[CharlesKu]

My systems are OPPO sonica DAC (which play as my player + pre AMP + DAC), a pair of NuPrime STA-9 power AMPs and KEF LS 50 speakers. It doesn't sound as good as my wish in this systems. I agree with Nuprime confirmed that the STA-9 did not work well with the KEF’s due to an interaction resulting from the addition of even order harmonics in the STA-9 and the KEFs. I will change the speakers for test later on.

[CharlesKu]

I found it if I change the speakers to B&W DM302, it sound better than KEF LS 50 with STA9.

[maty]

Maybe the problem the Nuprime STA-9 with the KEF LS 50: low sensitivity + low impedance at HF.

https://www.stereophile.com/content/bw-dm302-loudspeaker-measurements

I measured a B-weighted figure of 90.1dB/W/m—this mini will play quite loud with only a few amplifier watts. However, its impedance plot (fig.1) reveals that it drops below 4 ohms for much of the midrange, coupled with a moderately high phase angle in the upper bass. Wimpy amplifiers need not apply for the job of driving this speaker.

https://www.stereophile.com/content/kef-ls50-anniversary-model-loudspeaker-measurements

My estimate of the KEF's voltage sensitivity was 84.5dB(B)/2.83V/m, which is within experimental error of the specified 85dB. This is a little lower than average but 2dB higher than the LS3/5A's.

Somewhat optimistically specified at 8 ohms, the LS50's impedance (fig.1, solid trace) drops to 4 ohms at 200Hz and to 5.4 ohms at the top of the audioband. The electrical phase angle is generally mild, but the combination of 5.3 ohms and –41° at 135Hz, a frequency where music often has high energy, will make the speaker work at its best with a good, 4 ohm–rated amplifier.

[maty]

More class-D amps have an awful frequency graph like:

[CharlesKu]

I found that sta9 better pair with soft dome like coated textile diaphragm as ScanSpeak Revelator D2904/7100-03 Wide Surround, Black Face Plate. It is now become my refence speaker instead of KEF LS50.

[rustydoglim]

Please don't publish plot from other amps here, to speculate about the FR of STA-9, many users are not technical and could be confused.
Regardless of the reasons, we don't recommend STA-9 with LS50.  Again, the same reviewer who has LS50 recommended IDA-16 product of the year award.  So this is about matching the sound characteristic that we have intentionally tuned.

Or put it differently, STA-9 was internationally tuned to be on the warm side, whereas all other NuPrime amps are neutral.  NuPrime is probably the only class-D/hybrid amp company that has many different designs in-house instead of tuning some off-the-shelf modules.

If we are using someone else amp module (IcePower, Hypex for example), we will be limited to changing power supply or perhaps some opamp and capacitors for sound tuning. I asked our chief engineer why he come up with a new design for every new model. The answer is that we have a specific usage for each model, so it make sense to do so.  In the NuForce days, all we had was the same fundamental design that evolved over 10 years. In the early days of Class-D amp evolution, engineers were still learning how to control and improve upon a design.  Think about how car's engine evolve over the years.

It is interesting when you compare with cars. however advance is the gas powered engine, it will soon be surpassed by electric motor. Soon most of the electric family cars will accelerate faster than a million dollar sports car.  Can we take class-D/hybrid design to the next level? I am sure about this