Jon Nicoll wrote:
>
> 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 ;-)

Check out the Crystal Semiconductor's CS3310. -95.5 to +4.5 dB range.
 60 dB = 1:1,000,000.           (ratio of faintest audible to ear damge?)
100 dB = 1:10,000,000,000       (more than you'll need?)

To pick values off an R-2R DAC, you'd need about 2^34 linear steps.
So I doubt this is what they do.

Assuming you want stereo, you will need to have a pair of attenuators
matched to audiophile demands.

I'm told a simple good-quality pot adds less noise than an attenuator chip,
but you have problems and expense getting matched ones. They don't stay
matched, they wear out, etc. You could try getting a PIC to self-calibrate
a pair of stepper-controlled pots, and receive RC5 commands.

Electronics is not the main destroyer of sound fidelity.
Its the plain old mechanical speakers!

Cheers, Keith.

BTW, we make audiophile hi-fi here! :-)
Quick plug: check out our Alpha 10 amplifiers.
They grabbed a lot of stars in the magazine reviews.