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Subwoofer/ported speaker Crossover options?

8K views 16 replies 1 participant last post by  Garbz 
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#1 ·
Hey all. I'm a member of multiple audio forums around the internet and began lurking here last week having a look around at various discussions. Well now I'm here, and in with my first question.

I've built a subwoofer a while ago. It was a project to satisfy my curiosity and my first dive into speaker building. Not satisfied with how I've been running it I've decided to rip the crossover appart and start from scratch. I've been directed here from another forum for some of my queries.

Anyway my speakers are B&W Nautilus 805s (the small ones), and they are ported. I do know enough about speaker design to know that the phase at the port frequencies is somewhat chaotic. So I used to run them with the ports block but I didn't like what that did to my speakers.

Anyway. Ported speakers, port kicks in below 100hz, Sealed subwoofer which does a much better job at the bass than the speakers do, but the problem is I have no way of crossing over the speakers.

I am open to suggestions of how to cross over the subwoofer though. My current idea was a 2nd order variable low pass filter 20-100hz with a first 1st order filter tuned at just over 100hz. Or a 2nd order fixed filter over 100hz. or both giving me a total of 5 lol. This whole crossover thing is a mystery to me. Can anyone give me some insight?

Picture = thread sucess: http://www.garbz.com/blog/projects/subwoofer/test1.jpg
 
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#2 · (Edited)
If you are not using any hi pass filter for the B&W's I would recommend a fixed or variable(better) active 4th order low pass filter for the sub set at or just below the 805's -3db bass point.The steep cutoff will minimise the the amount of overlap between the sub and main speakers which should help in blending the two together.Some form of phase adjustment is also very important to achieving a good blend.
 
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#3 ·
Ok I have got an all-pass filter for phase adjustment on the circuit diagram already anyway.

I do have one further issue though. The -3db point on my speakers is 43Hz. However the bass sounds reasonably weak between the 43-90Hz. Should I potentially crossover my subwoofer higher, or cross it over lower and then boost it with the EQ? Somehow the latter seems more reasonable to me, but I've also had a suggestion to put the poles in different places, like a 2nd order filter at 43Hz, and another 2nd order at 80Hz so the sub does some work within the speaker range.
 
#4 ·
You may well find that once the sub is properly blended into the system in frequency and phase the 43-90 range will be better.Room placement also has a big effect on this region maybe experimenting with that will help.

EQing a sub is sometimes neccessary but I personally would'nt want to add EQ to a hi quality speaker like the 805's.Many EQ's will introduce objectionble noise and distortion.

Placing the poles at two different points may work.You could have a fixed 2nd order pole at 40hz and another 2nd order that is variable from 40hz-100hz.Try it with the 805's port plugged and unplugged.

Another option is to add an active hi pass filter to the B&W'at 80-100hz to relieve it of all bass duties and run the sub up to that frequency.
 
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#5 ·
I'll try it with the 2 2nd order fitlers at different places and make them both variable. That should give me the maximum flexibility.

I'm not too worried about EQ. As a student of electrical engineering and currently doing filter design, once I have a proper EQ sorted making it noise free, passive, or doing it via digital FIR convolution should be unobstrusive to the sound. I'd prefer eqing the response to eliminate the horrible resonances in my room anyway. I should've spent my money on a new room not new speakers lol, can't wait to move out :daydream:

The B&W's ports plugged destroy the sound between about 800hz-3k, though the results made the bass sound simply phenominal.

I also considered some kind of low-passing on the speakers. Passive would require inductors that are way to large to economically produce, and active would not work without modifying some equipment thanks to using a single Integrated amp (Electrocompaniet ECI-3), with no tape loops. Bloody audiophile equipment just seems to irritate me with it's constant belief that reducing the customer's options is better for them.:explode:
 
#6 ·
I like that Idea.With the 2 variable low pass filters and a variable phase control you should be able to get good results.Sometimes when trying to blend a sub with the main speakers you need all the flexibility you can get.
When you mentioned EQ I wondered if you meant adding something like the Behringer BFD.While it works great for subwoofers in my opinion it has too much noise for full range useage.I tried one briefly on my main speakers and had to remove it because of the noise it introduced.

If you wanted to filter some the bass from the 805's passively you could just use a large capacitor in series with it's woofer.

btw. Electrocompaniet makes great products although they don't appear to be available in North America any more.
 
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#7 ·
Being an EE I had to open the 24band eq unit I borrowed off a friend for a party. It was fully of cheap and nasties and would probably do more harm than good!

If I would go for an analogue EQ I would spend a bit of money on it.

