I'm building a KGSS and I'm not sure how I´m supposed to measure the bias voltage. Am I supposed to get 580v before or after the 4.7 Mohm resistor. It is better to get this right the first time since I do not want to fry my Omegas....
If you measure after the 4.7Mohm resistor, the high value current limiting resistor in your voltmeter is in parallel, which will change the reading. This is true for any resistor, but for most small value ones the change is minimal. Measure before and the 4.7Mohm resistor is out of the loop.
Afaik, the bias voltage doesn't have to be too exacting. a 10v variance is just under 2%.
According to Kevin Gilmore, Stax phones are nigh indestructible. As I recall, he did mention one scenario where the phones could be damaged: if both amp channels are pegged at -350v, then the bias voltage could go too high and damage the phones. Pretty unlikely though.
Since headphones take at least half an hour (according to Stax) to charge from a discharged state, my bias supply is on all the time run from a small transformer (allowing the main one to be switched off and thus taking out the significant idle dissipation of a large transformer), and also by a pushbutton allows the user to start a timer that applies a higher initial voltage (by switching in an additional multiplier stage) for a several minutes to make the charging much faster. I posted details at headwize.