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You fail to specify if the Nuprime is new or used. Pops on turn-on usually mean bad caps some where in the circuit. I took possession of a used BAT VK300-SE that hissed like spray paint through my speakers. The seller had me send them to BAT who fixed them at no charge (Vicktor, you're my hero!). BAT said it was a "bad voltage regulator".
NuPrime is a Class D amp.
Some ClassD’s do sometimes have a nasty (edgy) turn on sound, not a soft bump that some linear amps have.
And they can also depending on the output filters roll off point, emit HF continual noise from the tweeters.
I’ve seen results over time of these two noises continually combined cook (blue’d coil) a Wilson 8 tweeter by so called highly regarded Class D monoblocks, that had the output filter that rolled of too high, the owner would turn his system on and off quite a bit as he was a reviewer, it had a nasty sounding squelch at turn on from the tweeter and had continual hiss from the tweeter you could just hear from 1.5mts away.
The hiss noise is not volume control dependent on class D’s, which means it does not emanate from the gain stage.
It’s there on some that have too higher set a final roll/off filter, eg flat to 20khz or more. And this lets through the switching noise through to the speakers which is heard as hiss from the tweeters.
The Nad may have filtered it out by setting the final roll/off filter lower (<20khz), and thus subdue the hiss much more.
BTW I had to replace both tweeters on the Wilson 8’s (which weren’t cheap) as both were severely compromised after all that constant punishment over time, needless to say he got rid of the ClassD monoblocks after that, and purchased a good linear amp.
Once the technology is available in the near future to get the switching frequency 3 or more times higher than it is now to say 2 megahertz or more then the output filters can do the right job without getting too close to the audio band and effecting it with early HF roll off and phase shift, as it will be able to be done well away from the audio band and leave it untouched.
One of my aging Canadian class D amps sounds pretty rude upon powering up and down. It sounds perfectly well other than at those times. I had a hunch discharging caps might be the cause at power off, it does an almost typical sounding "pop" but its accompanied by harsh static or crackle. Ive grown used to with other amps that went on working well into the future. Never had one make a noise during power up, and its not a pop Im not even sure how to describe it other than brief and nasty. Im tempted to buy new caps and wire them in and take a blind crack at it anyway to see what happens.
You probably have a different version of output filter to the OP. Some other Class D manufacturers changed theirs every 12 months V1 V2 V3 all output filter changes, just to keep owners interested.
Resulting in different amounts of hiss,, less or no hiss = lacks detail and top end extension, distant. Much hiss = lots of detail, harshness, and extension, in your face. Other versions somewhere in the middle.
This is the crux of the class D, the "switching frequency noise" is still too low to be filtered out without effecting the audio band. When the technology comes in the future to get the "switching frequency" and it's noise up 5-10 higher so then the filters can do their job many octaves away from the audio band, then we'll see class D's take over and bury linear amps.