It all started with a couple of DS1809 by Maxim-Dallas I had come across. And the point is to determine how good they are for audio applications. Searching through the Web you can see quite pessimistic predictions from the DIYers. It is said there is much to complain of the DS1809 and of the other digital pots too. Some people say digipots 'kill' the sound or make it worse, the other think it is OK if you don't go too far with your ambitions... Anyhow I got myself to figure this out, any practice is practice you know...
There is an important rule when working on digipots btw, the input AC signal must be biased to half of the supply voltage. That is why a virtual ground supply for the circuit is organized.
Another disadvantage is the DS1809 needs the input signal to be decreased as low as 0,1V or so.
What is the best way to lower the input signal? A resistor devider? - Oh, no ! Just a simple attenuation makes you a big trouble - your regulator comes to catch all possible drawbacks and pick up a lot of noise!
The best and the most sounding way is to involve a junction FET limiter. The JFET I recommend is 2N5457. Actually the real way 2N5457 is used here remains a dilema for me. I'm not sure if it works as a source follower or as a drain follower/limiter. Though the drain goes down to the negative(ground) rail it is okay since the drain and the source pins of this FET are interchangeable. You'd better not replace 2N5457 with substitutes unless you dare run the risk.
The Buffer: You can clearly see it is a noninverting opamp buffer with gain of about 9, nothing special or unusual. The only two tasks we have to do now are to choose a well sounding opamp, and to make us sure we won't get any DC at the output. I tried a few parts like AD817, OPA637, MAX477 and THS4601 and found they were all nice candidates. But the winner was... MAX477 and this is what I recommend. Another thing is the MAX477 originally works with +/- 5V supply and keeps zero DC at the output fairly well.
