- Some suspect that the issue is caused by static spark... is plausible, not confirmed.
- Some says it's not likely to be the case based on the burnt position of the board.... not necesarilly true
- Static up to 5kV have not reproduced the issue at topping's lab... so it seems, however, the human body can hold around 20kV. Devices may be able to withstand 10kV but can fail at 20kV.
- Some people felt 'spark' or 'electricity' when they touched the knob or housing before the incident... the moment one feels/sees a spark the body discharges through the amp to ground (most likely via the connected DAC)
- The 'spark' could have been a current leakage, not a static, indicative of a short or a failure on the board.... I don't think so. A leakage current would always be felt, have no spark and would indicate that the amp is not grounded.
- The DC protect circuit is updated after '2012' batch .... yes, this will prevent the headphones from being blown up in case something goes wrong inside the amp leading to DC on the output of the amp. (there is a slight delay so a loud pop could still be heard)
- Some suspect that the board is lacking protection circuit for overcurrent / surge at power input part... some are wrong.
If my understanding written above is correct, could it be the surge from 15VAC input that have caused the short or break inside the amp?
No...
If so, can we add some breaker / fuse between 15VAC input port and 15VAC output cable, to protect the board from the surge?
No is not the problem.
There are regulators inside.