Another Voigt Pipe - with the Fostex FE207 by Dan Dicker OK, so I’m bumming around the NY Noise show for 2002, nothing to show this year so I’m just another spectator. But, as usual, I meet the most terrific people at these events, whether I’m an exhibitor or not......so.......besides the usual cast of characters, I come across Mr. Jonathan Weiss, who has brought a pair of Voigt pipes using a vintage 800ohm driver. Hmmmm, I think, this looks interesting. I’d already done a pair of Diatone P-610's in the de rigueur vented box and found them to be wonderful, but I had since moved on to other, larger projects and had sold the Diatones. Yet, I could not forget the coherence of the single point source and the great ability of Full range drivers to disappear completely. Add to that the fact that my basement seemed to be filling up over the past few years with completed amp projects the COMBINED power of which couldn’t light up a small cell: I now have 2 under 6W amplifiers and 2 more which can put out 15 Watts.......downhill.......with the wind at their back. >Needless to say, I’ve been looking for a simple, high-sensitivity, low anxiety full range project for quite a while since foolishly selling the Diatones, and after speaking to Jonathan, was convinced that the Voigt pipe might be a good way to go. Well, before embarking on this project I hit the internet, and found a wealth of information it would be silly of me to repeat here. The write-ups elsewhere in this DIY section will give you everything I had or anyone should need to complete this really simple project. I used Herb Jeschke’s dimensions and found them to be spot on, and his advice about construction really useful.......follow it. The only thing that I did differently is in the cutting of the rectangular port on the bottom of the pipe; I left out the 3/8" lip on the bottom, and installed a 3/4" base instead. A little easier to construct and more stable too. Let me add just those few points that I think might be slightly original and worthwhile: Driver selection: No great mathematical reasoning on these theories but this was my take. You want as smooth a frequency response as you can get, right? You know that the Voigt pipe’s main issue is a lumpy response in the lower registers, a lumpiness that can be somewhat ‘tamed’ by stuffing the pipe. The stuffing will, however, reduce the sensitivity of the speaker, but compromises must be made, yes? So, this was the ‘magic’ driver I sought: A driver with a high (93db+) sensitivity, Qts in the neighborhood 0.2-0.3 to be a candidate for horn loading, as flat a frequency response as possible to find, particularly in the midrange, as high a nominal impedance as could be found, Alnico magnet a plus, but, here’s the MOST important part........... I wanted the driver to have a response difference between its resonance frequency (Fo) and its midrange sensitivity to be NO MORE than 10db. In other words, when I looked at the frequency response graph, if the sensitivity of the driver was 95db/w, I wanted the response at around 50Hz to be no less than 85db. Why? I figured that the pipe could augment the bass response only so much, and I would have to reduce the midrange response somewhat anyway by ‘hardstuffing’ the pipe and flattening the ‘humps’. If you’ll look at the T/S parameters and response graph for the Fostex FE207 (available at Madisound), you’ll see that it meets my needs perfectly. ALSO NOTICE, that some more expensive drivers do not meet these criteria quite as well, including the FE208 Sigma, or even the new-fangled, just out FE207E. SO..... look for a driver with a more gradual rolloff to F(o), and sacrifice a few db of sensitivity for a whole lot more balanced response, OK??? I have since tried quite a few 8" drivers in the baffle of the pipes, and it seems that my intuitive guesses have turned out to be useful ones. Other drivers have been inferior to the FE207. Stuffing: As you might have guessed, I used more than the standard amount of stuffing. This won’t hurt your wallet, however....... fiberfil stuffing, available at any fabric or crafts store, should run you about $2 / lb. I used about a l ½ lbs. Per box, including behind the driver. My point is, don’t be afraid of using too much, you can only hurt sensitivity, which shouldn’t matter too much, unless your one of those guys with the 250mW spud amps. I can run my pipes to QUITE respectable levels with my SE 2A3 amps, which are good for about 2 Watts and I don’t even bother with a preamp and its added gain.............. OK, now let’s cut to the chase. Total construction time.......about 1 weekend start to finish. Total construction costs...........about $300 including drivers, less if you use lauan or MDF instead of birch ply as I did.......I ‘splurged’ for the extra $40, and also bought a pint of stain too.....wow....big spender. In terms of sonics, I put these pipes against my reference speakers and...........they very easily held their own. Of course, they do the transparency and disappearing act that all full range drivers do singularly well and, although not as extended on either end as my references, they were really, really natural and open sounding. If memory serves me, they are superior to the Diatones I built by a fair margin, although both speakers shared many of the same advantages, the pipes being much better in the bass and throwing a much larger stage. Build ‘em.......... you’ll love ‘em........... My thanks to Jonathan Weiss for the great suggestions and my great friend Ron VanTongeren, who had all the neato tools (rotozip, nail gun, monster clamps) to make assembly a 1 hour operation, as opposed to 10 hour ordeal. Top