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Juspur 8th June 2010 03:11 PM

The Small, Cheap, Good, Easy2Build OB
3 Attachment(s)
Long time no build :D

But I've for quite a time been tempted to try an OB. However money and space are limitations (listening room in cellar is 4 x 4 meter, with lots of stuff in it).

But could it be possible to build an inexpensive, cheap, goodsounding OB - which didn't dominate the room physically?

Just for fun, I picked this fullranger:

Attachment 175390

It's a Monacor SP-205/8. It's an 8", costs 11-12 euro each, and has a very small magnet (79 grams) an Fs at 100 Hz, 94 dB sensitivity and almost no other specifications.

Impedance (Z) 8Ω
Resonant frequency (fs) 100Hz
Max. frequency range f3-17,500Hz
Music power 10WMAX
Power rating (P) 8WRMS
SPL (1W/1m) 94dB
Magnet weight 79g
Speaker type
Mounting cutout 180mm
Mounting depth 63mm
Dimensions 205mm
Weight 370g

The very small magnet means that the driver has a very high q. I guess it must be over 1.0, but that's absolutely a guess:o

I had this theory, that besides the 6 dB/octave roll-off from (17700 Hz/bafflewidth in cm), the 12 dB/octave rolloff from Fs was dependent on the Qts. A Qts of 1.0 I guess will roll off at less than 12 dB/octave, and a driver with a Qts of 0.4 will roll off at more that 12 db/octave.

Placing an OB driver low (closer to the floor) increases the field of the "infinite baffle", the driver sees (the floor) - giving more and better bass. Placing the driver higher up on the baffle means less bass (a smaller angle of the driver sees the infinite baffle - the floor).

Okay. One thing is theory, another is practice :p

I found a bookshelf, 75x30x40 cm (HxWxD) and mounted a baffle, with the center of the cutout 55 cm from floor.

How did it sound? Awful. Not very much bass and far too much treble. Not recommended.

Next thing - I reversed the bookshelf, the center of the driver now just 20 cm from the floor.

This is good. Now there's bass, and it made several things in the room resonate, while I played Dave Holland's "Spheres". Dave Holland is an acoustic bass-player. The bass has a nice amount, detail and speed. And the "too much" treble was gone,as I now listen about 30-35 degrees off-axis (the driver situated 20 cm from the floor, me sitting 2 meter away with ears at about 95-100 cm from floor). It simple create a balanced sound with sufficient detail and bass. I guess the bass has full level to about 55 Hz, due to addition from the floor - but from there the bass is very fast decreasing.

Another good thing: the bass seems to join the room with more muscle than a traditional 2-way stand-mount with roll-off at 55 Hz. The bass seems more present from the OB

Music played:
Leonard Cohen's song: "To a Teacher": deep, deep voice with just the right amount of roughness.

kd lang's album "Ingenue" was just enjoyable all-through.

Paul Simon's song: "Homeless": the upstart s'es and T'es wasn't sharp, but just natural, when he starts singing, half through the song. Good test-song.

Dave Holland's melody: "Spheres" - a very natural bass, moving several things in the cellar. Impressing

Next thing was building easy and proper enclosures. I found some other bookshelves, 35x35x35 cm. Pics below

Attachment 175392

Note that you can rotate the box for varying driver-distance to the floor and in that way adjust the bass-level slightly. More adjustment can be done by changing the distance to side- and backwall.

Attachment 175393

Conclusion: These are keepers, enjoyable and very natural. I'm really wondering how those cheap drivers can produce that sort of sound :D

BTW: I'm using a tripath 2020 amp, great combo for almost no money

Recommended. Try it.

Have a nice diy-day ;)

Rullknufs 8th June 2010 06:32 PM

I might actually give this a try since the drivers were so god damn cheap, but that will be after my coming project. So sometime in the winter.

Juspur 8th June 2010 06:57 PM

Rullknufs, can you actually wait that long :-DDD ???

Rullknufs 8th June 2010 06:58 PM

I think i have to, unless the whole sh*t wont cost me more than 40$. I am really low on cash already and i got a project coming up already. Gotta sell my current setup and work like a ... like a ... like something to afford it! :D

Juspur 8th June 2010 08:12 PM


Originally Posted by Rullknufs (
Gotta sell my current setup and work like a ... like a ... like something to afford it! :D

- Like a polka-dotted grizzly bear with hangovers ?? :rolleyes:

Rullknufs 8th June 2010 08:14 PM


Originally Posted by Juspur (
- Like a polka-dotted grizzly bear with hangovers ?? :rolleyes:

Yeah, sort of...

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