Sorry I missed this message. Thanks for PDFs, i will read them. But 20dB gain is too low for me anyway. So could I omit those parts by increasing gain to lets say 26dB?
No. You still need Cc = 180 pF; Rf2 = 20 kΩ; Cf = 47 pF.
But why it is bad to increase LM3886 gain to 26dB?
Increasing the gain to 26 dB means 6 dB lower loop gain available -> 6 dB worse on most parameters. Andrew is not correct that the LM3886 will operate at compromised stability at 20 dB gain and reducing the loop gain as proposed in the .pdf by Bob Pease, will reduce performance as I described two sentences back.
You'll get better performance if you leave the LM3886 with a gain of 20 dB and put the remaining 6 dB up front in the instrumentation amp front-end. This is because any modern audio op-amp will have better THD and noise than the LM3886, hence, applying the max amount of loop gain possible to the LM3886 will result in the best performance.
What I mean, maybe this would be already stable:
[IMGDEAD]https://i.xomf.com/vxvrh.png[/IMGDEAD]
It'll be stable as long as the output voltage is away from the supply rails. Once the output approaches the negative rail, you'll get oscillation unless you add the stability components.
As Andrew correctly points out, R4-R7 need to be pairwise matched. I.e. R4/R6 = R5/R7 for optimum CMRR. If you use 0.1 % resistors, good layout, and good thermal matching, you should be able to get down near 60 dB CMRR. Considering that resistors R6 and R7 have much higher dissipated power than R4, R5 that may be tough to achieve.
Also: R3 should be 0 Ω.
For some reason i can't install Tina Ti... I want to try to simulate it.
Contact the applications team via the E2E forum on TI's website. They're paid to help you get it going.
Tom