|Mark's Project Pages/Naked Hi-Fi/Rogers Speakers/dB101|
These loudspeakers marked a radical departure from the traditionally conservative Rogers design, and were launched in 1996 amongst a blaze of publicity.
Unfortunately, at launch a pair of dB101's cost £400 - a bad marketing decision. A price reduction hastily followed but it was too late - all the reviews were written in the context of a 400 pound product, and hence they were somewhat luke-warm. At £250, they were rather more competitive, but the damage had been done and they weren't selling. After Rogers went bust, dB101's turned up in Richer Sounds stores for £110 (or less if you haggled), which was a complete steal! I bought 2 pairs...
They were designed by Andy Whittle, Rogers' chief design engineer at the time. Styling was by Peter Stevens who, according to the manual, is a freelance industrial designer with the likes of McLaren, Lotus and Aston Martin on his CV. The stunning case is made from a lightweight composite of ABS and aluminium, and the shape of the cabinet is designed to minimise internal standing waves and cabinet coloration. The aluminium side-cheeks was available in a wide variety of finishes, from the walnut effect you can see here, right through to some rather bold metallic and primary-colour designs.
The 110mm bass unit is reflex-loaded, and unusually, the tweeter is mounted in the centre of the port. Connection is via a pair of cheap spring-clips, but at least they can accommodate 4mm banana plugs. Bi-wiring is not a option - naturally, I approve!
As you can see from the manual, Rogers also had semi-pro applications in mind. They offered a mounting system that allowed two or four units to be mounted together - although I've no idea if they actually sold any of these.
The box and the front cover of the manual were finished in a striking silver finish. Upon opening the box, the speakers are wrapped in recycled cloth bags that you are advised to use to clean the speakers with. Here is a copy of my manual, minus said front cover as it's impossible to scan. Click the thumbnails to enlarge...
As usual, we start by dismantling the poor thing! Access to the interior of the dB101 is via the rear panel, as shown here. There are a total of 11 screws to undo, so reach for the electric screwdriver. You can see the spring terminals and the rather nice aluminium nameplate - I'd like to make something similar for the rear of my Audax mini-monitors.
There are four mounting points for the custom stands and brackets that are referred to in the manual. The round protrusion below the input terminals doesn't serve an obvious purpose, so I can only assume that it adds stiffness to the rear panel, or perhaps was required to add a small amount to the internal volume? Of those two theories, I feel that the former is the most likely.
The writing at the bottom of the rear panel says:
The sensitivity figure is slightly massaged by the low 4 ohm input impedance - very common these days. The "old fashioned" way of quoting sensitivity was to measure the sound pressure level that resulted from 1 watt of input. The trouble with that is the power drawn by a load depends on its impedance, and the impedance of a loudspeaker varies wildly with frequency. So, a load of 8 ohms was assumed, for which you need 2.83 volts. The problem comes with low impedance speakers like this one - it's a nominally 4 ohm speaker, meaning that 2.83 volts actually results in a power input of 2 watts - which effectively adds 3dBs to the sensitivity figure. This is fine because most amplifiers can cope with 4 ohm loads, but I still feel slightly uncomfortable about the state of affairs.
Once you've undone all those screws, this is what you see. I found stickers with a date of 12 October, 1996 in there, and as I bought mine in 1999, they'd clearly sat around in warehouses for a good few years. There is a foam gasket to seal the rear panel, and a small amount of damping material. Note how the wires from the crossover PCB are covered in a thick layer of foam to stop them rattling. I reckon that's a lesson learned from the motor industry, although I wish my car had been built that way!
This view with the damping material removed shows the rear of the bass unit and the tweeter and reflex port. The woofer is secured with large washers and vibration-proof "nylock" nuts. A word of caution - the bolts are molded into the plastic cabinet, and can become loose if you overtighten the nuts during re-assembly. They can be glued back into place, or the bolt can be heated and placed back in place - the plastic will reform around the knurled end of the bolt. I found that the nylock nuts didn't necessarily go back on easily, and replaced a couple with shakeproof washers and normal nuts. Be equally careful with the self-tapping screws that secure the rear panel and terminal panel, as it is easy to over-tighten these and strip the thread.
The inner port which contains the tweeter can be removed from the main outer port by carefully rotating it clockwise (as seen from behind).
