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Old 30th January 2008, 02:40 AM   #1
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Default Baffle Step Compensation Adjustment

I'm confused... I've been reading about BSC from the following sources:

http://trueaudio.com/st_diff1.htm

http://sound.westhost.com/bafflestep.htm

And I've come to the conclusion that a BSC passive filter looks something like this:

Click the image to open in full size.

Well, I'm attempting to adjust the BSC filter from the following crossover network to accommodate a wider baffle that the designer intended. I already have the cabinets built with the same driver compliment and would like to apply his crossover network to it.

http://www.htguide.com/forum/showthr...6&page=1&pp=35

Click the image to open in full size.

The design was originally for a 7.5" wide baffle. My baffle is 11.5" wide. I've calculated that this should drop the f3 of the diffraction loss from 608Hz to 397Hz.

My QUESTION (finally =) is....
How do I alter the crossover network shown to reflect this? I don't even really see the BSC filter clearly in the schematic. Help?
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Old 30th January 2008, 03:55 AM   #2
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BSC is generally not a discrete circuit. The XO as a whole incorporates it, mostly at the first inductor of the woofer.

Changing this value substantially obviously affects the XO to the tweeter, so it's not something that can usually be changed much without adjusting everything else. Small tweaks will be possible, but without modelling it, I'm guessing the diiference caused by going from 7.5" to 11.5" will be substantial.

I would contact cjd at htguide to get his advice since that is his design.
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Old 30th January 2008, 08:46 AM   #3
Thawach is offline Thawach  Thailand
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Hi

the crossover network that's show.i think that he does not use
some the bsc.


Thanks

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Old 30th January 2008, 09:02 AM   #4
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Quote:
Originally posted by Thawach
Hi

the crossover network that's show.i think that he does not use
some the bsc.


Thanks



It does use BSC,

The 1.8mh inductor serves as BSC.

If the circuit does not use BSC at all, The 1.8mh inductor should have a value no bigger than 1mh.
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Old 30th January 2008, 11:48 AM   #5
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Quote:
Originally posted by augerpro
BSC is generally not a discrete circuit. The XO as a whole incorporates it, mostly at the first inductor of the woofer.

Changing this value substantially obviously affects the XO to the tweeter, so it's not something that can usually be changed much without adjusting everything else. Small tweaks will be possible, but without modelling it, I'm guessing the diiference caused by going from 7.5" to 11.5" will be substantial.

I would contact cjd at htguide to get his advice since that is his design.

I did, he respectfully indicated that he was not interested in making the modifications. I can understand that. I was hoping that here was some sort of mathematical trickery I could use to lower the f3 of the given BSC. Actually, I'm still fairly certain there must be. Crossover software modeling programs are able to calculate the values somehow. I just haven't been able to figure out which components in the given crossover are involved in the BSC filter and what their mathematical relationships are.
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Old 30th January 2008, 12:55 PM   #6
rabbitz is offline rabbitz  Australia
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As Thawach has mentioned there's no full blown BSC in the filter such as the one shown here.

http://www.quarter-wave.com/General/BSC_Calculator.xls

There is also some other reading here as well:
http://www.quarter-wave.com/General/..._Articles.html

There are some tricks in the xo to help with BSC such as the first inductor for the woofer and probably some extra padding for the tweeter.

Just build it and tweak. The tweeter should have no problems as it already cooked to suit. It's only the woofer that has a new BS F3 which may just need a few less turns on that first inductor.

Working this out with software could be more frustrating and time consuming than tweaking in real life which is more fun and educational any way.

Then comes the question.... how close to the rear boundary are the speakers going to be used. Near the wall, no BSC required, 1.2m into the room maybe 4-6dB BSC.
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Old 30th January 2008, 01:05 PM   #7
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You can model it if you had his measurements. Having said that I do have RS150 measurements on 7.5" baffle and 1.8mh seems quite a bit less than full BSC, particularly with an 18g inductor as advised in that thread. My own design used 2.5mh for a full 6 dB of BSC. I would build it as is and see how it sounds.

If you find you must reduce BSC try this: 1.6mh on the woofer and change the tweeters series 6 ohm resistor to 5 ohm. This will result in a slightly higher woofer and tweeter response *away* from the crossover frequency, which will tend to remain the same level. Phase integration will stay about the same this way. If you want to risk a little bit of phase deviation in return for basically the same response as the original, do the above but also bypass the tweeter's series cap with a 0.5-1.0uf cap, raising the total value to around 8-8.5 uf.
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Old 30th January 2008, 06:27 PM   #8
rj45 is offline rj45  United States
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Default bsc freq less - confused?

Dudes,

I'm not a guru, but since the OP's question was what to do since
his baffle was wider than the original plans, so that the BSC
frequency went from 600 Hz to 370Hz.

So wouldn't he need more inductance, not less?

-Don
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Old 30th January 2008, 10:22 PM   #9
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Default Re: bsc freq less - confused?

Quote:
Originally posted by rj45
Dudes,

I'm not a guru, but since the OP's question was what to do since
his baffle was wider than the original plans, so that the BSC
frequency went from 600 Hz to 370Hz.

So wouldn't he need more inductance, not less?

-Don

Yeah my original feeling was to raise the 1.8mH inductor's value, but I wasn't sure by how much and if anything else needed to change...
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Old 30th January 2008, 11:01 PM   #10
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*In the circuit posted* if you raise the inductor value the woofer's higher frequencies will drop further, no? So bass is higher in relation (disregard DCR loss for the moment), thus it has more BSC correction. If we were to lower the value the high frequencies would rise in relation to the bass, thus less BSC. People really need to get out of their heads that BSC correction is a discrete circuit, and they certainly don't used generic textbook calculators for values.You'll see there is no resistor bypassing that inductor in the design posted, like the theoretical circuit shows, makes all the difference here...
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