It is a truth universally acknowledged, that a pair of LS3/5a loudspeakers in good condition, must be in want of a bit more bass. OK, so my days as Jane Austen’s stunt-writer are somewhat numbered, but you get the point. The LS3/5a (in all its guises, even including the LS3/5) is a little bit light in the bass, in that there is precious little content below about 100Hz. The usual way to boost this is with a subwoofer, and typically today that means a powered subwoofer. The Chartwell SUB3 by Graham Audio is the notable exception; it is a passive bass cabinet designed specifically for the classic BBC design, and adds a little bit more bottom end to the classic broadcast bookshelf. As Chartwell also makes both the LS3/5 and a version of the LS3/5a, it’s a logical add-on.
Like all Chartwell and Graham Audio designs, the SUB3 use the company’s own Volt-built design drive units, and is the work of Derek Hughes, son of Spencer and Dorothy Hughes (of Spendor fame), and one of the keepers of the BBC flame. The SUB3 is a small, bass-extension cabinet, effectively a sealed box upper chamber containing a 155mm Graham/Volt bass driver and a crossover, with the lower chamber containing a port constrained by a foam bung. There is no internal damping or wadding, and the thin-walled cabinet doesn’t even feature any bitumenised internal wall cabinet treatment. Connections amount to two pairs of speaker terminals; one from the amp to the SUB3, and one from the SUB3 to the LS3/5 or LS3/5a.
The front baffle (I’m not even sure if that’s the right word, given there is no front-firing bass driver) is sloping slightly, otherwise the SUB3 is a perfect cube, and sits on a quartet of spikes. Although sold in pairs, they are identical, and each is designed to sit between your amp and your loudspeaker, both electrically and physically. Ideally, the SUB3 should sit close to the wall behind each LS3/5 or LS3/5a, rather than next to it. This helps provide some boundary reinforcement, and – from a purely room acoustics point of view – the single worst place to put a bass loudspeaker of any description is the place where the loudspeakers typically sit in the room. So, having these bass speakers set back is a good thing; just make sure this isn’t an attempt to add small speakers into a massive room and have several metres between speaker and sub. As there is no phase control (no controls of any kind, in fact), setting the SUB3 far back in the room and having the main loudspeakers 3m or more into the room could put the bass slightly out of sorts with the sound from the BBC designs.
As the principle changes between the LS3/5 and LS3/5a involve the high-frequency driver and crossover, the SUB3 is designed to fit in with both, extending the bass of the speaker into the double-digit zone. Chartwell claim a response down to 35Hz, and while that might be the case, there’s not much going on at that bottom end of the bottom end. Instead, a real-world in-room response is going to be something nearer to about 50Hz. This is still a marked increase in bass compared to the 100Hz of the main loudspeaker.
This is a one-trick pony, but it’s a very good trick. The SUB3 is designed to only work with the BBC loudspeakers, and even matches the impedance of an LS3/5 or LS3/5a loudspeaker (although in fairness some of the more odd BBC-derived loads will work, but the impedance mis-match is not ideal), and Chartwell supplied a pair of LS3/5 (and a set of Something Solid stands designed specifically for that cabinet size). It uses a fourth-order bandpass crossover, meaning it has no interaction with the loudspeaker itself, and it just kicks in below the end-point of the LS3/5 or LS3/5a. As that design comes to a fairly abrupt stop around 100Hz, the SUB3 fills in the spot from 100Hz down to (theoretically) 35Hz. If you play the SUB3 without a loudspeaker connected, you hear just how effective that bandpass really is; the speaker has a sharp stop at 100Hz where almost no frequencies above that leak through. The character of the loudspeaker remains untouched through the SUB3... almost. If you don’t get the installation right, there is either slight bloom and exaggeration between bass and sub-bass, or the reverse happens: a very slight gap at the inflection point where SUB3 ends and LS3/5 begins, almost as if the two products were teenagers at a prom night, hugging the walls and afraid to dance with one another. Get it right, though, and it’s like someone put on the Barry White records (I’m showing my age here!). When it’s all done properly, the two interact seamlessly and the tonal balance is nigh on perfect.