Measuring HT supply ripple with your Sound Card

Despite having used the Pete Millett’s SC interface to measure HT supply ripple, it does have a limitation as the input capacitor is rated 400V. Morgan Jones recommends on his latest great “Building Valve Amplifiers” (second edition) three methods for doing this. I opted the one shown above as it does help rejecting common mode noise and also protecting the SC interface. I’m careful enough to not revert the input by accident. If that should happen, the transformer used in this case provides a 500VAC (700VDC) isolation. Normally I play around 400-600V, so should survive in case of misuse. I played with LTSpice to tweak the secondary resistor down to 200Ω when input capacitance is raised to 10μF. Other transformers may allow reducing size of capacitor and increasing resistor up to 10KΩ. I have an OEP E187F at hand so this is what I will use.  f-3dB is about 30Hz and provides flat response at 50Hz and 100Hz to measure accurately input ripple.

Will build and report 🙂