Hello all
        I'm thinking about making an attenuator ('volume control') for a
hifi preamp. I don't want to use semiconductor devices for this, and am
considering using a PIC and network of relays and resistors to get the
appropriate range of attenuation. Messy, perhaps, but you know what
these hi-fi types are like ;-)

I know about R-2R ladder networks, but as you know the necessary 'law'
for the ear is logarithmic. So depending on the actual arrangment I use,
(eg. a standard pot. arrangement, or a fixed series resistor and
variable 'shunt' to ground) I actually need a log or anti-log law.

I've puzzled over appropriate ways of doing this using the minimum
number of relays & resistors. The 'logDAC' type semiconnductor devices
seem to use a large R-2R ladder (like, 17 element pairs), giving 2^17
possible settings, and then internally choosing a much smaller set to
give the appropriate law. This seems a bit excessive in terms of relays
even for me! (I'm looking for around 30 discrete attenuation steps)

There is clearly room for experimentating with variation on the 'R-2R'
ladder, to change the law from linear. I'm a bit rusty in this area
however, and I'm not sure if what I'm after can be done. Can anyone
offer me any advice in choosing the resistor ratios, or point me to
references in this area?

Thanks for any help.

        Regards
        jon N