You may have seen my earlier ramblings on the subject of using PC-based DSP to provide an active crossover with filters for an open baffle speaker system. I got on reasonably well with Richard Taylor’s excellent notes on the subject however I was struggling to settle on an actual baffle design, along with the usual problems of actually building them. Enter an auction on ebay for a pair of ‘open baffle speaker’ with the following description:
‘Here is an open baffle project that could not get quite right. The bottom drivers are eminence alpha 15 drivers. the mid drivers are Vintage B&W 8″ drivers and the top drivers are Tang Band W4-657D. they are currently fitted with a 2 way crossover made with decent parts from Falcon audio. In their current form they sound bright to me.’
The crossover was not implemented correctly and would have given very poor results, as implied in the advert. The good news is the baffles are very solidly made. They appeared to be made from two slabs of MDF sandwiching some internal wiring (that I won’t use as it’s been hacked about) and of course ending up at about two inches of thickness. There’s some equally impressive bracing for the baffle and the whole thing is mounted on a massive chunk of MDF. It’s been fairly well finished and is in pretty good nick. I’ll likely repaint at some point but for now it’s brilliant.
I swapped out the B&W midrange unit with a pair of Vifa 8” (P21WO-20-08) as used in some of Siegfried Linkwitz’s early tests. I’ve retained the Tang Band 4” for ‘tweeters’ (yes, I know they don’t go very high, but neither do my ears) and of course kept the 15” Alphas as they should do the trick for me.
Building a six channel volume control
One thing that quickly became apparent was that I would need a volume control between the line outs of my sound card and the inputs of my amp. I’d bought an Alps 6 gang potentiometer months ago and after a trip to Maplins for an enclosure and some phono sockets I settled in for the evening and knocked up a little control box.
You can see from the pictures that I should probably have mounted the input and output around the other way or spun the pot around to avoid crossing all the wires, however I used a solid copper connector for the grounds and didn’t want to go unsoldering things. The most annoying thing was wiring in the six pots. For that I offer the reader some tips….
Soldering the Alps 6 gang potentiometer
First thing to note is that the diagram below (as is typical) shows a view from the top of the pot, ie as though you are looking through it from above with the pins pointing down. Sometimes this makes things hard to translate when you look at the bottom of the pot, however as you will note the pins 1,2 and 3 are all in line and therefore simply rotating either the diagram or pot through 180 degrees gives the layout as though are looking at it from above (harder to explain in words than do – just spin the pot around so the shaft points the opposite way from the diagram).
Pins 1 are ground – so I just wired all those in together. The outer pins on the left and right of the pot are just dummy pins for location – I chopped these off and the whole thing look a lot less intimidating. 3 are the inputs and 2 are the outputs (assuming you want the volume to go up as you rotate the pot clockwise). That’s about it. It took me about an hour of staring and checking with a meter to work out that nonsense!
The end (or at least, interim) is outstanding. At present they’re in a far less than ideal position and I can’t get far enough away from them to really listen properly. That said there is some astounding imaging and beautiful openness. The nicest thing is how they sound in the rest of the house, it just sounds like someone is playing in another room, live – there is not obvious source of the sound. Running some Peter Hurford organ classics through them is quite lovely.