B&W FST midrange... I've been given a pair

[Mos Fetish]

A mate has decided to give me a matched pair of 6" FST midranges from his B&W 802 speakers... don't ask, long story.

All I know about these drivers is what appears on the B&W site, which isn't much. XO of 350hz and 4k. I'll be able to measure the DCR when I pick them up. In the 802's, they reside in a 20 litre "marlin head" chamber.

Anyone ever measured these? Used them?

Any thoughts appreciated.

Mos

[badman]

They seem like really nice drivers. But I sincerely doubt they've been used in ANY diy efforts. Looks like you'll need to get some testing system together and let us know!

[ShinOBIWAN]

Hi Mos

Uli Brueggemann uses these drivers in a DIY design of his. See here for a photo:

http://www.diyaudio.com/forums/showt...75#post1457175

Try emailing him and see if he can offer any advice.

[5th element]

The lower Xover on those is obviously very important, as the drivers appear to have almost zero xmax due to the surround. I wouldn't mind betting however that the fs is low (in comparison to the 350hz) and the response might extend quite a bit below B&W's 350hz xover point. If so then I doubt you're going to be designing the box with regards to a proper "bass" alignment, more designing the box to eliminate any impact on the sound it might have. Or of course open baffle.

The upper Xover of 4khz puzzles me though, I'd have used it lower then this. Kevlar cones are quite stiff and can ring in their breakup, the FST is most certainly operating within it's breakup range @ 4khz and would be beaming some by then too.

A lower xover of 350hz sounds like a good place to start, but you're really only going to see what you can get away with by measuring the frequency response and distortion of the driver.

I've always been intrigued by the B&W topline Midrange unit myself, it looks very well built (from the photos on the website etc) and I've always like the yellow cones

[Mos Fetish]

Yeah, I figured the xmax would be very low. These things are dedicated mids. In fact, I've realised that I probably shouldn't apply the full MLS signal to them when testing Z and FR, perhaps putting a big cap on them to keep the lower frequencies to a sane level.

I have read an article--could have been in Stereophile--where they say that B&W have admitted that the FST is operating in the break-up range. They sound pretty good in my mate's 802's, but I'd have though an xo of much less would be good, especially since B&W seem to use pretty high order slopes in the 800 series.

I've emailed B&W asking for the TS params, but I don't hold out much hope.

Shin, thanks for the link.

[uli.brueggemann]

Uli Brueggemann uses these drivers in a DIY design of his

Hi Mos,

I'm using these drivers now since a while and I must tell you that I'm fully satisfied and happy with them.

Shinobiwan has already given you a link to a speaker picture.
I use my FST in combination with Scanspeak Ringradiators (3.5 kHz) and with Lyngdorf corner woofers (350 Hz).

I run all the drivers with active amplification (3 digital Tact amps) and without any passive crossover part (even the tweeter has no cap).
Crossovers (linear phase Neville-Thiele 2nd order), driver linearization and room correction are carried out by a PC based on spblinux, BruteFIR and my own FIR filters (Acourate). A very special application but GOOD

The NT filters are very steep and thus the breakups of the Kevlar cone are cancelled well. There are severe breakups especially in the higher freq range.

Unfortunately I do not have Thiele-Small parameters. I have not done any TS measurements because in my active system this is not very important. The speaker box is designed as an inverse exponential horn (folded Nautilus) with the target to kill internal frequencies as much as possible and to avoid an internal reflection to pass through the cone.

The frequency and phase response is linearized and measures perfect in the nearfield measurement. The baffle step is compensated too.

[rabbitz]

Originally posted by Mos Fetish
I've emailed B&W asking for the TS params, but I don't hold out much hope.

Hi Mos

B&W are usually quite good about giving out info and I've asked previously and the information was forthcoming.

B&W appear to like crossing over high to tweeters and there's merit in that with the right drivers for sure. I'm certainly not going to argue with their choices as they are a very switched on speaker company. I've noticed some of their crossovers are going for lower orders now and to me, that indicates very well behaved drivers.

I will be following your progress on these suckers.

[tktran303]

Hi Uli,

Do you have any measurements, at all, that you could share?

Your perfectly linearised, post-equalized measurements (obviously beaten into submission by good processing) is nice, but not particularly helpful.

Raw frequency response, harmonic distortion sweep data, that may be helpful for designing passive crossovers?

regards,
Thanh

[uli.brueggemann]

You can find some pictures in
www.acourate.com/FST.zip

There are pictures without smoothing and with windowing. You can also compare non-linearized to linearized behaviour.

As I do not have a passive crossover protecting especially for lower frequencies I have anyway applied a crossover in the game. In my experience it is not good to go below 250 Hz.

You can also see the breakups at higher frequencies.

The measurements done about 2005 (B&K) are made independantly.
Please note the very good behaviour of the impedance curve (quite flat even at high frequencies without resonances). This has been a reason for me to select this driver.

[ShinOBIWAN]

Wow, the off axis performance for a 6.5" is excellent. The rather deep cone with its non linear profile is doing a good job there.

Looks like a great driver, especially when you get a pair for free.

Thanks for the measurements Uli, your a gent.