The manual is not clear on whether it provides 500 mA at all voltage ratings. It’s more often the case that as the voltage rises the power goes down. It might be worth emailing their customer support to get clarity. For example, 500 mA at 9 V is not enough to power the H90. You would need to couple two 500 mA outlets. If the outputs are switched to 12 V, the question is does the outlet still give you 500 mA or is it less?500 mA might be enough if the Palmer allows momentary surges beyond 500 mA as other power supplies on the market allow like the Eventide PowerMax. At startup, the H90 spikes close to 600 mA, but the nominal rating is < 500 mA. If the Palmer can provide 500 mA at 12 V and it can handle a temporary surge, it will work.