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Old 2nd September 2014, 10:59 AM   #1
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Lightbulb BK103 measurements and filter

Hi all,

I understand that the official Fostex BK103 horn for the FE103En is not the most highly regarded design over here. Even so it is interesting that the German magazine Hobby Hifi measured it and proposed a filter to correct the response.

Measurements with and without filter, on axis or under 30 deg:

Click the image to open in full size.

I thought this would be interesting to share for your opinion.
Top left: 30 deg with filter (full line) and without (dotted line)
Top right: on axis with filter (full line) and without (dotted line)
Bottom left: with filter, on axis (full line) and under 30 deg (dotted line)
Bottom right: without filter, on axis (full line) and under 30 deg (dotted line)

The proposed filter is 2 (R // L // C) in series before the driver. They recommend to have connectors just before and after the filter, so you can hear the speaker without any filter as well, depending on the music.

It seems like a good idea to try the filter with other FE103En constructions as well.

Values: C1=33 microF, C2=10 microF, R1=8.2 Ohm 10W, R2=10 Ohm 10W, L1= 0.56mH, airgap, 1.4mm wire; L2=0.22mH airgap, 1.4mm wire.

and if you use a tube amp then they also recommend an impedance correction filter (R in series with C, this between the connectors - hence parallel with speaker and its correction filter), with values R = 10 Ohm, 10W, and C = 10 microF
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Old 2nd September 2014, 12:42 PM   #2
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Depends what you call the BK103. The BK103 is a kit available from Fostex in Japan, plans for which have not been published. As far as I know, nobody here has commented on it, positive or negative, since availability is very limited elsewhere. The plan provided with the FE103En driver was designed for the old FE103E, and is not actually called the BK103. It doesn't have a name, although I see some German sites are incorrectly (to the best of my knowledge) calling it that.

Be that as it may, the horn itself is not one of Fostex's better efforts when used with the 103. You can see the duff response from the horn in the lower registers -Hobby HiFi's parallel RLC circuit appears to be a broadband notch / midrange shelving filter from ~500Hz - 5KHz judging by the scans posted. This is not correcting the horn output (which like all back loads is operative only in the LF & below the lower corner of the filter), but the overall system behaviour -partly step loss & the inherently rising response of the 103. With the 103, you tend to get complaints of a rather boomy, hollow sound, some of the reasons for which you can see on the plots. It gets a bit better with some strategically applied damping, but there's a limit to how far you can go. Basically, it's a bit of a mismatch, at least to myself / many western ears (somewhat different priorities can exist in Japan). Oddly enough, the 126 can work better in it. YMMV.

Last edited by Scottmoose; 2nd September 2014 at 12:45 PM.
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Old 2nd September 2014, 01:23 PM   #3
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Looks like yet another FR speaker that would benefit tremendously by having a xover and tweeter.
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Old 2nd September 2014, 02:04 PM   #4
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Quote:
Originally Posted by Scottmoose View Post
Depends what you call the BK103. The BK103 is a kit available from Fostex in Japan, plans for which have not been published. As far as I know, nobody here has commented on it, positive or negative, since availability is very limited elsewhere. The plan provided with the FE103En driver was designed for the old FE103E, and is not actually called the BK103. It doesn't have a name, although I see some German sites are incorrectly (to the best of my knowledge) calling it that.

Be that as it may, the horn itself is not one of Fostex's better efforts when used with the 103. You can see the duff response from the horn in the lower registers -Hobby HiFi's parallel RLC circuit appears to be a broadband notch / midrange shelving filter from ~500Hz - 5KHz judging by the scans posted. This is not correcting the horn output (which like all back loads is operative only in the LF & below the lower corner of the filter), but the overall system behaviour -partly step loss & the inherently rising response of the 103. With the 103, you tend to get complaints of a rather boomy, hollow sound, some of the reasons for which you can see on the plots. It gets a bit better with some strategically applied damping, but there's a limit to how far you can go. Basically, it's a bit of a mismatch, at least to myself / many western ears (somewhat different priorities can exist in Japan). Oddly enough, the 126 can work better in it. YMMV.
Hi ScottMoose,

Indeed I meant the official FE103En enclosure. Which is called BK103 in the magazine.
I realise that the filter does only affect the frequencies above that of the horn, that is why I thought that it might be interesting to experiment with it in other enclosures with the driver. If I still had my Zigmahornets, I would try it.
And I forgot to mention that the magazine recommends, like you, some damping in the compression chamber and on the panels of the first upper stretch, but indeed not too much.
All in all the box may be too ambitious for such a small driver. I kinda liked my Zigmahornets for acoustical music but I simply need more, full bass in my main system.
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Old 2nd September 2014, 03:00 PM   #5
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Quote:
Originally Posted by vacuphile View Post
Looks like yet another FR speaker that would benefit tremendously by having a xover and tweeter.
Hardly a new thought. It depends on what you want. The FE103 has a cult following, especially in Japan, and has been around for 50 years in many variations, only one of which ever deviated significantly from the same basic response trend (which caused a minor uproar amongst its legions of fans). Given that, it's fairly evident that it's doing something right for them. I can see the appeal -I don't much care for it myself, but to each their own. If you want a perfectly flat response, wideband drive units (and a lot of tweeters for that matter) will not suit. But since they have legions of users, it is also evident that many highly intelligent and discerning people do not regard a perfectly flat amplitude response et al as the sole criteria by which they judge their loudspeakers.

Last edited by Scottmoose; 2nd September 2014 at 03:05 PM.
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Old 2nd September 2014, 03:04 PM   #6
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Originally Posted by talaerts View Post
I kinda liked my Zigmahornets for acoustical music but I simply need more, full bass in my main system.
They do have something, but I certainly agree, LF is not the 103's forte.
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Old 8th September 2014, 07:02 AM   #7
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LF is not the 103's forte. Click the image to open in full size.
You can see it in the upper right graph... the FE103E is a very nice mid high range unit... it is somehow,... "not the best idea" to "just knock the midrange down" you will only need an adequate woofer... and can built nice FASTs with that one... it does not need a tweeter, imho...
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Last edited by venomizer; 8th September 2014 at 07:04 AM.
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