fetal Doppler question

What's the earliest that you can pick up a heart beat? Just went to the doctors today and I have an ultrasound scheduled next week (I'll either be 5 or 6 weeks) and she said I'll be able to see a little speck of something, the yolk sac and possibly hear a heartbeat. I had a miscarriage in January so depending on how that goes, will determine how heavily monitored I will be. That being said, I have a Doppler at home and will be checking everyone once in a while. So what's the earliest time with an at home Doppler?