Bob Blick wrote:
> I'm in need of an audio attenuator (logarithmic digital potentiometer)
> IC for occasional home projects. Nothing audiophile-grade, although
> that would be a plus if it weren't too pricey. The DS1801+ is
> adequate for my needs but I would think for the price (basically $8
> in singles) I could get something dual supply so I don't have to add
> powerup thump muting.
>
> I need stereo, 1 db or better steps with 60 db or more range, two or
> three-wire control, readable as well as writeable, volatile memory.

Since this is for personal one-off use, you have some flexibility.  A long
time ago I wanted a analog controlled volume control for normal audio
signals.  Not so easy when you sit down and think about it.  A true
multiplication or anything even roughly like it is not easy to come by.
Most analog multipliers work on the fact that the voltage accross a diode i=
s
roughly the log of the current.  That's a long way from making a volume
control.

What I came up with is only for one-off use, but it worked nicely.  Use two
LEDs and two light-dependent resistors.  I had the analog circuit drive the
two LEDs in opposition smoothly.  When the voltage was at either extreme,
one LED would be on brightly.  In the middle they were both on somewhat.
This has the added advantage of the control and controlled circuits being
optically isolated.

If I remember right, CdS cells have a peak sensitivity around 600nm light
wavelength, which is orange.  I think I used yellow LEDs since that's what =
I
had.  I went thru the junkbox and selected LEDs that showed the best
response on the CdS cell, which in my case were yellowish (maybe they were
amber, it was a long time ago).

The result is of course non-linear.  I found it helped to put fixed
resistors accross each LDR.  It depends on how far off you want the volume
control to go.


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