Have you tested isolating the amplifier and preamplifier to find out where the problem is located? (remove the bridge on the back)
@Jakobsson, Good idea. I just did and it turns out that the startup/shutoff noise is coming from the preamp.
Illustrating that conclusion, let's start with the right speaker output at startup when the bridge between the preamp and the power amp has been removed:
The startup noise seems to be absent. (although that right spike looks like the intermittent noise I'm also experiencing but I'm hoping that fixing the startup/shutoff noise will fix that as well or perhaps adjusting the DC offset. In any case, one issue at a time)
Conversely, here's the right preamp channel at startup:
We can see by zooming in that it's a 50 Hz sine:
Interestingly, measuring the same right preamp channel at shutoff shows the same sine but it lasts about 2 minutes before tapering off like this:
which feels a bit like a capacitor discharging.
Here's the waveform zoomed in in the middle of the taper:
Not sure what more to make of this but I'm looking forward to hearing your thoughts on this @Jakobsson @kapitiaudio
Illustrating that conclusion, let's start with the right speaker output at startup when the bridge between the preamp and the power amp has been removed:
The startup noise seems to be absent. (although that right spike looks like the intermittent noise I'm also experiencing but I'm hoping that fixing the startup/shutoff noise will fix that as well or perhaps adjusting the DC offset. In any case, one issue at a time)
Conversely, here's the right preamp channel at startup:
We can see by zooming in that it's a 50 Hz sine:
Interestingly, measuring the same right preamp channel at shutoff shows the same sine but it lasts about 2 minutes before tapering off like this:
which feels a bit like a capacitor discharging.
Here's the waveform zoomed in in the middle of the taper:
Not sure what more to make of this but I'm looking forward to hearing your thoughts on this @Jakobsson @kapitiaudio