# BFD as a low pass



## sil180sr (Dec 11, 2007)

Hello there, I have a question concerning using a BFD as a low pass filter for my sonosub. I have a HK-3485 which gives a full signal subwoofer out and that gets passed to my ep2500 to power my sono sub. Is it possible to get a BFD and add a filter to use it as a low pass at 150hz without using a mic to test for SP? I did plan on eventually using the BFD for EQ just dont have the time to figure out how it works and was curious if I could use it as a low pass filter until I got the chance to properly set up the BFD to do EQ and low pass at the same time. Thanks!


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## brucek (Apr 11, 2006)

> Is it possible to get a BFD and add a filter to use it as a low pass at 150hz without using a mic to test for SP?


Yep, but it takes more than one filter depending on how perfect you want it and at what slope.

Why 150Hz? and not 80Hz..............

If you need help with the filters required, I can create them for you, but it's not too difficult to do yourself..

brucek


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## sil180sr (Dec 11, 2007)

brucek said:


> Yep, but it takes more than one filter depending on how perfect you want it and at what slope.
> 
> Why 150Hz? and not 80Hz..............
> 
> ...


I just used 150hz as a arbitrary figure, although im not sure if my mains produce that much bass to begin with. I did happen to read a similar thread where you responded with regards to using the bfd as a low pass, but was quite confused about .. pretty much everything you said :bigsmile:. I did happen to read through the BFG guide but It really just blew by me. If you could maybe link me to previous threads which have maybe gone through a step by step process in going about the whole process that would be awesome. Thanks again!


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## brucek (Apr 11, 2006)

> If you could maybe link me to previous threads


I don't know that we've ever discussed using the BFD as a low pass/crossover with enough detail that it would be useful for you.

Basically you need to first install the BFD between your receiver and subwoofer power amp and get that working with the BFD in bypass mode. Then simply enter the filters required to create the LPF (low pass filter). To enter filters into the BFD, use the BFD users manual obtained on-line from Behringer. It tells you how to master the front panel entry.

To create the filters required to setup a LPF in the BFD, download REW software into your PC and enter filters until the results you are looking for match the target you have set. As I said before, if you have trouble with this part, I can help you through it, or I can simply give you the filters to do the job............

brucek


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## sil180sr (Dec 11, 2007)

Im considering purchasing the FBQ2496 instead of the DSP1124. Just want to make sure the FBQ will do the job just as well as the dsp. Ive probably read 5 threads comparing the two and they seem to be neck to neck. I will most likely just be entering everything manually (not using midi) and leaving it on one setting. Which is one reason why Im opting for the fbq. Just want to get a go from you guys before I press the order button!


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## brucek (Apr 11, 2006)

Yeah, I think the FBQ is a good one to get given your requirements.

brucek


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## sil180sr (Dec 11, 2007)

sounds good, I put in a order for a fbq. In the mean time ive been reading the bfd and rew guides. I seem to understand the eq process but im still confused about the filters. Do I just select LP @ 80hz?


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## brucek (Apr 11, 2006)

> I seem to understand the eq process but im still confused about the filters. Do I just select LP @ 80hz?


I don't really understand the question. Can you rephrase it another way. Where are you talking about about selecting the LP?

brucek


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## sil180sr (Dec 11, 2007)

Sure. Im referring to the EQ Filters Panel, I would check one of the 12 filters and Select the filter type as LP and frequency at 80 hz to select a 80 hz lowpass filter.


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## JohnM (Apr 11, 2006)

You have the wrong equaliser type selected, the BFD does not have LP filters. Select FBQ2496 in the Equaliser menu.


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## brucek (Apr 11, 2006)

> Do I just select LP


Ahh OK, yeah, as John points out, you have to select FBQ, and then there will only be one type of filter.

Set a target in REW and enter filters until you see the filter line track that target. Be sure to deselect the check box of all the other lines on the screen except for FILTERS. 

It's a bit of trial and error, but you'll see it's quite easy to do with not too many filters.

brucek


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## sil180sr (Dec 11, 2007)

Happy Holidays! My order for the fbq ended up on backorder so I cancelled. Im thinking of ordering the 1124. Im curious if 12 filters is enough for setting EQ, lowpass filter @80hz and a highpass @ 10hz.


