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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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
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.



2007-01-24 2:28 pm
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.
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.


2007-01-24 2:28 pm
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.
Apparently we have different meanings for 'rough vibrato', especially if it's not volume dependent and isn't on every recording.

'Harsh' is one thing and is not uncommon with heavily tweaked drivers such as the Sigmas until broken in, or as I prefer to view it, worn out, but 'vibrato'? Out of the cab, this implies a defective driver, but it would be defective all the time. Hopefully, they will smooth out over time, but with no hands on experience with this driver I don't have a clue.

Sorry guys, you backed the wrong 'horse' this time. :( Thanks for the confidence though!

Right, buried in the BIB thread I discussed various ways to damp pipe horns with lining one of any parallel walls, behind the driver, the bottom plate and doing the major tuning by adjusting the packing of the closed end. Mouth damping includes the triangular pendant Lonnie tried (can't remember if you kept it or not) as well as covering it with a damped grill.

More recently, sims indicates a conic tapered 'top hat' like seen on Dunstall mufflers looked promising.

Note that my damping material of choice is 1" acoustic fiberglass insulation for lining and backless R-19 for stuffing and the only felt I'm familiar with is the 5/8" - 3/4" thick premium wool felt carpet underlayment I occasionally used for when very high damping was required, but it seems reasonable to me that your 'light bass' comment is due to too much of a good thing. I've only used foam for horn HF reflections, so can't comment about using it behind wide BW drivers.

Note that my damping material of choice is 1" acoustic fiberglass insulation for lining and backless R-19 for stuffing and the only felt I'm familiar with is the 5/8" - 3/4" thick premium wool felt carpet underlayment I occasionally used for when very high damping was required, but it seems reasonable to me that your 'light bass' comment is due to too much of a good thing. I've only used foam for horn HF reflections, so can't comment about using it behind wide BW drivers.



While you've mentioned 1 inch acoustic fiberglass, can 5/8 or 3/4 inch carpet felt be used (or doubled?) as an adequate substitute?

Best Regards,

Don't know about current felts, but the one's I used were way more dense than the fiberglass, so I only used them for vibration damping/isolation. Maybe you remember the felt pads used under large typewriters, mechanical computers? This is what I've used except I'd get it as high end carpet underlayment remnants pulled up when a major business got new carpet.

Frankly, except for the late 'itchless' fiberglass, Miraflex, nothing I've tried works as well as fiberglass overall.

Thanks for all the interest and replies.

My free time to play with this is far less than the rate ideas come in, something about work, baby and life...

So far, I did, as mentioned above, pull the driver out of the BIB and listen. the harshness is still there.

I may not have the right terms for things GM, I describe vibrato, but harsh and jagged are also accurate terms. When I say light in the bass, it is still awesome bass, I did not mean that in a negative way. I was damping some on purpose, as I expected this size BIB to be a little much and I think I did ok. The felt is thin, somewhat dense felt, like you put under a floating hardwood floor. So the goal was to reduce boom without suppressing too much. I think it worked.

Anyway, back to my issue.

What do I know:
It seems to come from the driver, not the box, so I think it is not a problem with the BIB, stuffing or build.

It comes from both drivers, which have no obvious damage, although I bought them used (lightly) here on DIY audio with phase plugs installed by someone else. Seems unlikely that any damage would be to both, but possible.

It seems that the harsh sound is only with certain music, but I think that is is just accentuated by certain music. I also think that it may be subtly audible in my other speakers. Muted to the point I have to listen for it. So maybe the Fostex, are sensitive enough to also accentuate it.

So i will take RJs advice and look at the components (I suppose it could be recordings too). Also, I will go run the fostexs until broken in (worn out, per GM), and see if that helps.

I will try the drivers separate from the crossover network, and with other cables and, more importantly, amps. I do wonder if it could be an amp problem. I did have a recent issue with my KT88 amp and had to replace a PS resistor. It is also old and I bought it water damaged. wasn't hard to fix, but maybe it needs a rebuild.

I will also try some other drivers, the crossovers will be off, but I should be able to tell something.

So, not the BIBs fault anyway. On to the next step.
You're welcome!

Understood, my quality time to respond on the various forums is severely limited also, though for other reasons, so these 'failures to communicate' are very frustrating.

Check to make sure the phase plugs are centered and that the gap is paper thin all the way down. Probably wouldn't hurt to remove them and make sure there's no dust in the gap.

Make sure there's no problems with the tinsel leads. They should move freely to the extremes of excursion without binding and/or touching the frame.

Yes, HE systems bring out the best/worst in the signal chain where every little change can be perceived if efficiency is high enough and having had a ~112 dB eff. (in-room) system, even squeaky clean McIntosh units needed the annual factory tune-up when they came to town. At this level, 'God' is in the details, so such pedantic things as proper torquing of connectors and the size/type connectors themselves become issues not to be ignored same as with any other ultra high performance system.

With a claimed ~95.6 dB mid-band eff. that's probably not actually quite this high, it's still enough to expose some of a system's more subtle weaknesses, but falls a bit short of being HE except relative to most consumer systems.

Happy hunting!

Thanks GM and Davecan. Interesting and helpful. I think there may be some issue with sensitivity and my amp. I am pretty sure that I hear the badness I am talking about with other speakers too, just less obviously. The most subtle are the Adagios (89dB), then the BIB with Hemp FR8.0 and the Fostexs just pop it right out.

Guess it is time to work up the supply chain. I am concerned it might be my amp (PP KT88) but it also might be that I just have too sensitive drivers for the current system. I have not had time to builf the amp I wanted to use with the Fostex BIB which will be a single ended 300b.

I will try some other amps just to see if it makes a difference. What I have available is a PP pentode EL34, which may have the same problem, then I have two SS amps, a Forte 55 and a mMcIntosh 2105, both of which may really just over power the whole thing. Worth trying though.

Do you thinke there is real benefit to taking a hint from tip 43 and rolling the bass off the Fostex a bit?
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