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Old 15th November 2013, 06:24 PM   #1
Bromle is offline Bromle  Norway
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Default Help with crossover mod for Seas Idunn

Hi!

Having built the original Seas Idunn (well, almost: made it a floorstander) I was not satisfied: I found the voicing a bit on the bright side. I suspect this may be related to surplus energy around approx 2 kHz, ref powerrespons by Seas.http://seas.no/index.php?option=com_...274&Itemid=248

Now, the listening room is re-arranged to a movie room with a custom bookshelf were I have made new boxes for the Idunn, see pic.

As can be seen the bookshelf shuld work as big baffle, reducing the need for bafflestep correction in the crossover.

So, what to do? Having looked at Truels Gravesen Curv design, I would like to try to change the woofer filter by the same philosophy. http://www.troelsgravesen.dk/SEAS-CURV.htm

I have a couple of nice 12awg foil inductors of 1.35 mH I hope can be used in this respect.

But what should the values in the RC and the LCR circut be?

Note that I would like to keep the tweeter circuit - will this be possible?

Any other suggestions or recommendations?
Thanks in advance!!

Bjarne
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Last edited by Bromle; 15th November 2013 at 06:47 PM.
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Old 17th November 2013, 05:57 AM   #2
system7 is offline system7  United Kingdom
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HI Bjarne. Let me get this right, you want to add the Philosophy of Troels' SEAS CURV to your Idunn?

Frankly, this will be difficult with entirely different drivers and a closed box wallmounter scheme.

I have run the Idunn filter with more regular drivers, as shown in the first picture. I would expect it to look much better with the H1499-06 27TBCD/GB-DXT, because the one I used here is more like the H0881-06 27TFFC. It is quite easy to tab between the frequency response of different drivers at SEAS to see the differences.

I'm not sure the problem you describe is entirely to do with a 2kHz presence. Bright voicing may just be a tweeter that is too loud to your taste. If I was going to reduce the bafflestep, I would go for something like the second picture with a 5kHz LCR notch. But I really can't guarantee it's going to be right without a more detailed sim.

The third picture is the Idunn crossover. It looks set too high (2.7uF is VERY small here) on the 2kHz tweeter crossover, but the midrange shout of the DXT horn seems to fix that. So I'd say SEAS did a good job, though you could attenuate the tweeter a bit more to fix brightness.

Hope that helps. Sometimes all you can do is ask the right question to get nearer an answer.
Attached Images
File Type: png SEAS_Idunn_Rough_Sim.PNG (23.8 KB, 108 views)
File Type: png SEAS_Idunn_Wallmounter.PNG (8.7 KB, 108 views)
File Type: png SEAS_Idunn_Orig.PNG (7.2 KB, 106 views)
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Well, there it is! Best regards from Steve in Portsmouth, UK.
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Old 17th November 2013, 12:17 PM   #3
Bromle is offline Bromle  Norway
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Hi Steve, thanks for replying!



Quote:
Originally Posted by system7 View Post
HI Bjarne. Let me get this right, you want to add the Philosophy of Troels' SEAS CURV to your Idunn?

Frankly, this will be difficult with entirely different drivers and a closed box wallmounter scheme.
Yes, correct. I know, this may turn out impossible without real measurements and simulation tools. But I have some crossover components and the possibility of trying qualified guesstimates based on similar designs/ drivers and advice. So your input is very much appreciated!

By the way, the boxes are approx 22 l, ported. I'm using 50 mm (2") tubes where the length can be adjusted. Currently they are approx 20 cm. I'm running the original Idunn filters at the moment, and it sounds ok. A bit too much bass (they go looow, subjectively matching my 12" peerless xxl subwoofer), but the "brightness" is still there. I have tried decreasing the tweeter level (lpad), and yes, the brightness is reduced. But then the details and sparkling are gone...


Quote:
Originally Posted by system7 View Post

I'm not sure the problem you describe is entirely to do with a 2kHz presence. Bright voicing may just be a tweeter that is too loud to your taste. If I was going to reduce the bafflestep, I would go for something like the second picture with a 5kHz LCR notch. But I really can't guarantee it's going to be right without a more detailed sim.
Ok, just what I was looking for! How did you calculate the values for the notch? It would be nice to know how different values on LCR will change how much and at which frequency band the circuit work.

Quote:
Originally Posted by system7 View Post

The third picture is the Idunn crossover. It looks set too high (2.7uF is VERY small here) on the 2kHz tweeter crossover, but the midrange shout of the DXT horn seems to fix that. So I'd say SEAS did a good job, though you could attenuate the tweeter a bit more to fix brightness.
Yes, agree. However, the above mentioned 2kHz area do have some additional energy... Looking at Mark K's ER18dxt design he uses a LCR notch to dampen this "bump". The Seas ER18DXT ported two way
This comparison may not be relevant sice Mark filter is 6 dB electrical?

