| beanbag |
Hello,
I have a lm3886 amp built as a non-inverting amp. Currently the only thing between the signal source and the amp is a pot. However, I recently got a subwoofer, and now need to build a crossover. I decided to do a 4 pole L-R filter. However, I would like to degrade the signal to the high frequencies as little as possible. So the thing I don't want to do is build separate HP and LP stages, coz the high frequencies will now go through a buffer +2 op-amps.
My current idea is to buffer the input to the amp. Then build a LP L-R filter that has one amp inverting. Then sum the signals together at the input of the LM3886. (The low frequencies will cancel out, mostly) So now the high freq's only go thru one buffer, and look at the output of a LP filter, which should be nothing at high frequencies anyway. What do you guys think of this idea? Is there a better way, or does it even matter? Thanks |
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| pinkmouse |
| What's the accoustic roll of of your main speakers? |
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| beanbag |
| The woofers are sealed, so it should be 12dB/oct. The speakers claim to play down to 45 Hz, but I called the company and they suggested I cross over at 80 Hz. |
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| BobEllis |
Yes, the highs only seem to go through one opamp in your schematic. However, any grunge from the low pass section is also mixed in with the highs. The low pass section will be a 4th order electrical filter with 2 op amps. - so your top end gets 3 op amps' distortion products mixed in.
If your speakers really have an F3 of 45 Hz, then you can use just a second order 45 Hz. electrical filter (one op amp) to achieve a 4th order acoustic response. Whatever the sealed F3 of your mains, set a second order electrical filter to the same F3 and you'll havea 4th order acoustic response. You'll have to play wiht the filter's Q to mate with the speaker's Q to get true Linkwitz-Reilly response. You ought to be pretty close if you use a filter with component values chosen for a 4th order L-R response.
Did the manufacturer recommend a 4th order or second order 80 Hz XO? they may have said 80Hz thinking that you were planning an electrical 4th order high pass. The most accurate thing to do is to measure the response of your mains and design your electrical filters accordingly.
FWIW, I have an active XO that has 7 sections of NE5532 in the high end and midwoofer sections, and 5 in the subwoofer section. The top end is Buffer-HP-phase correction-EQ-HP-buffer/inverting buffer. I listened to it without the phase correction and EQ, then added each one stopping to listen. In each case, I did not notice any loss in clarity or imaging. In fact, the improvement due to phase correction was just short of amazing. My then 14 year old son stopped playing video games to ask what I had done to make the speakers sound so good.
While plan to try other op amps, at the moment I enjoy listening too much to tear it apart. The amps are modified Pass/Thagard A75s (higher rails for increased output) and the speakers are Focal 6W4252 woofers and either Focal TC120TD5 or Fountek JP3 ribbons, in identical cabinets.
I also tried the setup with a pair of app note LM4780 amps (2 3886s on one chip) and my Leach amps. The amps had much more impact on the sound than the additional opamp stages - I could tell the difference between amps.
In short, don't sweat the addition of an op amp or two in the signal path. Give fully active XO's a shot. |
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| sagarverma |
anyone tell me the ideal frequency for subwoofer.i have built 12db/octave lr xover from rod elliots site.xover at 350hz.mainly for subwoofer.
is 350hz too high.should it be lowered.what is best value so woofers give clean bases only.no voice etc desired only that THrrrrrrrrrrrrrrrrrrrrrr THrrrrrrrrrrrrrrrrr sound desired that comes outta cars with great system
i suppose i made myslf clear.
thanks in advance:D |
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| beanbag |
Thanks for the reply, Bob.
I thought about the fact that the HF signal "looks into" the output of a 2 op-amp filter. My expectation is that the output has no noise (except white) at the higher frequencies. The only effect I can think of is output impedance vs frequency effects for the last op-amp in the filter.
When I asked the manufacturer about crossover freq, the guy said that yes, he does suggest crossing over at a higher frequency to make the load on the speakers easier. When asked about the slope, he said that 2nd order is a maybe, and 4th order is fine. |
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| sagarverma |
anyone tell me the ideal frequency for subwoofer.i have built 12db/octave lr xover from rod elliots site.xover at 350hz.mainly for subwoofer.
is 350hz too high.should it be lowered.what is best value so woofers give clean bases only.no voice etc desired only that THrrrrrrrrrrrrrrrrrrrrrr THrrrrrrrrrrrrrrrrr sound desired that comes outta cars with great system
i suppose i made myslf clear.
thanks in advance |
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| BobEllis |
Well, Beanbag, it sounds like you ought to go 4th order electrical. From the manufacturer's response, it seems that your speaker's response extends to the mid 40s but does not have much output capability at the low end.
Sagaverma - The ideal crosover frequency depends on your sub driver's high end respnse and main speakers' low end response. Any frequency within both driver's capabilities that allows smoothly blending the response is "ideal." At 350 Hz, you should be using stereo subs placed to time align them with your mains. Is this a car or home application?
From your description, it sounds like you are after "one note" bass. :( This is not a crossover issue, it is a driver/box issue, although a High Q HIGH pass filter can achieve something like that by boosting the low end response of the sub. Try a high Q driver in a too small box, or a narrrowly tuned bandpass box. If you go bandpass, then 350 Hz is definitely too high - shoot for under 100. Not high fidelity, but if it gives you a thrill, go for it (but not where I can hear it) ;) |
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| carlmart |
| quote: | Originally posted by sagarverma
anyone tell me the ideal frequency for subwoofer.i have built 12db/octave lr xover from rod elliots site.xover at 350hz.mainly for subwoofer.
is 350hz too high.should it be lowered.what is best value so woofers give clean bases only.no voice etc desired only that THrrrrrrrrrrrrrrrrrrrrrr THrrrrrrrrrrrrrrrrr sound desired that comes outta cars with great system
i suppose i made myslf clear.
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350Hz is definitely too high. Something in the 100-200Hz should be much better. It will all depend on how low your mid-bass can handle well.
Carlos |
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| sagarverma |
bobellis,
i need that sound which rattles surrounding.people should turn and see when my car drives by.+
i built Mr Elliots xover adhoc bec site says biamping improves amp performance dramatically.
from your reply i im making that u advising me to try something like this http://sound.westhost.com/project48.htm
thanks for telling me the freq.i will retune xover for 100hz.saw sony222
in friends car.its lp filter is at 80hz for sub applications.
carlos
thanks for your reply too
i have chosen 106hz now(its so bec 15k and 100nf for my filter gives this freq. which is nearest to 100hz desired)
:devilr: |
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| beanbag |
| I found out that this is called a subtractive crossover. I read an article on ESP's site saying why it was bad. Anyway, I just built a "normal" 2 way LR crossover and even though the HF signals now pass thru 3 op-amps, I can't really notice any difference in sound vs. a signal that doesn't pass thru any. |
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