The crossover can't be separated from the rear panel just yet, despite the four screws being undone. The wires must first be disconnected from the drive units - these use Lucar-style push-on connectors. The picture above shows that this is easy in the case of the woofer, but much harder for the tweeter.
But if access to just the crossover PCB is required, there's enough slack on the wires to allow you to remove the crossover without having to undo the rear panel.
These two pictures show the crossover. As you can see, it's very simple - quite possibly the simplest crossover I've ever seen in a "quality" loudspeaker. The two inductors are wound on plastic formers, but as there is a ferrite insert, they're not the air-cored devices that they first seem. The glass component looks like a light bulb, and indeed it does light up given enough current. It's a protection device, commonly found in PA applications. It's wired in series with the input, and is designed to heat up under heavy drive conditions - the filament has a positive temperature coefficient, so the increasing temperature results in a higher series resistance being placed in series with the speaker. Under particularly heavy drive, it can light up, illuminating the inside of the enclosure according to the manual. I must admit that I've never seen this happen!
The crossover frequency is 5KHz, using just a single inductor forming a first-order electrical filter for the woofer which is fitted with a phase plug. The 8 ohm tweeter requires a second order crossover, and a small amount of attenuation. It's a model of economy - even the inductors are the same value. I measured them on my Marconi bridge - as normal, they aren't marked in any way.
I also made some measurements of the "light bulb". At room temperature, it measures 0.6 ohms. When fed with 5V DC, it glows reasonably brightly, and consumes 1.25 amps. Translating that to input power, 1.25 amps is only around 6 watts at 4 ohms, so I'm perhaps surprised I've never noticed it glowing. Like me, you might be thinking about removing it - after all it can only add distortion and it could affect the bass damping and efficiency. Providing you listen at sensible levels, I can't think of a good reason not to remove it. As I said above, I do have two pairs of these speakers, so it would be easy enough to A-B test the difference it makes. This is definitely something I will consider in the future.
To quote from the manual, "Rogers designed two new, unique drive units". So imagine my surprise when I saw that these two drive units are made by Audax! That's not a bad thing as such, indeed Audax make some great speakers. But I think I would have preferred a more honest "spin" from Rogers. It's possible that Rogers had some design input, but generally a manufacturer like Rogers would approach an OEM (original equipment manufacturer) like Audax with a specification, and the OEM will do the design work.
According to the manual, "The fully-shielded 110mm woofer is built into a rigid polymer chassis with negligible flux leakage. The woofer diaphragm is manufactured from lightweight high definition aerogel (HDA)." Sound familiar at all? Check out my Audax AP100Z0 projects! Minor differences aside, this is a slightly larger version of the woofer that I used.
The surround is actually quite neat. By slightly altering the shape of the basket, the rubber suspension also forms the seal with the enclosure. This works because the drive unit is rear mounted in this enclosure, but this wouldn't be so useful for the more commonly encountered front-mounting.
Rather than a conventional dust cap, this woofer has a phase plug - in this case made from rubber. While I know that manufacturers like these because they make the crossover simpler, I don't like the idea of the magnetic gap being open to the elements. The lack of a grill significantly increases the risk of contamination. If anything magnetic finds its way in there, you can say "goodbye" to your woofer - this makes me slightly uncomfortable about using them in the workshop, and explains why they are on high shelves. Something to bear in mind when buying these second-hand.
These two pictures compare the dB101 woofer (on the left) with the AP100Z0's mentioned above. Despite the different terminals, you can see that the chassis has the provision for either type. You can also compare the rear of the cone, and see that they do indeed share the same HDA material. The front of the dB101 woofer has been coated with a black substance that is slightly sticky and therefore is very difficult to remove dust from. Another reason why I feel the lack of a grill is a serious problem.
The dB101 has a very high power rating of 125 watts, whereas the AP100Z0 is rated (by Audax) at 30 watts. The dB101 woofer uses a kapton voice coil former instead of aluminium - perhaps this is responsible for the higher power handling?
The sticker on the rear of didn't come out in any of the photographs, so I popped it on the scanner! The "Made in France" statement belies its' origins.
