Rail splitter for variable state filter VGND

[abbey road d enfer]

No other than distraction honestly, sorry for that. But it is so much better to get its precise function now (rail decoupling, immediate supply source...) Is ceramic recommended/enough? Or polyester film can do noticeably better? I'm thinking on a 100n value.

Film has no advantage over ceramic here.

On the output capacitor, I understand that the build-out resistor (R100) in combination with the bypass cap (100u) form a low-pass filter with its knee at 16Hz. The little extra output impedance of the opamp would bring this down a little bit further down, leaving everything above it clean for audio AC current to flow freely after. Am I right?

Remember that a 1st-order filter still lets higher frequencies pass. They are indeed attenuated, but they may pollute a sensitive node.
As it depends on the rest of the circuit, you need to experiment.
You have to keep in mind that it's not a unidirectional situation. The rail splitter delivers DC but it also receives audio currents from the circuits.

 

[Newmarket]

It depends on several other factors.
With 100nF, the impedance seen by the opamp's input is low enough (1.6k @ 1kHz and decreasing with frequency) to null the effects of Input Noise Current on most opamps (except those with sub nV/sqrtHz INV), so I don't think there's any serious noise issue.

Can't argue with the numbers there. Agree no significant noise issue in the terms outlined. My instinct to 'Go Large' is really because I can - that voltage doesn't need to be going anywhere fast (ignoring power up delay) - and probably a background in defensive design for difficult electronic environments, in particular heavy duty off highway vehicle stuff. Not really applicable to an audio eq I know

To the OP:
On the capacitor on the op amp output - this brings us to the 'classic' problem of OpAmp capacitive load. If you want to explore beyond the 'Resistor Outside the Loop' solution as discussed here then you can look at more complex solutions. eg Resistor in side the DC feedback loop with a capacitor in parallel.
See below for an image of this sort of thing albeit from a different application. But it shows the basic idea. Having the resistor inside the DC feedback maintains DC accuracy at Vout. Meanwhile, hf feedback via the feedback capacitor is isolated from the load capacitance. Obviously values required vary according to the detail. You could probably lose the 100K (ie direct connection) but it's a good idea to allow for a resistor in that position with some opamps.
Of course, I'm not saying this would be of any real benefit in your application.

(on the capacitor type - just avoid X5R, Y5V ceramics. Otherwise X7R Ceramic / Poly all good.)

 

[FIX]

Not sure what you were trying to say here. The AD711 is not a current feedback amp, it is a traditional voltage feedback design, so I don't see that adding a resistor does anything except add an additional pole with the input capacitance, which would tend to worsen stability, not improve it. For a unity gain buffer the easiest layout is just a short trace from the output back to inverting input directly under the op-amp.

If you increase the feedback resistor, the gain drops from full blast, but still unity, which is the lowest pole frequency, depending on the amp can be as low as 160Hz, dropping the gain moves that pole higher, and it's easy to compensate, and in audio amps, it sound really sweet, with DC stuff, the amp is a little happier. Externally, the amp is unity and the pole is way out, internally, the gain is full blast until it isn't... This is why old op-amps needed to be compensated (the 2 pins) if run at unity. In one conversation with a well known designer that also designed his own op-amps, I told him what I did for unity gain, "I designed and run my op-amps with a feedback resistor at 10K, and don't care about unity gain stability, I optimize the amp for anything but unity" his reply, after he thought for a second, was "Spending all that time making my amp stable at unity is probably my biggest regret"...

 

[FIX]

One is enough.

I'd rather keep the one that goes to ground.

YUP, the rule is always source grounds go to the quietest point, and always source caps like that to the quietest point, which is ground.