Naim has been making integrated amplifiers since 1983, and the Nait line has always been the point of entry — the thing you buy when you've heard what Naim sounds like in a dealer demo and you want that sound without selling the house. The 5si landed in 2014 and stayed in production through 2017, sitting just above the humble Nait 2 descendants in the lineup and below the XS series, which costs considerably more and rewards you with marginally more grunt.
It puts out 60 watts into 4 ohms. That number sounds modest until you actually use it.
What Naim Means by Quiet
The 5si runs a classic Naim topology — discrete transistor output stage, no global negative feedback in the conventional sense, a DR (Discrete Regulation) power supply inherited from further up the range. The DR supply is the detail that matters. It showed up in the 5si around the 2014 revision and it's the reason this amp sounds faster and quieter than you expect something at this price to sound. The noise floor drops, the transients sharpen, and suddenly you're hearing the room mic on a jazz recording instead of just the kit.
Naim wires the 5si in a very specific way — they use 5-pin DIN inputs rather than RCA as the preferred connection, which annoys approximately everyone the first time and which you'll eventually come to regard as right. It's not affectation. The circuit is designed around that topology, and the DIN inputs genuinely sound better than the RCA alternatives on the back. Get a good DIN cable, stop arguing.
The character of this amp is neutral to a fault. It doesn't bloom in the bass, it doesn't add a warm haze to the midrange, it doesn't soften transients to make bad recordings listenable. What it does is get out of the way while maintaining exceptional timing — pace, rhythm, timing, the PRAT thing Naim people talk about incessantly and which is nonetheless real. Music through the 5si has a forward momentum that makes other amps at this price feel slightly lazy by comparison.
This is why it pairs beautifully with ESL-57s. The Quad electrostatic doesn't need help sounding detailed — it needs an amp that won't smear the timing or fog the midrange with second-harmonic flattery. The 5si obliges. The combination is almost unfairly coherent: instruments sit in air, transients arrive on time, and the whole thing sounds like somebody removed a layer of gauze you didn't know was there.
The Honest Caveat
The 5si will not be kind to a rough digital source or a cheap phono stage. It doesn't compensate. If your front end is mediocre, the 5si will tell you that, politely and without mercy. Some amps forgive upstream sloppiness. This is not one of them. Budget accordingly, or be prepared to hear things you don't want to hear.
There's also the power question. Sixty watts into 4 ohms is enough for most speakers in a normal room, but if you're driving difficult loads — or if your ESL-57s have been recapped and are running a bit hungry — you'll want to make sure the room isn't too large. In a 12-by-14, it's perfect. In a ballroom, less so.
Used prices have settled in the $1,800–$2,800 range depending on condition and provenance, which is genuinely fair for what you're getting. A newer unit with the DR supply and the original remote is worth the premium. Don't buy one that's had the volume pot worked on by someone who doesn't know what they're doing — Naim's Alps pot is finicky and a bad repair shows up in channel imbalance at low levels.
The 5si doesn't ask you to love it. It asks you to listen.