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A potential issue with running 2 switching PSUs in series to make a dual rail supply is heterodyning due to the slightly different, out of sync switching frequencies, which can beat against each other in the audio range. It might be possible to mod one PSU to clock from the other, instead of its...

I'd add LCSC to my top supplier scan list, in addition to the usual Mouser, Digikey and so on. They're reliable and usually cheaper than others. Resistors are about a dime a dozen there. Literally.

I've been getting my MN3207 from an ebay seller, Polida I believe. Dozens of chips spread over several orders across the years. They all worked. I only threw away one chip because it was significantly noisier than the rest. They're probably not genuine Panasonic, but whoever makes them must have...

Valid questions. R7 is often used after an op amp to insulate its output from the effect of a capacitive load. Otherwise it might oscillate. R6 is just weird, just another path around the op amp. Second op amp not really needed, but sometimes they use a dual part when a single is enough just...

Woohoo, no blend! But no THAT chip in That compressor? Click bait 😁

Now that I took a better look, I wonder what C4 is doing there. A 100nF cap right across the output? That's odd. Can the op-amp drive that without oscillating? (Not that I haven't done much worse myself 😂)

An attempt at an RF filter. You can probably omit all 3 (jumper the resistors) and nobody will know. C2 value is too low to have much of an effect anyway.

While it's squealing, try spreading these wires apart. Also try rerouting the yellow in/out wires.

Consider also TINA-TI (free from TI) and SIMetrix Elements (also free). In addition to pots, Tina also has switches that you can flip in real time as you're monitoring a waveform or whatever, so you can A/B circuit options on the fly. Very very useful. And most TI opamps have detailed Tina models.

Reading in Angry German's voice: AktivBuffahSplittahBendah!

The circuit diagram itself is fine, I have built it in the past and so did quite a few other people around me. It makes a great mic pre, on par with just about anything out there in the pro audio world.

Any mic pre will make more noise (hiss) when there's nothing plugged into the input. Mic pres need to have a low impedance source (on the order of 200 ohms or less) to achieve their lowest noise. Short the input XLR pins 2 and 3 together. Still have noise? I'd try troubleshooting with a clean...

Also, you have a DC-DC switching supply extremely close to the most sensitive audio path in the whole circuit: the gain loop. That really won't help with noise. Try disabling it and using an external +/-15v supply for troubleshooting.

Does it do the same thing with the input shorted or with a mic plugged in?

^ This. Sometimes you just need a low impedance Vref. This means that it's able to supply (or sink) significant current without moving up or down with the load. A simple resistive divider made with sensible values, such as 100k or 10k, can't do this.

It's safe to use the low level 1/4" output with a simple mono cable and just call it a day. Shorting the unused Channel to ground in this fashion does not harm the unit at all. Don't do this with the 1/8" headphone output though. There is a subtle difference between the L and R outputs: one...

^ there's a weird path to ground from the emitter of T1 which might explain why it works:

Well, I'll be damned. It seems to work in Tina! You should have taken the bet, you'd be up $20 :) The only way I can imagine it working is if the PNP transistor is simply used as a passive clipping device. Maybe someone more knoweledgeable could explain how the darn thing actually works??

Ok, now I'll really put it into Tina if I find some time. As drawn, it really looks to me like it shouldn't work. Will let you know.

Easy. Put the whole thing in Tina-ti and see if it works. I have $20 that says it doesn't. But what if I'm cheating and I've already done that? 😁 (I haven't)