Hear the heartbeat?

Catrina • 25. Married to my other half. 5-15-15. 1 angel baby named River. Daughter named Remington. Son named Journey.
What is the soonest you can hear the heartbeat? 
I am going to buy a Doppler to have at home but I'm not sure when we can hear it. 
We lost our first baby in October so I'm a little paranoid.