>    At that point, the thing would make a TREMENDOUS racket as
>    dozens of relays frantically clicked on and off.  Worse, there
>    were big volume spikes whenever we reached one of the power-of-2
>    steps in the volume sequence... One relay would switch from "off"
>    to "on", and all the lower-order relays would switch from "on" to
>    "off".  Since they all switched at slightly different rates,
>    there would sometimes be NO attenuation for a few milliseconds,
>    and you'd hear loud pops from the speakers.

Sound to me like it needed a PIC to sequence the relay break before make
order (or make before break).  Could you not mute (or switch in the maximum
attenuation) first, then move all the other relays to the new setting, and
then unmute?  I guess it wouldn't be very user friendly to have the volume
mute every time you turn the knob...


[snipped discussion of Dallas digital pots]
>    However, there were three big problems with them:

What about maximum input level?  Aren't they picky about high level input
signals that go beyond the power rails, or have they fixed that?  I guess
you could attenuate the input, at some cost in noise.


>1.  Spend some time thinking about how to implement the
>    relays-and-resistors volume control.  If you can't came up with
>    a better way, use 2N relays, then select (with the aid of a
>    computer program that you can run overnight) 2^N more-or-less
>    equally-spaced steps out of the 2^2N possibilities.

I wonder how the newer signal level photorelays (HP, IRF, somebody?) would
work.  I think they're just an optically driven fet, so the switching time
should be minimal, the on-resistance is probably low enough not to be a
problem, and you might even be able to 'fade' them in and out over several
tens of milliseconds, if you wanted.

I bet they aren't cheap.


newell