SP 20 does not switch on when connected via the 3.5 mm jack
SP 20 turns on when it detects a bias voltage on the microphone terminal. If you just play music for example, the device may turn off the bias to save power.
SP 20 can be forced on by pressing and holding the red and green buttons for 5 seconds. This can also be used when listening to music from a MP3 player.
SP 20 does NOT turn off automatically, if it has been turned on manually.