Tuesday, 10 December 2013
Quasars - Open Baffles Par Excellence
Okay, okay, it's time I wrote about some proper DIY audio, enough shirking and flattering to deceive. And if you're thinking "he's a bit full of himself - first blog about DIY and he's telling us how wonderful his stuff is" then you'll be relieved/disappointed to know that I make no claims to progeny of these speakers at all. No, speakers are not my "thing". These open baffles are the design of James Doddington and we are fortunate as a community that he has been happy to share his design. Mine are fairly rough prototypes, but these are James' own speakers
The drivers are fullrange AER Mk 1s which cross over around 150Hz to Supravox 285GMF bass helpers. Both are very nice drivers and not cheap either, though the AERs are no longer available.
James has spent a long time fine tuning the shape of the baffles. By their nature baffles need a large surface area for low bass notes, even allowing for the pi/2 effect of using the floor. The shape of James' baffles use the cardiod effect to improve bass response for the given size, down to 40Hz. Nevertheless these are still large speakers at 1350mm high and 420mm wide. The size is the downside of baffles, but the upside is the lack of boxy honky colourations that cabinets usually have, and well built baffles shouldn't.
Whereas I used yucky 18mm thick MDF for my prototype build James' finished articles use 20mm thick perspex which being see through minimises the visual intrusion compared to wood. The front baffle is shaped, the sides of varying depth and the base profiled as they all enhance the sonics. There's some discussion too as to whether the perspex enhances the sound too. Here's the dimensions of James' Quasars.
In comparison my prototypes are an easy build. The front baffle is flat and 450mm wide x 1350mm tall sloped back at 5 degrees, the base is 500mm deep, and the sides are a simple triangle. It's important that the joins are all air tight. Sadly two baffles won't come out of a single sheet 2400mm x 1200mm, but spare MDF is always handy. The bass helper is mounted approximately 100mm above the base, and the full range driver at 750mm.
The crossover is usually a series one as this offers benefits over parallel. It's fairly simple, or rather there are few components: an inductor, a large polypropylene capacitor, a much smaller paper in oil capacitor to bypass the PP and bring back a little tone, and likely a resistor to balance the relative efficiencies of the two drivers. The inductor wire benefits from a cross sectional area of 2.5mm^2 as this minimises the DCR of the coil, though smaller diameters will work.
Currently I have a pair of Alpair 12Ps mounted as they're new and I thought I might as well run them in in the new baffles. My intention is to make some MLTLs for the 12Ps and use my Fostex FE208E sigmas and T90A supertweeters in the baffles, but one job at a time. I'm waiting for the crossover components to arrive, it'll probably be in the new year now unfortunately.
According to the Mark Audio datasheet for the 12P the drivers should be treated gently for the first 100 hours, then slowly increase volume and bass content and the drivers will improve for up to 800 hours! So at the moment I'm trying to resist the temptation and turn the wick up and listen gently. But the speakers already show tremendous potential.
I wasn't expecting much soundwise as the 12Ps are new and I don't have any crossovers yet but even now they're really very nice, albeit quiet. The pedigree of the 285GMF is clearly evident; it's a wonderful driver - clean, fast and tuneful. A number of James' Quasar designs have used 15" Eminence Beta drivers to excellent effect and are substantially cheaper, but the Supravox are really nice. It's early days with the Alpairs, but initial signs are good; they sound a sweet driver and very capable.
So only another 95 hours to go and I can turn the volume up a bit :-). It will be interesting to compare the 12Ps with the 208s in the fullness of time.