50V DC Power
DC-DC Power Supplies
Since the HR50+ is so small and light, it seems like a great choice for a portable/mobile amplifier. That is until you try to power it. Fortunately there are inexpensive DC-DC converters available. I have had good results with this 1200W Boost Converter and it may seem like 1200W is overkill for a 100W amplifier but when you dive into the ratings you can see where the problem is. It's rated at an input of 10-60V and an output of 12-83V. The HR50+ needs 50V at 4 amps, which is well under the device's output rating of 10 amps. But if the input voltage is 12V and the supply is 90% efficient, the input current will be 18.5A wich is dangerously close to the 20A max input rating of the device. If you are using a decent 13.8V power source this converter should be fine but if you intend to use a battery that will drop below 12V as it discharges, you might want to lower the output voltage to 40-45V to prevent blowing the input fuse or tripping the current limit.
You can use the PowerPole jumper removed from the Hardrock-50 on the input side of the converter to connect to the low voltage supply. Use the Hardrock-50+ connectivity kit has everthing you need to connect the 50V side to the Hardrock-50+.
AC-DC Power Supplies
There are several choices for AC input power supplied that supply a 48-50V 4A DC output.
The Mean Well EPP200-48, which is available from vendors like Mouser and Digikey, is a great power supply for the Hardrock-50+ you can adjust the voltage to 50V and it's small size is a real bonus. For higher power dute cycles it requires a small fan but it has a place to connect a 12V fan that adjusts the fan speed based on the power supply load.
For a full enclosed power supply ready for you to install the correct connector, the Mean Well GST220A48-R7B is a good choice. It is a bit more expensive than the other choices but it a good turn-key solution that does not require additional cooling. There are also cheaper 48V 4A AC-DC supplies and I'm sure that some of them are just fine, I can endorse these supplies and I have verified that they woul with the Hardrock-50+