[M.Gher.]

Hi. From the beginning, I apologize for my English. I'm a 50'er east Europe guy, 80-90's  music lover . Actually, I like the sound more than the message of the music, and i'm improvement addict. Lately i use NAD C658 and Audiophonics LPA-S400ET for auditions (with SPARKOS LABS SS3602 as replaced OPA's). The only thing to complain about is the noise from the speakers when turning off the power station, which has already burned a tweeter ( I made a speaker protection by myself after that, but it also produces a not very Hi-Fi sound when the relay is turned on). I'm thinking of an upgrade. Help me decide between the NuPrime ST-10 ( ex-demo 1000 Euro offer) and the NAD C268 (650 Euro offer ,which I could also use as a monobloc because I also have a NAD C368). Thanks.

[FullRangeMan]

Welcome to AC 

[AllanS]

Welcome aboard! 

I have an NAD AVR I like but I can't offer any experience with these models.  You might find the NuPrime website has a good reference that describes the difference in amplifier sound characteristics.

https://nuprimeaudio.com/guides/amp-comparison/?v=7516fd43adaa

Best of luck!

[toocool4]

Welcome to AudioCircle M.Gher.

One solution to the thump when turning the amp Off or On is to mute or have the volume to the lowers setting before you turn Off or On.
With regard to which amp is best for you, my best advice is to take your amp to the dealers or better still borrow the amps you want to try and try them in your system at home then pick the one you like best.

[M.Gher.]

Thanks for the reply. Every time I reduce the volume to mute before closing the power amp, but the Audiophonics LPA does not have a 12v trigger, but a compromise solution:12V Slave Power Supply Device (https://www.audiophonics.fr/en/diy-kits-and-boards/audiophonics-trigger-12v-230v-slave-power-supply-device-p-10661.html?search_query=12v%20trigger&fast_search=fs).So when remotely close the preamp it's like pull out power corde from power amp and the crackling still appear no meather the volume are ( the capacitors discharge). Regarding the try of the product: i live in a small town and the dealers are miles away, so i cannot try any. Thanks again for your answer. Respect, M.Gher.

[Phil A]

Welcome!  How big a room?  How efficient are your speakers and what are they (and what's in the rest of the system)?  How loud do you like to listen and to what types of music?

[richidoo]

Your broken tweeter is a strong sign that you have uncontrolled oscillation. Only a high frequency signal can make it through the  tweeter's high pass crossover filter. The damaging signal must be ultra sonic because you don't hear it, and with large enough voltage to fry the tweeter voice coil, meaning it's probably full amplifier voltage, which means most likely the amp is oscillating, not the preamp. Probably the amplifier is running hot?

Most amps have an input capacitor which blocks DC on the input signal. And most preamps have a output capacitor to block DC on the output, so it's highly unlikely that the DC offset causing the thump or pop on shutdown is coming from the well engineered NAD preamp. IMO, the Sparkos upgrade opamp in the amplifier is the culprit. Opamp replacements must always be checked for oscillation while working in the actual amplifier circuit with an oscilloscope. Please replace the sparkos with the original opamp that came with the amplifier to see if there is improvement.

Oscillating amp can overheat and even catch on fire, so only run it for testing until it's fixed.

DC offset is what causes thump or snap when powering off. The DC offset can be caused by oscillation.
Test for DC offset:
Disconnect the amp from everything - standalone. Use a voltmeter set to VDC to check the output voltage of the power amp on each channel. It should be zero +/- 0.05VDC
You can have oscillation without any DC offset, and it will still cook the tweeters, but it probably wouldn't pop on shutdown.

If the amp behaves fine when running alone by itself, even turn on and off while connected to speaker without pop, then cause is preamp or signal source.

As for your question about class D amps, I have Purifi and it is amazing. It is better than ICE and HYpex UCD. I have not heard NuPrime. Denafrips AB even better than Purifi.

[ArthurDent]

Greetings & Welcome to AC M.   

[richidoo]

I see that Audiophonics installed the Sparko opamp, not you. I have friend with Purifi amp with sparko, doesn't have any problem.
Still check for DC offset with amp standalone as it might be broken anyway.

Try shutting the amp off with amp power switch, not the power strip. This will make the amp shut off before the preamp shuts off. Maybe that will avoid pop if the NAD is making a transient signal on shutdown.

[M.Gher.]

18m2. Technics sb-ca1060 modified ( i replace all the electrolitic capacitors with non polarized) , with supertweeter Visaton TL16H( that's the broken one) , with high pass filter 17000Hz.
The crackling was present until the opa replacement ( i do it by myself with audiophonics specialist guidance). And is about confort: one button start the system and shut it down. I already said the problem with Audiophonics was cosmetic. I just want to know short time( flying offer) what to choose: NAD combo or Nuprime ST-10. Thanks.

[M.Gher.]

I took the NuPrime ST-10. Time will tell if was a good decision. Consider close topic. Thank again all  those who spent their time for me.