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There are a couple of issues I see here besides the ones that Matt and John already pointed out.
The issue with relays is how do you get quiet changes in gain using relays. I haven't looked at the specs of the chip, but the issues of break before make are not insignificant here, especially when deal with high amounts of gain. Are you planning on switching in a single resistor or are you building a pot out of a series of resistors?
Also, getting usable steps with only 12 positions is virtually impossible. We are talking something like 5 or 6 dB per step. A little course for most applications. This requires a high gain position with a single gain stage (Like the Hardy M1) which makes the gain steps non linear in one or the other gain range or a multi stage preamp which has a second stage with a fixed 18 or 24 db of gain. I am of the school of thought that the 2 stage preamp has some real advantages over the single stage WRT front end headroom and amplifier linearity/noise.
All the best,
-mark
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