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## JohnM (Apr 11, 2006)

The BFD doesn't allow filters below 20Hz, and you can't even approximate an HP response with peaking filters. You might want to consider a DCX2496 which is aimed more at crossover applications and has a better set of filtering capabilities to suit.


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## sil180sr (Dec 11, 2007)

Ive been reading a few threads regarding the DCX and DEQ models and it seems that im in over my head. I know that I want to be able to EQ, have a low [email protected] 80hz and prevent my sub from bottoming out. Ive stumbled across a thread where brucek mentioned that a HPF at 10hz probably already exisists. Which has me thinking if a subsonic filter is really necessary, and if i really need to be spending more money on seemingly confusing gear.

As for the low pass filter, I read from another thread where brucek also mentioned that with roughly 6 filters on the BFD you can create a LPF that is mathematically the same as a LPF on crossover. Just thought I would share my thoughts to help clear any confusion I may be having. Thanks!


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## brucek (Apr 11, 2006)

> I know that I want to be able to EQ, have a low [email protected] 80hz and prevent my sub from bottoming out. Ive stumbled across a thread where brucek mentioned that a HPF at 10hz probably already exisists. Which has me thinking if a subsonic filter is really necessary, and if i really need to be spending more money on seemingly confusing gear.
> 
> As for the low pass filter, I read from another thread where brucek also mentioned that with roughly 6 filters on the BFD you can create a LPF


Well, the EQ part is fine, and you can certainly make a decent resemblance to a low pass with the BFD, but the high pass isn't a function that can be created with filters that the BFD provides.

If you consider your sub audio chain (including the processor and BFD), it's entirely possible you may indeed have sufficient rolloff to protect your sub, but I wouldn't rely on it if you don't know for sure. You would have to test your own system with REW to find out. Ported subs will unload pretty quick, and so you want to check if you're OK before not using a subsonic filter.

brucek


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## sil180sr (Dec 11, 2007)

maybe ill just get myself a 1124 and just start from there. Any chance I could get those filters created for 80hz?


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## brucek (Apr 11, 2006)

> Any chance I could get those filters created for 80hz?


Yep, here's an 80Hz LPF that uses five filters. I guess it could be done in four, but I used an extra since I extended out quite far (which is probably a good idea).

Certainly in the area of interest the signal is down 45dB. That's certainly enough I would think.

I attached the REW .req file that you can load into REW and play around with yourself if you want, or you can simply read off the filters in the pic and enter them by hand.. I also attach a jpg of the area that your interested in so you can see how it tracks a standard 80Hz 24dB/octave target slope. Close enough.....

Anyway, you can also attempt to make your own when you become more familiar with REW. It's like a jig-saw puzzle to make one of these.

You can simply load these five filters into filter #8-12 of the BFD and forget them. Then you can use filters #1-7 to equalize. If you ever need more than seven filters to EQ a sub, you're doing something wrong... 

brucek

--------------------------------------------

80Hz filters








View attachment 80Hz crossover.req


60Hz filters








View attachment 60Hz crossover.req



brucek


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## sil180sr (Dec 11, 2007)

awesome! thanks brucek!


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## sil180sr (Dec 11, 2007)

Finally had a chance to punch in those filters and test it out, seems to work perfectly! Although I am a little puzzled when using REW. Im using the OSX version and when I load up the filters I get a strange looking graph, not to mention the values are different from the ones in the screen shot you posted. Maybe John can chime in on what im doing wrong? Thanks!


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## brucek (Apr 11, 2006)

Mmmm, don't know what happened there, sil180sr, but when I look at the req file that I attached in the post #17, it matches the wacky entries you have shown above. I have no idea how that happened.

I went to the file in my computer as I saved it and reattached it to the post again and deleted the old one.

"Save target" on the 80Hz crossover.req and load it into REW. It seems fine now. Go figure.

Anyway, the file (when it's correct) still looks somewhat like the one you show. For a sub we're interested mostly in the low end up to about 400Hz and then that's it. I did add filters up to 10K, but after that it really doesn't matter.










brucek


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## myn (Sep 19, 2006)

Thanks for posting the 60hz crossover


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## jakenz (Jun 18, 2008)

*Re: BFD as a low pass [with Marchand xm-46 High pass]*

Hi,

First time poster - not sure of protocols re resurrecting old threads, but I have a question squarely related to the above discussion re using the BFD as a LP filter so hope this is OK. 

