Originally printed in the September 1991 issue of Gramophone.
Image: On the wall, the DM600 and DM610. On the ground, the DM620.
This report is in continuation of that last month which dealt with the two new junior models in the B&W '600' range and gave some details of the thinking behind them. The DM620 differs from the other two in that it is an entirely 'in house' product (apart from the cabinet) and in addition is one of the few B&W designs to use a passive radiator, sometimes known as an ABR (auxiliary bass radiator). This is basically a specialised example of a reflex design which substitutes an additional unit with a normally suspended diaphragm, but no voice-coil or magnet, in place of the more usual air mass retained in a pipe or tube; in doing so it avoids possible pipe resonances, puffing noises from the considerable to and fro air motion and, most importantly, effectively shuts off unwanted audible contributions from inside the box. As our picture shows, the bass/ midrange driver and the ABR have much in common in this B&W design, both using identical die-cast chassis, cones and suspensions, but in the latter the voice-coil has been replaced by a weight.
The active driver unit has a 31mm diameter voice-coil of 4 ohms DC resistance and a free-air resonance of 32Hz. Its outside diameter is 207mm and the actual diaphragm is 152mm across. The cabinet follows the construction and appearance of the other two smaller models but at 750mm (30in) tall is intended to be floor-standing. There are two internal frames to brace the longer cabinet walls and restrain any possible movement and in this design, which might appear more prominently in a room, an alternative walnut style finish is offered. The active bass/midrange is the upper Unit adjacent to the same metal dome tweeter used in the other models. However, the crossover frequency has been raised slightly to 3kHz and there is an additional pole in the network which remains divisible to permit bi-wiring or bi-amplification if required. The complete loudspeaker, in confirmation of its membership of the reflex family, exhibits the usual two band-pass resonances, in this case occurring at 25 and 60Hz.
How it performed
In spite of the quite considerable differences between this model and the smaller DM600 and DM610 the family resemblance is still strong, allowing for the fact that the bass extension in this version is more obvious; in addition to its lowered resonance in the bigger box, there is some additional reinforcement from floor reflection. Although the sensitivity is only a single decibel greater than that of the DM610, the subjective effect of the enhanced low frequency response suggests rather more than this and, given a suitably powerful source, a pair of DM620s will produce enough volume to satisfy most 'head-bangers' as well as those who choose the front three rows of the stalls at the Royal Festival Hall. At the same time there is sufficient delicacy in the response to reveal the wonderful orchestral colour in the recent Philharmonia/lnbal recording of Stravinsky's Petrushka (Teldec CD 2292-46327-2). As is all too common (and probably unavoidable) where bass loudspeaker units are near the floor, there is a marked degree of false chestiness to male speech. This was considerably relieved by raising each loudspeaker by about 230mm on some old display cubes which were to hand, and I must say that these produced an all-round improvement. The remainder of my listening was done this way, after a few modifications (involving bricks!) to produce a more stable assembly.
I realise that the price gap between the DM610 and this DM620 is a rather forbidding £110 and I several times had to go back to the former to convince myself that one could justify this differential. There is no doubt that it can be justified though, particularly if one takes account of the hidden factor — the possible need for a pair of decent stands for the DM6I0s: they are not cheap these days so the decision is far from clear cut. Both these pairs of loudspeakers, in fact all three pairs in this new B&W range, have their strong points and each fills a particular need and circumstance in a way that, once again, offers something of a challenge to the opposition. No doubt that is exactly what B&W intended and as they have so obviously succeeded they should easily top the success of their distinguished forbears.
Enclosure type: two-way reflex enclosure with ABR radiator. Frequency response: 58-20,000Hz ±2dB on reference axis. Drive units: 200mm bass/midrange. 26mm metal dome tweeter. Sensitivity:90dB/watt/metre. Amplifier requirements: suitable for amplifiers rated at 25-100 watts into 8 ohms. Nominal impedance: 8 ohms. Dimensions (W x H x D): 236 x 742 x 302mm. Weight: 14.2kg. Manufacturer: B&W Loudspeakers Ltd.. Meadow Road, Worthing. West Sussex BNI I 2RX. UK retail price: £309.95 per pair in "Zelda or Walnut finish.