[RodB]

To get the C voltage, you can use a 9 volt battery with a voltage divider. That way you can use high value resistors and adjust their values to get any voltage in between. For example you can use 330k to get -3v, 470k to get -7v and 200k to complete the divider. Remember, these are bias voltages and have a fuzzy tolerance. No need to be exact. Something like this, the battery would probably last a long time.