Thank you, to everyone, for helping me!
I forgot to add to my original post that I am just a hobbyist. That means that adding details (of both why and how to do it) to replies really helps. I am fortunate to have a rather nice lab, but the equipment is way smarter than I am. So, I can probably make most any measurement that would be helpful. Although a spectrum analyzer would likely be useful, as this is an audio project, I only have a scope. But, it has FFT, which may be useful, to some degree.
David's confirmation of my concerns with the filter being before the 1512 led to
- moving the filter to after the 1512,
- adding back
- a) Caps 1, 2 and 3 and
- b) The Bias Resistors R1 and 2.
Regarding C1/2/3 and R1/2. I had removed these components, because I thought (incorrectly) that the filter, in its former pre-1512 position, was also fulfilling the purposes of these components.
So, here are some new considerations.…
The Data Sheet does not clearly state that C1/2/3 are simply to filter noise, as would any other decoupling cap. I believe this is their function. Do I understand correctly?
Is there significant benefit to using a precision resistor array for the biasing resistors, R1/2, to warrant its approximately $8USD price?
Figure 4, of the 1512 DS, clearly shows the use of two types of ground…
- "Rake" symbol. This ground is true Earth. It is provided by the XLR Input Connector's case. It provides the exist for (only) C3. Do I understand correctly?
- "Inverted Triangle" symbol. This is the null point of the +/-15V supply. All grounds except C3 return to this point and exist via
Pin3 Pin1 of the XLR Connector. Do I understand correctly?
Benta did not expound on why he suggested that trim pots not be used to tweak the filter. I take it that there are two reasons...
- The setting is not critical.
- Trim pots are subject to thermal drift.
Do I understand correctly?
If so, I have left the trim pot footprints in place, to provide some options. They may be used for trim pots, a second fixed value resistor to be used in series with the first resistor, to give flexibility in attaining a particular value, or simply bypassed with a jumper wire, if not needed.
I would like to add David's EMI filter. See Reply #10/Figure 18.9, of the document that he provided.
- I believe it consists of the pairs C3/100pF + R7/100K(not 100R) and C4/100pF + R8/100K(not 100R). Do I have 100K, not "R," correctly?
- Which Ground does the EMI filter return to, Earth or Null? Earth is shown.
- Exactly where is it inserted into the circuit? After the biasing resistors RBIAS1/2?
Again, I am grateful for your help, expertise and effort to reply. We have a great community.