Hello, everyone, I just scanned this forum, and was unable to find any threads that discuss the problem I am having with this Parasound A21 amp. So, I hope someone can help. John Curl? The problem is in the protection circuitry. It is far from straight forward like so many protection circuits I have worked on. At the moment I am trying to understand the function of D18. The amp powers up, but does not switch out of protection. Both amps function with a clean signal, and no DC offset. All the power supplies are looking good and stable. This leads me to believe there is some issue with the protection circuit. Come to find out, D18 is not developing a voltage across C73. Taking a closer look at that circuit, the AC feeding that diode is not grounded! The D19 bridge is working perfectly, so the AC is there. Yet D18 is not developing any voltage across the cap C73. I think this is due to a lacking ground on the AC feeding D18. Any thoughts?