Monitoring On The Cheap: Part 1

In the nearfield monitor reviews I write for Sound On Sound I sometimes find myself explaining that the monitors in question are frighteningly expensive and for the majority of readers will probably remain, “an aspiration rather than a realistic option”, or some other comfortable phrase intended to soften the sad news that most of us will never have the chance to own a pair. In the last six months or so for example I’ve reviewed monitors from Kii Audio, Geithain, Manger and Amphion, each of which demand many thousand of pounds, and each in their way triggered a pang of regret when they were reclaimed by their respective manufacturers or distributors. The Kii Three, for instance, is easily one of the best speakers I’ve ever heard and in particular had me wondering if I could get away with an innocent exclamation of, “Sorry? What speakers?” When the man came to collect.

So, partly as a kind of antidote to all the listening to expensive monitors, and partly in response to a friend who’s just going through the process putting together a studio on a restricted budget, I’ve been wondering just how little can be spent on a genuinely capable pair of nearfield monitors. Of course the market for new active nearfield monitors, extends right down to a couple of hundred pounds or even less these days, but not only do I have grave suspicions when a price appears too good to be true (decent speakers fundamentally need to be constructed from properly engineered materials and components, and these are not cheap), the notional budget I came up with was even smaller: £100. With £100 to spend, and a viable pair of monitors the aim, the most likely route to salvation it seems to me is a pair of classic passive hi-fi speakers from eBay or Gumtree twinned with a similarly inexpensive amplifier – either another previously loved eBay purchase, or that old Rotel, or NAD, or Yamaha, (for instance) that’s been hiding in a cupboard for years.

dm550
Bowers & Wilkins DM550 eBay Special

I’ll perhaps look at buying an eBay amplifier another time but to satisfy my cheap monitor curiosity I have for the last few weeks been keeping an eye on eBay to see what can be had for a £100. As you might imagine, the choice is huge and the pitfalls varied, but I finally took a punt on a pair of Bowers & Wilkins DM550s for £75 (+£13 for delivery). The DM550 dates from the early 1990s (Gramophone magazine review) and comprises a relatively advanced (for the time) 25mm aluminium dome tweeter combined with a 140mm paper cone bass/mid driver. The bass/mid driver is entirely conventional in style but is neatly constructed to a good standard within a custom designed die-cast chassis. The industrial design of the whole DM500 range was by Kenneth Grange, who worked regularly for Bowers & Wilkins over many decades, and to this day I wonder why he didn’t have the big B&W logo on the front panel aligned centrally.  The DM550 cabinet is a closed box constructed from 15mm board but with a notably rigid thermoplastic front panel. The small crossover attached to the inside of the rear panel comprises a first-order low pass filter for the bass/mid driver and a second order high pass, with some resistive attenuation, for the tweeter. Estimating the component values from their size suggests the crossover frequency is, conventionally placed, around 3kHz – although with the very gentle bass/mid driver roll-off from its 6dB/octave filter, there will be significant driver overlap. The capacitor in the crossover network, in series with the tweeter, is a reversible electrolytic of a type known to degrade over time. So if the speakers do turn out to be basically sound, it would probably be worth replacing the capacitors. Relatively unusually, the crossover incorporates a tweeter protection fuse. While such things are I think relatively benign in themselves (although not entirely), corrosion and the general grime of 25 years on the fuse clip and fuse body will quite possibly be a source of distortion. So, as with my instinct that the electrolytic capacitors should be replaced, I think the fuses ought to go too.

Diagram 1. DM550 Amplitude response and harmonic distortion.
Diagram 1. DM550 200Hz to 20kHz amplitude response and harmonic distortion. Microphone at 0.5m of tweeter axis.

Of course the first thing to establish on a pair of speakers perhaps nearing their twenty-fifth birthday is whether they still, even approximately, work as intended. So I fired up FuzzMeasure to examine the basic performance of each and what I found is illustrated in Diagram 1. This is the basic amplitude response from 200Hz upwards of both speakers overlaid. Also shown in the diagram is each speaker’s second and third harmonic distortion. The amplitude response curves reveal speakers that have a reasonably flat character with just a few small deviations and my gut feeling is that they are probably not much different now than they were 25 years ago. That gut feeling is partly reinforced by the pair match between the speakers being pretty good. There is a minor Dumbo in the room though: the one significant divergence between the two speakers is around 4kHz where their responses differ by as much as 4dB. Interestingly, the harmonic distortion curves (basically as expected for both speakers) also diverge at the same frequency. Taken together, the divergence at 4kHz in both amplitude response and distortion suggests that something is slightly, although not terminally, awry. Maybe one tweeter is slightly below par, or maybe one of those electrolytic capacitors is misbehaving. Some further investigation is required……

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