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Old 8th March 2006, 10:15 PM   #1
berthej is offline berthej  Italy
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Default Fostex 206 minimonitor filter

As one of my first DIY projects, I went for a BR design using the Fostex FE206E drivers. Mainly due to the WAF, I went for David Dicks mini-monitor design, placed on stands.
The speakers are made of 19mm (0.75) marine-grade plywood. Internal dimensions are 17.5x9x9.75 inches. The bass port is 3, the length being that of the front baffle (0.75).

The speakers are driven by the small SI t-amp. I must say that I am pretty satisfied. The speakers are wonderfully detailed and dynamic. The bass is not great as one would expect, but then I am not a heavy bass freak. Modding my t-amp (the stealth mod, see class-d forum) that improves the amps bass response gave a noticeable improvement.

My only complaint is that the speakers can be rather pitchy, especially with violins and female choral voices, which I understand is often seen with the 206s. So I thought of adding a correction filter to smoothen the high frequencies, something like the correction circuit used in the MJ King ML TL design. Problem is that I dont know what values to use.

Any suggestions to values or alternative filters would be greatly appreciated

thanks
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Old 8th March 2006, 10:42 PM   #2
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As a start you may use the following paralell notch filter:
1 mH
2,2 mF
6,8 Ohm
these three in paralell and the package in series with the 206e

Then you can play with the values of resistor and inductor as room and personal taste also play into that.

LC

btw. nice speaker!
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Old 8th March 2006, 11:00 PM   #3
EC8010 is offline EC8010  United Kingdom
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Did you mean "spitchy" rather than "pitchy"?

A problem that all moving coil drivers have is the dustcap. The dustcap is there to prevent dust from entering the gap and causing scraping noises. Unfortunately, it inevitably causes an HF resonance as the HF wave propagates from the voice coil to the centre of the dustcap and is reflected back to the coil. At worst case, the velocity of sound through the cap material is such that the reflected wave is in phase with the exciting wave, causing resonance. Dome tweeters deal with this problem by doping the material so that the reflected wave is zero (soft dome) or pushing the resonance above audible frequencies (hard dome).

Another possibility is to remove the dust cap (and associated resonance) altogether. "Phase plugs" are the alternative to dustcaps, and Planet 10 sells them for Fostex drivers. I haven't yet tried them, but I intend to savage my FE103 and fit his phase plugs.
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Old 9th March 2006, 10:46 AM   #4
berthej is offline berthej  Italy
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[QUOTE]Originally posted by EC8010
[B]Did you mean "spitchy" rather than "pitchy"?

Probably I meant "high pitched". Has been described by somebody I think in this forum as violins that start sounding as dentist drills !!
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Old 9th March 2006, 10:53 AM   #5
berthej is offline berthej  Italy
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Quote:
Originally posted by lovechild
As a start you may use the following paralell notch filter:
1 mH
2,2 mF
6,8 Ohm
these three in paralell and the package in series with the 206e

Then you can play with the values of resistor and inductor as room and personal taste also play into that.

LC

btw. nice speaker!

Thanks LC

Just to be sure I understand you correctly I have attached a schematics of the filter you propose. Is it correct?

For the Caps, I think you intended 2,2uF, right?. I have a couple of Solen MKP 2,2uF lying around, they would be perfect to use in this setup.

thanks again
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Old 9th March 2006, 11:46 AM   #6
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Hi berthej

the schematics are correct, and yes it should be microfarad uF.

This filter will lower the attunation of the mid and high frequencies, starting at around 500Hz up to around 12kHz.

I'm just working n a filter for another driver, and it's always a very good idea to take your time listen to alot of music and try alternative values for the components. To widen the filter effect, use smaller values of C and larger values of L. To make the filter narrower, use larger values of C and smaller values of L. To change the attunation change the R.

hope that helps and please post your results.

best, LC
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Old 9th March 2006, 01:39 PM   #7
berthej is offline berthej  Italy
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I will start ordering components of various values.

I will post results here once I am done

thanks a lot
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Old 9th March 2006, 01:41 PM   #8
berthej is offline berthej  Italy
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Quote:
Originally posted by EC8010
....Another possibility is to remove the dust cap (and associated resonance) altogether. "Phase plugs" are the alternative to dustcaps, and Planet 10 sells them for Fostex drivers. I haven't yet tried them, but I intend to savage my FE103 and fit his phase plugs.

I had a look at these phase plugs. Looks like an interesting mod. I would be interested in hearing about your results with the 103s. Anyone tried these with the FE206?
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Old 9th March 2006, 02:53 PM   #9
Dumbass is offline Dumbass  British Antarctic Territory
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Quote:
Originally posted by berthej
I had a look at these phase plugs. Looks like an interesting mod. I would be interested in hearing about your results with the 103s. Anyone tried these with the FE206?
I installed the Decware-style plugs (wrench sockets) in my FE207E. The effect was subtle, it certainly didn't change the overall tonal balance as much as improve the quality of high freqs.

Before your coils arrive, you could experiment simply with anywhere from 2 to 10 ohms series resistance in front of your speaker. This changes the Qts of the speaker, would make the FE206E more suitable for the vented design. I find it odd that anyone would sell the FE206E and recommend a bass reflex without substantial frequency compensation. It's not about being a "bass freak", it's about wanting reasonably accurate tonal balance. In your cabinets, the FE207E would require much less freq compensation. I have my FE207E in 45 liter vented cabinets, with maybe 2 ohms or so of series resistance, as well as the phase plugs, and I don't think anyone would describe the sound as "spitchy". That said the FE206E can be shoe-horned with series resistance, you do lose efficiency is all.

A couple other ideas:
Place the speakers right against the back wall. This will reduce baffle step losses.
Overtoe the speakers so their axes cross in front of the listening position. That is, they will be raked at quite a dramatic angle. You will find that stereo imaging improves over a wide area. Also, you will be listening to the speakers slightly off-axis, which IMO is how almost all full-rangers sound best. The nature of the beast is that full-rangers beam high-freqs, by overtoeing you use it to your advantage.
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Old 9th March 2006, 10:10 PM   #10
berthej is offline berthej  Italy
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Hi Dumbass,
Thank you for the socket phase plug hint. I read the threads about this one, seems like a fairly easy mod, will do that one first.


Quote:
Originally posted by Dumbass
. ...It's not about being a "bass freak", it's about wanting reasonably accurate tonal balance. In your cabinets, the FE207E would require much less freq compensation.
Sorry, I misphrased that one. I just meant that having chosen this cabinet layout I knew beforehand that the bass would be somewhat sacrificed in comparison with the bigger floorstanders. I was much in doubt in choosing the FE206 or 207, but the vendor recommended the former. I am not unhappy at all with the result; I just think that there is plenty of room for improvements.

I will definitely try your suggestions for placement and resistances. Have to order also the resistances, though.

Looks like I will do a lot of listening trials in the near future. Funny, before this DIY thing hit my head, I was very close to order a pair of B&W speakers. Might have been good, but that would be it. Right now I am having a lot of fun trying to make and optimize speakers and amps. And in the end I think the result is quite rewarding. So thanks to all the helpful people at this forum that make us newcomers interested and going.

Jens
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