My situation is that I have an SVS SB12 sub room equalised with a BFD 1124p and REW - per the great instructions on this site. The sub/mains crossover is currently set in my Yamaha receiver at 80hz, with the SB12's internal crossover disabled. This setup is working well. However I'm looking to change my setup to use a much higher quality stereo integrated amp (LFD Integrated zero LE) that I'd like to put back in service - but still make use of the BFD and the SB12 - putting aside any home theatre duties for the time being. From what I've read the optimal way to do this without compromising the higher frequencies fed to the mains but whilst easing the load on the wee LFD is to use a high quality line level crossover arrangement.

I appreciate I could just feed a line level stereo feed to the SB12 then use its internal crossover (variable LP & fixed 80hz HP) to feed the High passed > 80hz signal to the LFD to in turn feed the mains. That said I've read the SB12's LP filter is 12db/octave which I understand is not ideal if I set the crossover point as intended to 80hz and also that the quality of this crossover is probably OK but probably not realistically going to be as clear as a good external crossover. Also it'd mean long runs of line-level cable back from the sub to the amp. Instead I'm thinking of somehow using the Marchand XM46SB passive line-level high pass crossover - by all accounts I've read it's as transparent a crossover as you can get.

So, I was thinking of doing this - do you think it would work and any complications I should be aware of:

1. source/pre-amp line level stereo outs into 2 x Y RCA splitters (are splitters bad/ok?)
2. one side of each Y output feeds Marchand XM46 high pass filter set at 80hz (24db/octave)
3. Marchand feeds the > 80hz signal to LFD amp driving main speaker pair.
4. other side of each Y output feeds line level input to BFD... apply set of BFD low pass 80hz filters described by BruceK above
5. retain BFD previous room EQ settings in addition to new LP filters per 4
6. line level feed to SB12, with SB12 internal crossover disabled.

One slight variation of the above I've thought of would be to make use of the SB12's 12db/octave slope LP crossover at 80hz and supplement it with an 80hz 12db slope LP in the BFD... not sure if there's any benefit to doing this.

Note also: I'm aware there are a few volume level setting complications inherent in the above set-up... but for the mo am just focussing on getting the system-level crossover design clear in my head.

Sorry for the long first post but any feedback much appreciated!

Cheers,
Jake


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## brucek (Apr 11, 2006)

> one side of each Y output feeds Marchand XM46 high pass filter set at 80hz (24db/octave)


Why wouldn't you use the low and high pass of the marchand to do all the bass management?

Why 24dB on the high pass? Normally the high pass would be 2nd order and the low pass 4th order.



> One slight variation of the above I've thought of would be to make use of the SB12's 12db/octave slope LP crossover at 80hz and supplement it with an 80hz 12db slope LP in the BFD


That's a no-no. The sub's own filter should be dialed out.

brucek


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## jakenz (Jun 18, 2008)

Thanks for the quick response!



brucek said:


> Why wouldn't you use the low and high pass of the marchand to do all the bass management?


In a word - cost. Marchand's XM46 comes in several flavours - the standard XM46 is US$1000 which includes both the LP & HP you refer to - agree, with that one I'd just let it do all the work. They also do a cut down XM46SB which includes only a HP (or LP) filter for $250.

Given I already have the BFD I hoped I could make use of that for the LP filter, meaning the cheaper Marchand would suffice on the HP side.



> Why 24dB on the high pass? Normally the high pass would be 2nd order and the low pass 4th order.


Ah to be honest despite my throwing around a bit of HP/LP lingo around I'm very much a newbie to this... I'd assumed the LP rolloff rate (db/octave) should match the HP rolloff rate (db/octave) so give a flat frequency response around the crossover point... but from your response I take it this is not the case/a bit too simplistic?

Jake


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## brucek (Apr 11, 2006)

> Given I already have the BFD I hoped I could make use of that for the LP filter, meaning the cheaper Marchand would suffice on the HP side.


Makes sense.



