Microphone Amplifier: Shielding

My friend owns a ribbon microphone, which have notoriously low output levels even after their internal transformers steps up the output, and always need to crank the gain up to the max and use noise reduction to get anything usable. A common solution is to use an in-line preamp such as the Cloudlifter and the FetHead. I wanted to see if I could make my own amplifier after seeing a small snippet on ribbon microphone amplifiers in the Art of Electronics. 

After a little bit of Googling, I came across reverse-engineered schematics for aforementioned amplifiers. They both use JFETs in a differential pair, which allow for the gates to be biased at 0V straight from the microphone, and are powered directly off of phantom power with the phantom power resistors as the load. The Cloudlifter has a cascode configuration of JFETs, while the FetHead is just a single device. Someone already made a FetHead clone, and to keep my project simple I essentially cloned this. I added a few changes to my PCB:

• Added a footprint for a two channel ESD diode device
• Added places for emitter degeneration resistor if one wants to add them
• Used the connector that has the shield separated from ground both the input and the output XLRs
• Added jumpers that allow for configuration on how the shields are connected to each other/to ground

I was really interested in seeing how to properly shield the amplifier and the effects of grounding the shielding at the amplifier versus not. From several videos I watched on connecting up the shield in general for cables (not just audio and XLR), the idea is to form a continuous Faraday cage to keep out noise. You want all the shields joined through the cable to the chassis of all the devices in line. Whether you ground this shield at all points vs just at one point or only to earth or not at all (no DC connection but AC coupled) I'm still not clear on. With all the jumpers I wanted to test what happens under different conditions.

KiCAD Schematic.

I sent the boards out to made and ordered the parts from Mouser. A week later, everything showed up at my door and I filmed a making-of video just for fun:

I still got the footprint wrong for the ESD diode; I originally chose a SOT-23 part but changed to a lower voltage part just before sending out the design and forgot to change the footprint for SOT-523. Thankfully the part still soldered into place without much issue.

PCBs.

Parts.

Assembled.

The metal box I originally thought of putting the amplifier into ended up being just a little smaller than I thought, causing some interference with the connector screw holes. For now there is no enclosure, making proper shielding tests impossible. However, I can still generally test the amplifier. I handed off the amplifier to my friend and we saw significant gain (I don't really have any equipment setup to do a proper measurement of the absolute gain at home anymore and definitely don't have any to do a portable measurement.). There was very audible noise when there was supposed to be no sound but we'll need to figure that out another time. Some initial experiments with joining the shields on the board and to the ground gave inconclusive results along with a jank foil shield. The cable being used is also of questionable quality.