I collect data from VLF radio stations for the Stanford SOLAR Center for detection of Sudden Ionospheric Disturbances (SID).

I've been using an RPi3 with the audioinjector zero sound card feeding the signal into the mic input.

I recently purchased the audioinjector stereo card and I am feeding the signal into the line in input.

With the stereo card on the RPi3 I can get very good signals above the noise floor as shown below:

I transferred the stereo card and the SD card to a RPi4 and I get this:

I've tried two RPi4's and two different power supplies (both from Canakit).

The RPi3 is working very well with the stereo card but I do not understand why the RPi4 does not work as it should.

Any thoughts would be appreciated.

Bill