I actually have a million questions, but the one (well, two) on my mind at the moment is this: How do you determine what gain setting is optimal for your application? Is there any reason you couldn't use variable resistors for R3-R6 so you can change your gain values on the fly?

I would hazard that very few (if any) variable resistors would supply a signal as good as a fixed-value resistor.

Well... what you could do is wire a DPDT switch to say, switch the value of R4 to adjust from high gain (say 10) and low gain (3-5, whatever).

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Then that would give the quality of fixed value resistors and still offer multiple gain settings. No reason why it couldn't be limited to two settings, right?

BTW, is ten a pretty high gain setting?

How about a stepping potentiometer (I think that's what its called). I saw an online store in singapore, I think, that made them using Vishay-Dale RN55's. I don't have the url, but it looked like it was good quality. A bit expensive, though.

A stepped attenuator is merely a rotary switch that's connected to several resistors, for the application of volume control.

It would be more practical to get a 8PxT rotary switch ( (L+R)(R3, R4, R5, R6) ), in x possible gain settings that you want, and put in the resistor values of your choice.

If you were going to go the switch route, a 2P6T rotary would be the best plan. You would only be switching out R3 or R4, because the inner loop gain shouldn't have to change. If you wanted to fiddle with that, too, you'd use a separate switch and then connect it to R5 or R6. Switching all four resistors at once is much more difficult than it's worth.

But, what's wrong with putting a socket on the board and then trying different resistors that way? The switch method is good if you plan on changing the gain of the amp frequently, but if you're just trying to home in on the perfect gain level for your setup, swapping socketed resistors is probably going to be easier in the long run, and cheaper too.

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Good point. I'll probably just stick to that. Much more compact too...