We have BOTH systems in our fleet.
In the Leased birds, the CVR starts automatically after 1st engine start [N2 >50%]. The acft is also provided with a "Voice Recorder" switch which has 2 position - ON and AUTO. As part of our Pre-flight procedures, the switched is moved to the ON position. As the 1st engine accelerates through 50% N2, it automatically moves to the AUTO position. The CVR stops automatically 5 min after last engine shutdown.
In our own acft, the CVR runs as long as AC power is avbl. And the "Voice Recorder" switch is also not installed.
Hope this helps