NuPrime AGM STA stereo power amplifier
Review samples provided by Dimex
Retail Price: 1.595 euro
Regular readers will know that I have very good experience with NuPrime amplifiers. After years of use with all sorts of speakers and countless comparisons with other amplifiers, I still feel that the ST-10 stereo amp with its proprietary Class-D technique offers a unique proposition and amazing value for money. It’s not without reason that I still use it even in the context of equipment costing tenfold.
Of course, at 1.595 euros, it is not going to outperform state-of-the-art, high-end products, but in certain important aspects it equals some of them and in other aspects, it comes far closer than it should when considering its modest cost. At the exact double rate of 3.190 euros, the ST-10M mono versions retain all of what makes the stereo model such a great amp while addressing the main areas in which the stereo model falls short in my opinion: treble refinement and overall resolution.
Up for review here is another NuPrime amplifier but, this time, one with an enhanced working principle: the AMG STA that offers a different sonic perspective and costs exactly the same as the ST-10.
AMG STA Technical
The ST-10 and ST-10M amplifiers are pure Class-D designs albeit using a proprietary circuit. The AMG STA amplifier uses a similar circuit but with the switching frequency raised to 700kHz and fitted with an analog input with a 1M Ohm input impedance. Another important difference is that the ST-10 uses a high-end opamp for the input stage whereas the AMG STA uses discrete Class-A transistors.
I asked the manufacturer for more information. Jason Lim replied:
“Our design is different from other Class A + D designs, because we designed the Class-D amp ourselves, which enables us to combine the Class-A preamp stage in series with the Class-D power stage. For other manufacturers who have to purchase an OEM Class-D module, the principle of current dumping is to take a Class-A in parallel (bridge balanced) to supplement the sound quality of Class-D. This method addresses the harmonics/warmth issue of Class-D so that it can sound richer, but it still doesn’t fix the sampling rate problem of Class-D. Over the past 15 years, the switching frequency/sampling rate of Class-D amps has steadily increased, but for off-the-shelf amp modules (ICEpower, Hypex, and other lower quality class-D amps using reference designs from MOSFET suppliers), they have cost consideration and other priorities. Nuprime continues to push the envelope of Class-D design, such as using Class-A transistors and also raising the switching frequency to 700KHz (far above other off-the-shelf Class D modules). Therefore, the approach taken by Nuprime Class A+D design is not only to provide “richness”, but also improve the resolution.”
There is more to read more about this, as well as other techniques, on the Amp Design page of the NuPrime website.
Powered by a large toroidal transformer and with 2 x 130 Watts into 8 Ohms and 2 x 200 Watts into 4 Ohms, the AMG STA will be powerful enough to drive any loudspeaker, including Apogees, Logans, Magnepans and Magicos. And yes, that includes the rather difficult Q5’s, as I will report on further down. Naturally, if you push it far enough I imagine that there will ultimately be a limit to the available current. But for the playback levels that I can tolerate and with any of the speakers that I tried it with, it is more than sufficient.
With a smart front panel but an otherwise simpler enclosure, NuPrime managed to keep the price of the AMG STA the same as for the ST-10, in spite of more complex electronics. The front panel is machined from a solid piece of aluminum with a protruding NuPrime Brand name. Unusually, the power LED is mounted underneath and behind the power switch, lighting it in a nice and understated fashion when viewed from the listening position.
When standing up and looking at it from an angle, however, it is super-bright. Between this solution and the highly dimmed deep blue LED in the ST-10, I prefer the latter.
Jason Lim responds: “Thanks for your feedback. We will make an adjustment by adding a filter to the LED”.
The speaker connectors are very tightly-spaced on the ST-10, and the same is the case on the AMG STA. There simply isn’t much real estate, to begin with. But due to the louver-like extension on the rear, spades can only be inserted from below, meaning that the AMG STA needs to be positioned at the edge of the shelf or component underneath it. Thankfully, NuPrime did clear enough space to allow the use of beefy high-grade IEC power connectors.
Bananas work just perfectly… but when using Spades, especially those with long metal parts such as with the Jorma cable, one has to be very careful not to make a short
Connectivity matters and working principles aside, from the moment that I switched it on, I heard that the AMG STA is something special.
I will listen to the AMG STA, first using different setups in the same room, consisting of either the Aqua Formula xHD DAC with V2 output board and Audio-GD Master 1 combo, or the CH Precision C1 DAC. The source in both cases is the Antipodes CX running Roon via HQPlayer to the Aqua LinQ, connected via AQLink I2S cable to the Formula xHD and via a Jorma AES/EBU cable to the C1 DAC. The loudspeakers are the Magico S1 MkII’s and, in a later stage, the WvL SONs. I will also briefly involve the AMG HPA, an analog preamplifier and headphone amplifier.
Lastly, I will also listen to the AMG STA power amp in audio buddy Niels’ setup, which in its core is formed by the Aurender S10 music server, Mola Mola Tambaqui DAC and Spectral DMC-30SV preamp. Here, it will drive the Magico Q5 loudspeakers.