I hear AC701 is easily self-destroy by oversupply of heater voltage. What may happen after oversupply?
I was to careless to use AC701 with around 6V heater voltage for some minutes. After that I can hear much white noise or random noise with 4V heater. (I didn't check the noise before that) Other than that, it seems to work well.
If problem is only the noise, I'd like to use it for "not noise sensitive application" like loud vocal or something.