I've made an amplifier from a circuit I found on the internet (below, link). I made it more or less exactly as described (same components, big heat sinks) and it seems to work great (can run at a loud volume for extended periods of time). Sometimes, however, for no apparent reason, the fuses blow and the MOSFETs have become completely conducting. I understand this can happen if a high (> 20V) voltage is applied to the MOSFET gate. No other component is damaged.
I've completely re-made the circuit about 6 times and it always happens on the same channel. The amplifier attached to the other channel might last for weeks, until I test it on the bad channel, then it breaks within a day or two. So I think what's happening is that one channel of the sound source (a cheap bluetooth receiver powered by a questionable 5V supply) occasionally has short voltage spikes, causing an over-voltage on the MOSFET gates and breaking them.
I know I could get a better source, but I want to make the amplifier better. So my questions are:
- Is this a plausible diagnosis?
- If yes, what can I do to make this circuit more robust against a "bad" input?
My first idea is to put Zener diodes between the gate and source of the MOSFETs (say 15V threshold diodes, the gate voltage should never get close to that during normal operation). Is this a bad idea? Or is there something else I could do?