Finally, while the woofer was out, I measured the TS parameters of the woofer. Admittedly, I didn't do VAS as I didn't feel like cutting a new baffle for my test boxes. I found the following:
In the enclosure, the system resonance is 105.7Hz, and the port is tuned to 66.3Hz
The tweeter might be mounted in an unorthodox fashion, but it's a basic 14mm Audax polymer model with ferrofluid for cooling and damping. I used a similar model many years ago in the first "proper" speakers I built for my GCSE coursework, and people like JPW, TDL and Wharfedale have used them in their cheaper models. To be fair, I'm sure it's been subject to improvement over the years, and it's a perfectly acceptable performer when crossed over high enough. I imagine the faceplate was a custom-job to facilitate the port mounting.
This picture shows the inner port removed from the main enclosure. The next step is to push the tweeter from within the inner port - it's basically a friction-fit, albeit with small lumps to help the tweeter clip into place. For ease of mass-assembly, it forms an excellent case-study.
This picture shows the tweeter removed from the port. This is similar to my memories of the older tweeter I mentioned, which was available with round, square or rectangular faceplates - the actual tweeter simply unclips from the faceplate. This newer version is magnetically shielded, so the rear cover is steel and incorporates an extra "bucking" magnet to cancel the leaking flux from the main magnet - indeed the woofer employs an identical arrangement.
These speakers make a forward, exciting sound. Although not at first. I found that these speakers really do need a good few days to run in - in fact this was the first time that I really noticed this phenomenon - before then, I'd dismissed the effect as "hi-fi reviewer-speak". But I got to hear this for myself three times in a row.
Why three times, when I'd only bought two pairs? Simple, I'd lent my first pair to a colleague for a few days and he decided to buy his own pair. So we went to Richer Sounds, and at this point he said offered me the new pair as it would save having to put mine back in the box! So I had to run them in... I bought the second pair some months later, and found the same thing all over again. Even now, if the workshop has got cold overnight, they sound decidedly "thin" at first. I documented the run-in of six AP100Z0 drivers during my Audax experiments, and it seems that these Audax drivers are particularly temperature sensitive.
From new, they are a bit bass-light, but this only lasts for a few hours. The treble also seemed splashy and undetailed at first, but right out of the box these have a captivating midrange presentation and superb imaging. Driver integration is seemless. And there really is something in that unusual enclosure design, as there is no hint of box coloration.
When on song, these have a combination of beguiling qualities. They are fast and detailed, yet easy to listen to. The bass is quite extended for their size, and is reasonably clean for a small ported design. But they are not exactly neutral. The midrange is rather forward, presumably due to the lack of baffle-step correction in the crossover - I'm guessing that this was omitted on grounds of cost and efficiency. As such, vocals are spot-lighted, but at the expense of other details in the mix.
The tweeter is the weak point with this design. I can imagine that Rogers had a hard time finding a model that would be suitable for mounting in the bass reflex port, and that this was probably the best they could find. But I also believe that they took the correct compromise when "voicing" the speaker - rather than draw attention to itself, the tweeter rolls off gently over the higher octaves. The net result is that they lack a little detail and "air" at the top end compared to my usual references.
From building my mini-monitors, I know that the Audax HDA models make for great sounding bass/midrange drivers, and this unit is no exception. I keep wondering just how good this woofer would sound given a crossover with BSC, and mated to a better tweeter. I would also like to try it in a sealed box, as there are occasional hints of "muddy reflex bass" - personally I prefer to sacrifice efficiency and extension for cleaner bass. But I would re-iterate my concerns about the exposed voice coil gap - and I'm not singling out Rogers here, because there are dozens of speakers out there with this same problem.
I feel guilty for criticising them - please bear in mind that these are minor problems when placed into context! The bottom line is that for £100, these were a complete bargain. In the right room, these can sound fantastic - just avoid using them in a "live" acoustic. They're particular good in the workshop, where the acoustically treated walls mean that when placed in the corner, the bass and midrange become very well balanced.
Unfortunately, they are hard to find now. When they appear on eBay, they sell for more than I paid for them. If I had an ounce of business acumen, I'd take the hint there! They occasionally appear in the small-ads, but people generally want to hang on to them - not only for the sound, but for the unique aesthetics. Before you ask, no, mine are not for sale!
©2004 Mark Hennessy
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