At 11:22 AM 7/05/98 +0100, you wrote:
>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
>
>


Use a Dallas serial LOG pot!