch all new attenuator steps into circuit, wait a few ms, then switch
old attenuator steps out of circuit; for any given step.  This is to
avoid *nasty* transients going from say, 16dB to 15dB with binary steps.

> (I'm looking for around 30 discrete attenuation steps)

  Five relays.

  Cheers,
        Paul B.