Quote:
Originally Posted by system7 View Post
Hope that helps. Sometimes all you can do is ask the right question to get nearer an answer.
Thanks a lot Steve! Hopefully I will have some time during the day to try out your suggested LCR notch, fingers crossed that I have the right filter components...

Last edited by Bromle; 17th November 2013 at 12:29 PM.
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Old 17th November 2013, 03:06 PM   #4
system7 is offline system7  United Kingdom
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MarkK's ER18DXT design is certainly interesting. He used notches a lot. It's one way to do it.

The H1456-08 ER18RNX and H1571-08 U18RNX/P are remarkably similar drivers in many ways. You could almost substitute without modification.

Notches for 5kHz are some combination of 5uF and 0.2mH that keeps the product the same. So 10uF and 0.1mH would do good stuff too. Technically the resonance is at 1/Root LC, so if you want it lower, like 4kHz, you'd use 6.25uF and 0.25mH, or 8.2uF and 0.2mH. I find 1.5R resistance in the LCR keeps the Q about right, but it's another variable. 2.2R is good too.

I say that because your woofer might well work optimally with a 4kHz notch looking at the frequency response.

The problem with making the bafflestep coil smaller, is that it is harder to filter steeply and have low crossover. That depends on the modelling. It also makes the mids louder, which needs compensation in the treble filter.

It may be that really you don't get on with the SEAS DXT tweeter. It's clever, but is doubtless a double-edged sword if you follow the idiom. I have had some good results with the TW 70 - 8 Ohm which is an unlikely star. I am working on a Morel CAT 298 Tweeter this afternoon too.

Good hobby, eh? Ha det!
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Well, there it is! Best regards from Steve in Portsmouth, UK.
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Old 1st December 2013, 05:47 PM   #5
Bromle is offline Bromle  Norway
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Yes Steve, nice hobby this is

Attached photo of current xover, still trial & error ongoing. Started with your (Steve's) proposal in post 5, basically reducing the baffle step compensation for the woofer. Didn't like it, harshness somewhere in the crossover region. Then I rebuilt the tweeter section based on Mark K's Er18dxt. Some minor changes: serial capacitance is 5.5 uF instead of 5.6, serial resistor is 2.2 ohm instead of 2.5, and finally only one notch at 1.5 kHz were the inductor is 0.56 mH instead of 0.6. On the woofer I used the 1.35 mH coil, bypassed with 66 ohm resistor, and a notch filter with r=1.5 ohm, 5.5 uF and an unknown inductance (!) - should be in the region 0.2 to 0.3...Improvement, but still harshness.

Then changed the serial woofer coil from 1.35 to 2.2 - success!! Think I can live with this, but from experience I will let it sink in for some time before the final conclusion...

No it's time to continue listening to some music
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Old 1st December 2013, 08:27 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

The design is odd because most of the tweeter attenuation is
done by the reactive elements of the tweeter crossover rather
than the minimal L-pad added to the tweeter.

Your new cabinet arrangement completely changes BS issues.
By redesigning the treble part of the x/o you should be able
use a fairly simple low pass x/o on the bass mid.

I'm not one for conjecture and really wouldn't like to predict
the effects of the driver placement and what sort of treble
x/o is going to suit the effectively higher sensitivity it causes.

Click the image to open in full size.

Depending on what axis you compensate for, (with your ahead
arrangement on axis doesn't make much sense), will change
the perceived power balance somewhat.

The Seas ER18DXT ported two way

The treble x/o used here doesn't help much either except in principle.

A very tricky tweeter to use properly IMO.

rgds, sreten.
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Old 1st December 2013, 09:30 PM   #7
Bromle is offline Bromle  Norway
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Hi Sreten



Quote:
Originally Posted by sreten View Post
Hi,

The design is odd because most of the tweeter attenuation is
done by the reactive elements of the tweeter crossover rather
than the minimal L-pad added to the tweeter.
Yes, in the upper treble the sensitivity is modest, any more "normal" attenuation would strangle the top octave- we dont want that

Quote:
Originally Posted by sreten View Post
Depending on what axis you compensate for, (with your ahead
arrangement on axis doesn't make much sense), will change
the perceived power balance somewhat.
You are right, in my arrangement none will be seated on axis of both speakers, one at the most. Acually I just watched Star Wars episode IV (the kids loved it, funny that these films still can make an impression on todays CGI spoiled youth) sitting (layin)g in the right corner - on axis of one and approx 30 deg off on the other one. My attention was more on the sound then the picture and I noticed that the tonal balance was quite similar, despite this "extreme" listening position. So I still think my Modified Mark K's filters show some promising signs. Wish I had measuring equipment to verify my guesswork

Thanks for your viewpoints and advice!
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