> I'd assumed the LP rolloff rate (db/octave) should match the HP rolloff rate (db/octave) so give a flat frequency response around the crossover point


Well, the standard (and I believe as per THX) is that the sub LPF is -24dB (4th order) and the mains are -12dB (2nd order). This allows the natural rolloff of the mains (that is in the order of -12dB) to be combined with the bass managements HPF to result in an overall -24dB...... so that hopefully both sub and mains are down -6dB at the 80Hz crossover....

brucek


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## jakenz (Jun 18, 2008)

brucek said:


> ...This allows the natural rolloff of the mains (that is in the order of -12dB) to be combined with the bass managements HPF to result in an overall -24dB...... so that hopefully both sub and mains are down -6dB at the 80Hz crossover....


That makes good sense too thanks - I hadn't factored in the natural mains rolloff. I guess rather than second guessing my mains' in-room natural rolloff I could take a REW measurement or two of the mains only (sub-turned off) fed a full range sine way signal to figure out whether a 2nd or 4th order HPF is the better fit. (That's not going to damage them is it?) The manufacturer of my compact floorstanders claims they are "flat" in-room to 40hz but I seriously doubt that.

Jake


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## brucek (Apr 11, 2006)

> I could take a REW measurement or two of the mains only (sub-turned off) fed a full range sine way signal to figure out whether a 2nd or 4th order HPF is the better fit.


Why would it damage them. Simply take a normal measurement of one mains from 0-20000Hz. The problem you face with the results is room influence. The low end response of speaker is dominated by the room. 



> The manufacturer of my compact floorstanders claims they are "flat" in-room to 40hz but I seriously doubt that.


I don't know of too many receivers or processor that don't use an 2nd order HPF and 4th order LPF for their bass management. It's standard practice. 

brucek


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## jakenz (Jun 18, 2008)

> Why would it damage them. Simply take a normal measurement of one mains from 0-20000Hz. The problem you face with the results is room influence. The low end response of speaker is dominated by the room.


OK cool re REW mains signal - will do as you suggest. Re room influence on mains showing up in REW... just mulling this over - surely this is not a bad thing per se as it'd mean I'd be doing a more complete measurement of the speakers' real life bass extension/roll-off. The only obvious way I can only see this would be bad would be if my current room significantly reinforced the bass such that there was negligible measurable mains roll-off between 80hz (likely crossover point) and 40hz and this led me to purchase a crossover with a fixed 4th order HPF... and then I later moved to a much larger room with much less room reinforcement resulting in an in-room natural 2nd order rolloff at 80hz which when combined with the 4th order HPF would result in a net dip in bass response below 80hz. Or are there other risks/complications here I'm missing?



> I don't know of too many receivers or processor that don't use an 2nd order HPF and 4th order LPF for their bass management. It's standard practice.


This is interesting. I've done a little googling here to understand this better and I've come across criticism in some quarters of this standard practice of receivers arbitrarily having a 2nd order HPF to accompany the 4th order LPF. My understanding of the basis for this criticism is that it is a one-size-fits-all approach that makes an assumption the main speaker will always have a 12db natural rolloff from the crossover frequency i.e. that the main speakers are always small or sealed. 

If what I've read is accurate THX certified main or satellite speakers were designed to always have this 2nd order natural rolloff at 80hz which combined with the THX mode's 2nd order HPF at 80hz gives the desired combined 4th order rolloff you refer to. The whole point of a THX standard I guess.

I assume cost and simplicity of design and operation drive the above approach both re THX and receivers' HPF more generally - i.e. what will work for most people most of the time. That said surely the ideal (cost and simplicity aside and assuming no quality compromise) would be user selectable order of HPF rolloff to allow the speaker to perfectly match their mains to the sub in the room they are using it in. i.e. effectively what I am contemplating in measuring my mains in-room and selecting a HPF rolloff order to best match the sub/BFD's target 4th order LPF.

Another thing I've read over the last week.. (its been a bit of a cram session on crossovers  ) is that all things being equal a steeper 4th order HPF is preferable to a 2nd order HPF in that it takes more low frequency load off the mains amplification and minimises phase effects to boot. This makes intuitive sense to me but to offset this I've also read 4th order crossovers are tougher to build well.

Hopefully I'm not way of track with the above and in any case its all proving to be a very interesting and steep (6th order?) learning curve...


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