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Old 24th September 2010, 08:23 PM   #1
jrenkin is offline jrenkin  United States
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Default Tweaking FE208 sigma BIB

I am looking for a little advice in regards to my newly built BIB for the Fostex 208 sigma. Pictures below.

Generally they sound great, full and rich. Very real in resolution with a light bass (light in a good way) that is quick and where it should be.

With certain music, however, there is a rough vibrato to nearly all frequencies (it seems, although I doubt it is all). I would best describe it like it would sound if there were a tear in the cone. This is not the case.

It is in both channels.
It is not volume dependent.
It is really annoying.

The music I have noticed it with is Shawn Lee's Ping Pong Orchestra, Marcus Miller and a Van Morrison tribute CD (I haven't had time to listen to much more yet). Most other CDs of all qualities do not have this problem at all, but again, I haven't tested that many. Clearly it is certain music only, with the caveat that the new baby prevents really loud testing...The offending music is fine on my Acoustic Zen Adagios.

Speakers: BIB built to spec for the 208 sigma (per the calculator, thanks!) using Baltic Birch ply glued. Entire inside is lined with flooring felt, with 3/8 inch acoustic foam pad behind the driver. The 208s have phase plugs, they are crossed 1st order, to Fountek CD 2.0 ribbon tweeters crossed 2nd order (all butterworth) at 8500kHz. I have tried a zobel on the 208s, can't tell a difference. No BSC. I have fairly lightly stuffed above and just behind the 208's and even more lightly stuffed inside the base. Probably too light.

System: iMAC using itunes and Audio Hijack Pro to USB input of Red Wine Isabellina DAC to an older Sophia Electric KT88 PP in triode mode.

I am wondering if the drivers just aren't broken in yet, or, more likely(?) these particular music selections excite some pipe resonance that screws with the cone movement.

So far I plan to try moving the speaker out from under the overhang (see pictures), stuffing, stuffing and more stuffing. Finally, I have two other 8" drivers to try, Hemp FR8.0 and Viiton B200s (which will not really be optimal for the box....) just to see if they have the same problem.

I am hoping for more suggestions. Thanks!
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Old 25th September 2010, 01:33 PM   #2
jrenkin is offline jrenkin  United States
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I am surprised. No comments from this group? I could use a little advice, this is my first try building. Thanks!
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Old 25th September 2010, 03:39 PM   #3
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OK, what the hey....just remember: Free advise, worth every penny you paid for it.

Break in period: Some VERY bad sounds can come out of FR drivers during the first 100 hrs, and things don't really start to settle down until about 400 hrs.

Up stream gear: Sometimes a nice piece of gear just won't play nice with a driver; don't know why, but I've heard it for myself. Try some other gear.

Cheers, Jim
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Old 25th September 2010, 04:04 PM   #4
GM is offline GM  United States
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Hmm, I typed a response shortly after you posted yesterday, but have no clue why it didn't register.

Anyway, pipe horns don't like its walls damped much, so not surprised your bass is 'light' and until the whole pipe is stuffed it will have a certain amount of 'vibrato' since it has no built in low pass filter chamber (you create one by stuffing its peak to 'taste'), but it will be volume dependent, so factoring in it's in both channels, my SWAG is that you're getting some strong cavity resonances from its large open cave-like location that's modulating the drivers similar to what happens when a driver's face is recessed too far into a horn's mouth.

GM
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Old 25th September 2010, 04:12 PM   #5
jrenkin is offline jrenkin  United States
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Thanks guys! Both very helpful.
Maybe I should pull out some of the felt on the sides then. Just would be painful to put back if I decided I wanted it.
So more time and more stuffing I suppose.
I will be curiosa if pulling the speakers out from under the overhang will help.

Any suggestions for stuffing to try?
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Old 25th September 2010, 04:42 PM   #6
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Hi jrenkin,

Wow beautiful build. Those look like the 208E Sigma to me (vs. the older 208 Sigmas). I wonder how thick the felt is (it sounds like a lot of felt if the whole inside is done but whatever works is right of course).

The only way to solve this is to do something like this:

1. Isolate the exact audio phenomenon. Rip the offending tunes, open the WAV's in Audacity (free), make a little loop that plays the bad sound. Save that loop and open it in whatever.

2. Eliminate everything a step at a time. Put your ear up to the speaker to make sure it's really coming from there (and not elsewhere in the room), use a stethoscope to see if it's a cab resonance (thanks GM!), swap out every single item one at a time, no matter how seemingly unrelated or insignificant.

This would include swapping out the source, the cables, the amp, the DAC, etc. etc. even if it seems like a waste of time. You have to break it down to the simplest possible scenario which does, and does not, produce the problem. The difference is the culprit (der).

Eventually you'll find it.
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Old 25th September 2010, 05:27 PM   #7
jrenkin is offline jrenkin  United States
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Ow, thanks, and I do have a stethoscope to use. And a reflex hammer, maybe I can beat on it a bit...
The felt is not thick, perhaps 1/8 inch. Should I remove it from every where except perhaps above and behind the woofer?
When I get home, the first thing will be to pull them into th open so nothing is above. If that helps, I will know it is a reflection and hopefully can eliminate the upstream components.
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Old 25th September 2010, 06:46 PM   #8
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I defer to the BiB doctor (GM) who's message I did not see while I was typing mine.

But my newbie advice would be to pull the felt out last, because as you say it's not easily reversed. Once you have the audio loop running, you can easily do the ear-to-the-cone thing, the stethoscope and the one-by-one elimination of all the other items, like amp, cable, DAC, tweeter etc.
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Old 25th September 2010, 07:23 PM   #9
jrenkin is offline jrenkin  United States
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Good point. Besides, the felt is probably not causing the vibrato sound, rather damping the bass a bit, which is not really a problem. These are big enough that too much bass could too much.
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Old 25th September 2010, 09:02 PM   #10
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In re-reading GM's advice, I think he's saying (a) pull it out from that "cave", and (b) stuff the part above the driver rather than rely on the "felt on all inner surfaces" approach.

One would have to take his advice before mine!
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