Fostex 206 minimonitor filter

[berthej]

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 ø 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 amp’s 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 don’t know what values to use. 😕

Any suggestions to values or alternative filters would be greatly appreciated

thanks

 

[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!

 

[EC8010]

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.

 

[berthej]

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 !!

 

[berthej]

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

 

[lovechild]

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

 

[berthej]

I will start ordering components of various values.

I will post results here once I am done

thanks a lot

 

[berthej]

....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?

 

[Dumbass]

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.

 

[berthej]

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.

. ...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

 

[Dumbass]

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

Jens,

Yup, DIY is a cool thing, it trains your ears for sure.

When you order your inductors and caps, order a range of nice non-inductive resistors (like those available from madisound.com). Resistors are the cheapest component, so you might as well get a range of values for fine tuning.

Best,
STeve

 

[Dumbass]

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.

Incidentally, I was initially very reluctant to cut into the drivers. With a sharp hobby knife and some patience, it's quite easy.

Planet10 also sells those nice wooden plugs, that's another option. AFAIK the exact shape of the plug isn't nearlyas crucial as simply having something occupying that space in the middle.

 

[berthej]

I already tried to remove the dust cap on the woofers of my old speakers when redoing the foam surrounds on them. They were thick and heavily coated… quite amazing how much “abuse” such speakers can take, and then awake back to life (and sounding good). 😎

The only sockets I found around here have a different shape, they are a bit longer and the diameter gradually decreases versus the ½” opening (photo). Something in between the standard socket and the cone shape. Who knows, might create a new trend. 😉

 

[Kofi Annan]

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.

For the record, this has been my experience as well. I have the 206Es and I have found that room placement is very important. I also installed Planet 10's phase plugs and they're great.

I would say that the most noticeable difference the phase plugs bring is that vocals sound much more open. People have noted the "tin can" resonances with the Fostex drivers at certain frequencies and I've found that this has disappeared upon installing the phase plugs.

Kofi

 

[Dumbass]

Here are just some meanderings:

I have a conjecture that full-rangers tend to work better with placement
right against the wall than, say, a typical two-way monitor or what. A wide-
dispersion dome tweeter will have more problems with diffraction, interference,
etc, than the more directional cone driver. So the typical advice with the
two-ways is to keep them far from any wall, so that direct signal predominates
over reflections.

With more directional sources (full-rangers, horns, etc) it's not nearly the
same problem. And with wall placement, you don't need to electronically
compensate for baffle step losses.

Again, the "disadvantage" of full-range beaming can be viewed as a relative
advantage. You just need to implement the speakers correctly. When I first
started out with the full-ranger deal, I didn't quite understand this, I think
it's one reason for some people's dissatisfactions with full-rangers.

 

[no xo]

I would also recommend damping the basket with someting like the asphalt damping sheets sold by Parts Express. I`ve modded my FE206e`s with the Planet 10 phase plugs and basket damping, though I feel the phase plugs are the best place to start. Dave at Planet 10 seems like a cool guy and is patient with greenhorns like me. Testimonial from another satisfied customer!

 

[berthej]

So, yesterday I finally found the courage to plug in the sockets in my 206s. At first, I modded only one of them, then placed the speakers next to each other for a head to head comparison. I must say the effect of the socket was anything but subtle. Gone was what I described at the high-pitched sound, but also the overall sound pressure went down slightly, but noticeably. I thought that perhaps the socket (see above) was too long, so I swapped it for a normal “decware-like” socket, but the effect was much the same. Actually I preferred the sound from the none-straight socket, but that may be a very subjective opinion.

Finished the job on the other speaker, and went for listen-test. The sound was much more smooth and balanced, in choral works showed a much better balance between female and male voices, but also Norah Jones and Peter Gabriel came out of their tin cans. And Dire Straits showed a bass extension I haven’t noticed before. Very pleased, a thank you to EC8010 and Dumbass to point me in this direction.

This is definitely a long towards a good sound, next I will see (or hear) what filters can do. 😎