Tha value of the resistor after the 1 uF determines the LF cutoff. IIRC in my second stage I have a switch there with 1 M having a 0.2 Hz cutoff for ultra low sq wave tilt* and two much lower values to permit a 6 dB/oct HPF for the real world at 20 and 80 Hz. Note that the 2k value to produce 80 Hz is the minimum load the 1510 will drive.1. There's a 1M resistor right after the 1uF/250V cap. In the other circuit it is 100K instead. What value is correct?
2. In the filter section there are two optional caps. If they have to be omitted, should they be short-circuited?
3. I've noticed that the trim stage is inverting. Does it mean we get the inverted signal at the output (I don't see any other inverting stages)?
I was looking for a phantom power switches, and most miniature switches are rated for 5A @ 120V AC or 28V DC. I wonder if it's safe to use them @ 48V and small phantom current?
*(Tell me how that one sounds on kick drum please.)
Those optional caps are there in stage two just in case they're needed. I expect that stage is best made a follower without gain but I also placed an optional resistor from the inv input to ground in case someone wanted a bit more post-gain. Post gain offers higher values of Rgain in the 151X stage and can also allow the 151X to drive a lower impedance termination to move the HPF upward. (The 151X minimum load is limited by drive current.)
Good catch. Yes, the trim stage does invert. It's not obvious, but I flipped the 1646 outputs (redefining them) so that the 1646 effectively re-inverts the inverter. Overall it should follow correct polarity.
Have you considerded a relay or dual photoMos relay for switching phantom? A PS7113-2A (dual) works very well.
http://www.necel.com/opto/en/pdf/PN10269EJ02V0DS.pdf