The voltage needs to be the same, the mA is like the amperage, problem might be if the adapter can only handle 600 mA and you need it to handle 700 mA, that could either overload the adapter and damage it, or the voltage may drop because it is overloaded and it could then damage the device. To be safe you need an adapter that has a 9 volt output and can handle at least 700 mA or more.