This is example of sound card modification in order to work as a oscilloscope or another voltage sensing device. This works only with few sound cards which does not have hi-pass filter implemented in digital processing unit. This is the case of sound card based on chip CM119 made by C-Media Electronic, this particular modification have been performed on ultracheap Axago ADA-20 card, but should work for other too.

Basic modification was performed in 3 steps:

1. remove power supply bridge for microphone (R6 on picture)
2. bypass DC filtering capacitor
3. break out voltage reference as a new (relative) ground (pin 7 from left corner on piture)

Modification shown on picture above have one small bug, it takes signal before filtering coil (L5), but should be placed closer to original microphone input in order to filter out high frequencies.

Card input have voltage offset (2.25 V) in order to simplify gain stages and ADC conversion. This is the reason why voltage offset output is used as a ground for measurements. In order to protect the card against damage the input should be decoupled. This is well described elsewhere and is not used in our modification because is not required.

Another variation of this modification