The capacitor in series with the woofer idea would be nice except the results would be a single pole high pass filter. That doesn't really allow for any decent integration. Inductors are not easy to source at the required value to make it 2 pole.

I'm hooked on EC. It's a pity if you can't get them in America although that may be a temp supply issue since they were just recently aquired by another company.

Anyway I'll get to work on this crossover and post my thoughts when the first version is done. Thanks a lot for your help. :)
 
#8 ·
Being an EE I had to open the 24band eq unit I borrowed off a friend for a party. It was fully of cheap and nasties and would probably do more harm than good! :)
I completely agree,thats why it lasted all of five minutes in my main speaker chain and four of those minutes were to install and uninstall the unit.:yikes:
But it does serve as a very flexible and economical tool for EQing a sub were the noise will be far less of an issue.

The series cap when combined with the woofers natural rolloff may actual be much steeper than just 1st order.Ofcourse this depends on its value and the frequency were it begins to take effect.

Anyway Good luck and do let us know how it works out.
 
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#9 · (Edited)
Well initial tests worked out very well. I'll post pictures and schematics after I've cleaned them up a bit. Currently there's a few errors here and there, but the results blend in very well and sound very tight. Unfortunately I won't be able to post EQWiz results since I don't have a phantom power mixing desk and won't be able to borrow one before leaving town next week :(

/EDIT: It didn't actually take as long as I thought. No pictures yet but the complete schematic is here: http://www.garbz.com/blog/projects/subwoofer/preamp2.pdf for those DIY minded people out there.
 
#10 ·
Very nice :T I really like all of the flexability of adjustment you included in that design.With that circuit and some tweaking along with measurements you should be able to successfully mate any sub /main combo.

I will print a copy of the schematic for future reference. (if you don't mind)

Also why did you make the input buffer/summer into a LP at 130hz?

What opamps did you use OPA2134's?
 
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#11 ·
Of course I don't mind. Most of those individual parts are textbook implimentations, there's nothing originial or trademarked in there :).

I turned the input into a LP filter just for the fun of it. I could only really make it a fixed filter and since I doubt anyone would intend to run a subwoofer higher than that it does ensure that the entire frequency range is a further 5dB down at 200Hz, so if you like crossing over very high it may make the sub more invisible. Removing the 120uF capacitor on the input summer will eliminate the filter if need be.

I used OPA2604s. But you could really use any opamps that take your fancy. With the deep sub bass and the slow waveforms it produces even a cheap TL072 will surfice for this. It really is not an application where expensive chips are warrented.
 
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#13 ·
Lol I am strict DIY man. I make my own PCBs when a project calls for it, but this is a perf-board type project :)

I did not consider DC offset as my subwoofer amp has an AC coupled input so DC offset does not matter. At full volume using OPA2604s and feeding a signal in from an Electrocompaniet ECI-3 Pre-outs (which I believe are also AC coupled) this circuit has 30mV DC on the output. But suffice to say most low-noise opamps will suit this task.

If you are interested in customising the cutoff frequencies here are the pot values for the 2 Sallen-Key Filters:
Code:
Rx Val  (kOhm) : -3dB point (Hz)

0  : 345
5  : 134
10 : 88
15 : 63
20 : 50
25 : 40
30 : 34
35 : 31
40 : 26
45 : 23
50 : 21
And the input filter follows the standard w0 = 1/RC rule so -3dB Frequency = 1/ ( 2*pi*10000*C), 132Hz for a 120nF cap.

I wired my pots backwards, AND I used log pots, so for 80% of the rotation gets me to 45hz and then it suddenly jumps up to 360hz :hissyfit:.

Anyway here's the money shot: http://www.garbz.com/blog/projects/subwoofer/preamp2.jpg
 
#14 · (Edited)
I wired my pots backwards, AND I used log pots, so for 80% of the rotation gets me to 45hz and then it suddenly jumps up to 360hz :hissyfit:.
ouch:eek: that would give some interesting results.:bigsmile:

My Brystons don't have any gain at DC but they will gladly pass it on to the speaker.I found this out the hard way.I did an internal change in an active crossover and didn't double check my work.The result was a toasted voice coil because it got hit with more than 20volts DC.
 
#16 ·
Ouch now that would hurt. I guess unless you know the circuit following it one should always just put a cap on the output for safety.
Yes it destroyed the woofer in a nice little SEAS coaxial driver.:crying: I can assure you that I quickly installed some 10uf film caps at the output of that crossover to protect my speakers from my carelessness.

Thanks for sharing the schematic.
 
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