music, guitars & other obsessions

OB Speaker Build 1

It took awhile, but I finally managed to complete the OB speakers for my study. A prototype was made with MDF first and tested to verify its performance (listening tests only), I then had the carpenter make another pair but this time with plywood, alas at the end it was still not done properly, as I will discuss shortly.

The  OB design was done with Martin King’s great Quarter Wave software. The speakers used were Fostex 126En and Eminence Beta 15A. It’s an active 2-way design, Bessel 2nd order crossover with LF at 200Hz and HF at 400Hz. Below are the screen shots from Quarter Wave, one thing to note is that, while the frequency response looks very flat, it is very sensitive to placement in the room, in the simulations, small movements of the speaker caused the peaks and dips in the mid band to change pretty drastically, primarily due to wall and floor reflections. But the transition from LF to HF showed the design had a pretty good chance – it won’t “suck”. When I get a chance, I will try to measure in-room response to see how closely (or not) the actual frequency response matches the simulation.

Thiele-Small Parameters

Frequency Response

Impulse Response

As mentioned earlier, the construction could have been done much better – the mistake I made was being talked into “saving money” by the carpenter, he suggested using layered 12mm plywood for the woofer and a single 12mm for the full-range, instead of my original idea of just using a sheet of 18mm plywood from top to bottom. Oh well, that was another penny-wise, pound-foolish decision I regret making. The next build will be either 18mm or 20mm plywood, it will cost more, but should sound better too from the increase rigidity of the panel, and better time-alignment of the drivers. Here are some pictures of the finished speaker:

The front was covered with “white spruce” veneer then clear coated, I think it looks ok, but perhaps a bit IKEA-ish… You will also note that both drivers were rear-mounted, which was not based on my original design – the Fostex should be flush mounted on the front, to better time-align the two drivers. By the time, the carpenter brought over the speakers for me to check, it was already too late to correct the mistake, unless I wanted to scrap them and start all over again. Since it had taken more than a month just to get these darn things made, I decided just to take them as-is.

Front View (Left)

The transition from the lower 2x12mm woofer panel to the upper 12mm panel is disguised by the use of tapered cuts of plywood, I think it looks pretty cool and adds some rigidity to the overall construction.

Side View (Detail)

Full side view clearly shows the upper section is only 12mm, which is sufficient for the small Fostex driver, although as mentioned, it would be better had the whole panel be constructed with 18mm plywood top to bottom. Next time…

Side View

The signal chain is as follows:

Source -> Foobar 2000 with DSP crossover -> M-Audio Revolution 7.1 Soundcard -> S/PDIF carries the LF to digital amp -> Eminence Beta15A, and analog out carries the HF to DIY 6P1P amp -> Fostex 126En

I found the setup particular well suited to blues and rock music, well balanced… Not so good with electronic though. It shows up poor recordings easily, the difference between MP3 vs. lossless sources can be clearly heard. It is surprising how many albums are very poorly mixed, which usually lacking highs and rather compressed. But on properly mixed material, these speakers are very good sounding indeed – open, transparent, dynamic with decent soundstage. Overall, I think they are very easy to listen to, but perhaps lacking a bit in stage depth, I need to investigate further, the lack of depth could also be caused by the soundcard or the cheapo 6P1P amp that I am using. Current grade: 85/100.

For comparison, the 10-year old Usher speakers that I was using before would probably only score 70/100, it sounded boxy and lifeless compared with the OB. So at a minimum, the key feature of  a “no box” design, and all the online ravings that prompted me to build them in the first place, has not gone un-rewarded. It was well worth the effort. Now I am ready for build 2 😉 Better wait awhile and enjoy these first…


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