No single best amplifier here. What decides it is what you have to drive, what you need to plug in, and how much of the system you want in one box.
| Area | Better | Why |
|---|
| Power | A | Arcam A5 has more power - 50W against 40W into 8 ohms. That is about 1dB, which is a small step in loudness but real headroom on peaks. |
| Room size | depends | We rate Arcam A5 the better fit for a very large room, and NAD C 328 for desk or near-field listening. Match the amplifier to the room you actually have, not the one you want. |
| Grip on hard loads | A | Arcam A5 holds up better into a 4-ohm load: its output increases strongly to 75W, where the other barely increases. That matters with demanding speakers far more than the 8-ohm figure does. |
| Noise floor | A | Arcam A5 is the quieter design on paper - 106dB against 100dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Analogue inputs | A | Arcam A5 takes more analogue sources - 3 against 2. |
| Room to grow | B | Only NAD C 328 offers a subwoofer output - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Amplifier class | depends | Arcam A5 is Class A/B and NAD C 328 is Class D. Class A/B runs warm and is the long-established mainstream choice; Class D runs cool and draws almost nothing at idle. Modern examples of both measure well - judge by the rest of the page. |
| Shelf space | depends | NAD C 328 takes up about 15% less shelf area (1349cm² against 1580cm²), and it is 54mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | A | Arcam A5 is the cheaper watt - about $14 per watt against $17. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Arcam A5 B NAD C 328
depends neither is better in the abstract - it turns on your room, your system or how you will use them. Only specs published for both amplifiers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
Arcam A5 has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - Arcam A5 is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating.
What a spec sheet cannot show: how either sounds with your speakers, how the controls feel day to day, and what the app is like to live with - which is where most all-in-one amplifiers are actually won or lost. Power figures are the manufacturers' own and are not measured to a common standard, so treat small differences as noise. If you can hear both, that will tell you more than any row above.