laurencebarker wrote:I got permission from Abhi to publish the schematic of the Mic preamp board for the G2.
Laurence,
Thanks for posting the schematic. That's a good solution to stabilize the op-amp's Vcc/2 bus. It's strange not to see a ~22pF compensation cap between NE5534 pins 5 and 8 to prevent oscillations. It's not a unity-gain stage so it's probably forgivable.
It would be best to replace FL1 and FL2 with a bifilar-wound SMD common mode choke. Mount it physically close to the NE5534 input.
High mutual bifilar coupling between each L on a common core result in 2x the series inductance in common mode and complete inductance cancellation in differential mode to preserve input waveform integrity. Preserving waveform integrity is certainly much less of a concern in bandwidth-limited systems, but why not use the right choke and get superior RFI abatement?
I'm currently using the device linked below in my homebrew front-end audio designs. They're commonly used in datacom equipment where it's imperative to preserve a sharp-pulse digital waveform without fear of inductive overshoot and ringing.
Like all RF chokes, a peaking frequency occurs depending on ferrite composition and turns. Choking Z for the Bourns 475 type is 10K-ohm at 160m, diminishing to 2K at 10m. Optionally, adding a 513 type in series offers superior rejection through 6m. Price is $0.50 USD in quantities.
https://www.mouser.com/datasheet/2/54/dr331-777616.pdf
Paul, W9AC