AVI is a company that makes electronics and speakers - the two had to combine, one day. The company recently announced its Amp-Pak, a power amp module that 'bolts' onto the back of its popular Neutron IV or Pro-Nine-Plus standmount speakers, or the Duo floorstander, turning them into 'active' speakers - it's the active version of the Neutron IV that we're reviewing here.

To some audiophiles, 'active' speakers have the amplifiers connected directly to the drive unit, with the electronic crossover sitting between the pre- and power amps. This is 'active' in the Linn or Naim sense. Instead, 'active' in AVI-speak is taken to mean moving the power amp next to the loudspeaker, in a similar manner to PMC speakers sporting Bryston power amp packs.

As well as being a perfect partnership, this means no more chunky speaker cable running around the room. Instead, a line-level phono cable and a mains lead - one of each per speaker - are needed to feed and drive the amps at the rear of each speaker.

The 150-watt Amp-Pak is fitted to the back of each speaker in the factory, or can be retrofitted to existing two-way AVI speakers, for £599 a pair - it's not a complex upgrade, but not one that's sanctioned for DIY installation. It's a fully linear amplifier module, with its own toroidal transformer, so it's no low-rent bolt-on option.

It would be logical to assume it is a digital design because it's so small, but it runs its bi-polar output devices in good ol' Class AB operation. The Amp-Pak isn't exactly complex from the outside though, with just a mains socket, a power switch and a single phono socket on the top of the black powder-coated, folded aluminium box.

Boxes on boxes

Using single-ended line-level signals, rather than either balanced line-level or speaker level connections, does put a strict length limit on cables connecting sources or preamplifiers to the power amps. Anything up to five metres should be fine in most cases, however.

The Amp-Pak sits just below the rear port of the Neutron IV (where the loudspeaker sockets usually live), but it doesn't appear to create any nasty turbulence in the process. Otherwise, the Neutron IV is unchanged; it's a diminutive two-way speaker that uses a 25mm fabric dome tweeter and a bass/mid driver with a 125mm doped paper cone and 25mm voice coil.

This all sits in a very slim little five-litre box that is best used about 30cm from the walls. This isn't a bassy speaker - AVI rates it from 100Hz-23kHz and puts the -6dB bass response figure at 65Hz, meaning deep bass is almost totally non-existent and the upper-mid bass will sound slightly lacking compared to many speakers. But that's nothing new among classy mini-monitors and the Neutron IV both looks and sounds like it's an LS3/5a on a diet.

First and foremost, this is an excellent speaker. On its own, the Neutron IV delivers a remarkably transparent performance. It is revealing and dynamic, and goes surprisingly deep for so small a cabinet, ultimately trading really deep bass for speed and accuracy.

The addition of the Amp-Pak improves the Neutron IV's performance still further. With the Active Neutron, the upper midband is kept in check, where before this was amp-dependent. It's still a little etched-sounding in the mids and top, but this is forgivable, because of the stunning amounts of detail on offer.

There's a sense of directness and purpose about the Active Neutron sound that is very hard to achieve with passive designs. This comes across as both speed of attack and solidity; the latter is interesting, as people often equate solidity with powerful bass drive. Here, the bass is tight and controlled, but not especially powerful, yet sounds are rooted firmly in their own three-dimensional space. All of this contributes to the 'disappearing' effect the Active Neutrons do so well.

On the end of a good CD source and preamplifier, this active speaker system delivers an impressively detailed, precise and accurate sound. A loud sound, too - those Amp-Paks really drive the speakers up to a useful loudness ceiling that is more than enough for small rooms or close-up listening. You get good transparency, excellent detail, fine imaging and a natural sense of rhythm, too.

But a CD player and preamp is only the start. AVI suggests that by hooking these speakers up to an iPod or a PC, you can do without the hi-fi system altogether. We did just that and AVI is right... disturbingly so. We hooked the lengthy listening session sound; it was more natural and musicians seemed even more tangible. But the fine differences between the hi-fi and a laptop seemed comparatively small.

We also went iPod hunting, searching around for an Average Joe with an Average iPod, who recorded their music in Average AAC. We found one... and the little iPod Nano looks faintly daft in this context, dwarfed by even the minijack socket connecting it up.

But it did not compromise the sound - in fact, it sounded very close to the sound of the computer, and that of the CD and preamp. Not identical and not superior, but unless you happen to have the CD and amp nearby to check, more than good enough to be musically entertaining.

AVI's bold claim may hold this speaker back, disenfranchising audiophiles who think iPods are the work of the Devil. If so, these hi-fi fundamentalists are missing a trick, because this is a great amp and speaker system, whatever it plugs into.