I'm trying to block phantom power from a mic preamp. Phantom power is supplied as 12 or 48V (most commonly) from the preamp so as to power a condenser mic. A dynamic mic will ignore it. I do have a dynamic mic but the rest of my circuit (not shown here) cannot handle the phantom power.
I've found 3 versions of this on the interwebs, they all have
fundamental differences of opinion. Not just different component values. 2 of them are definitely outright wrong. I am not sure which 2.
In "my" version, attached, R6&R7 are described as load resistors, and R8&R9 as discharge resistors. This, I believe is wrong or rather incomplete. Sure, R6&R7 do in fact present a load, but we don't need that. We are just blocking the DC. Sure, R8&R9 do discharge the cap, but not on their own -- they require R6&R7 to complete the discharge circuit.
Q1: So, in fact I could remove all the resistors, yes? I just wouldn't then have a way to discharge the caps.
By removing the resistors I also no longer have any concern about the RC filters formed and the impedances presented to either side of the filter.
Here's the tricky part. I do want to discharge the caps. At some point the battery will need to be changed and I don't want 48V/10mA of charge there waiting to surprise someone.
My device is self-powered, with an LED indicator, so I would discharge the caps via that LED and an SPDT power toggle. (on -> switch out the discharge resistor; off -> poweroff but switch in a discharge resistor "R_d)". Bonus: LED fades to dark indicating cap was discharged. In the off state it won't actually go off until you also disconnect from phantom power.
Q2: Now, if I discharge the caps that way, do I have to worry about polarization while still connected to phantom power?
When R_d is switched in, the negative legs of the caps are open circuit (I'll be sure to use break-before-make switch).
The 48V circuit is not complete as there is no path to ground, however what is the effect, if any, of the 48V being now present on the negative side of the cap, across R_d? That negative side was previously referenced to the same ground potential.
My guess is that even though that negative cap leg is at the same "charge" potential as the 48V ground, there's no circuit because it doesn't present a return path. And even though we are now presenting the 48V potential to the negative side of the cap it's via R_d acting as a load, not as a wire. The + side of R_d sees 48V, and the - side 0V.
Q2.5: The cap in fact will not discharge as long as it's connected to 48V. Is that correct? But will discharge after removing the phantom power.