A question on low pass filters

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Hi there,

For most subwoofers, there is a crossover control knob but it's more accurately known as a low pass filter.

But I've noticed on quite a few subwoofers that there are high level speaker connections to the subwoofer. What is the purpose behind this type of connection and why would anyone want to use it ?

Second, do most subwoofers have built in high pass filters and again, why would anyone want to use these ? As far as I understand it, all that one needs to use is the built in high and low pass filter on the AV receiver and then turn the low pass filter on the sub all the way up.

I noticed on one system where the main speakers are set to large and all speakers set to small, 80hz crossover on amp. And yet on the subwoofer, the connection used is 'line in' and low pass control is set to 80 hz. The user won't be experiencing all the bass according to this setup because won't both low pass filters be added in series ?

Doesn't this cause cascading ? Using an 80 hz low pass on sub with an 80 hz low pass on AVR would cause frequency response abberations, would it not ?

I look forward to discussing this more. Thanks !

--Regards,
 
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The reason you have high pass filters on the amp is to allow the use of satellites by running the output wires from the plate amp instead of your regular amp . The high pass filters only work when you use the high level inputs (ie: speaker wire not RCA) If you use the RCA inputs, those speaker outs are inactive. It just cuts down on the amount of wire used.

Why high level inputs you ask? Because not everyone has an amp with a subwoofer RCA output. As far as missing some of the music; many will not even notice the gap.
 
So if I set my main speakers to large and set my other speakers to small (80 hz x-over) and set my subwoofer low pass filter to 80 hz, what is essentially going to happen ?

I can only imagine that two crossovers are going to combine. But I may be wrong. What are your thoughts ?

--Regards,
 
Why would that be ? Why would 80 hz low pass filter have any effect on the main speakers ? They have been set to large. So by definition, there is no crossover placed on them. They are handling a full range signal which has it's own pro's and con's.

But the rest of the speakers are set to small and 80 hz crossover. So the question is, if I set 80 hz low pass filter on sub and in AVR, will the person with this setup experience all the bass in those other channels or will both crossovers combine and cause cascading ?

Thanks !

--Regards,
 
If the mians are getting a full range signal, and the sub is getting 80Hz and down, then you have both the sub and the mains playing the same tones. This can cause the bass to be too loud, or even to be less loud due to cancellation. If you really like running your mains as Large then set the subs crossover to a lower point.

Keep in mind that for home theater aplications it is widly recomended to run the mains as small and let the sub do all the heavy work - regardless of the main speakers bass capabilities.
 
Vaughan said:
So the question is, if I set 80 hz low pass filter on sub and in AVR, will the person with this setup experience all the bass in those other channels or will both crossovers combine and cause cascading ?

By doing this you may risk cascading the crossovers. Whether or not this be soniclly degrading depends on many factors. Best solution is to set the AVR to send <80hz to the sub, >80 to every other speaker and disable the subs crossover (use a bypassed input, or if unsure set the subs crossover to maximum)
 

If the mians are getting a full range signal, and the sub is getting 80Hz and down, then you have both the sub and the mains playing the same tones.

I thought the main channels were seperate from the LFE channel. So the information in the main channels are not unique compared to the LFE channel ?

Are you suggesting that both the main channels and the LFE channel puts out the same information so that both channels voice 80 hz ?

--Regards,
 
Vaughan said:
Why would that be ? Why would 80 hz low pass filter have any effect on the main speakers ? They have been set to large. So by definition, there is no crossover placed on them. They are handling a full range signal which has it's own pro's and con's.

Yes so if the mains are playing 80Hz and below, you don't want the sub playing upto 80Hz as there will be bump in the crossover. This is for music without a separate LFE track. You set the crossover low-pass to meet up to where the mains fall away.
 
I think the reason why I disagree with the "lowering crossover if you have a floorstander" option is that the information voiced by the speaker at, let's say, 40 hz will not exactly be cancelled by the subwoofer provided it's also reproducing 40 hz content in the LFE channel.

But if both channels are different with different information contained therein then it is highly improbable that cancellation (or reinforcement) can happen. Just my thoughts on it.

--Regards,
 
Yes. Exactly.

I still have no idea why I see people use the subwoofer low pass filter instead of using the crossover in the AVR. I've encountered so many setup's where the crossover control on the sub is at 80 hz but then in the AVR, crossover is set to 80 hz, speakers set to small.

People don't realize that they are adding two low pass filters in series. I even tried test tones today and found that the level of the tones go down dramatically when set to 80 hz (and 80 hz low pass filter used at the back of sub).

Once the crossover control is set to maximum freq, there is greater output. Just shows you how much cancellation goes on.

--Regards,
 
My new setup (to be), which I'm slowly in the process of having a go at - haven't actually bought the parts yet, still thinking about options, will be:

- AVR x-over set to 80Hz (50Hz, 80Hz, 100Hz, .... are my options)
- Floor standers are bi-amped, MTM section using AVRs mains output, Woofer a separate amp.
- The AVRs sub setting turned off, mains on "Large", other speakers set to "Small", so all bass (+LFE channel) will go to mains
- The mains pre-outs will go through an active x-over @ 60Hz or so, < 60Hz to sub, > 60Hz to woofers.

Advantages I can see:
- The woofers in the mains (8"s) sound nicer for tight bass and music etc, but cannot deliver truly deep bass. The sub doesn't sound great at the higher bass freq. In a sense it's the best of both worlds.
- The surrounds/centre etc will effectively be high-passed at 100Hz (or maybe 80Hz) allowing for a smaller speaker - bit of a compromise really. The bass to them will be split between the mains woofers and the sub.
- The mains will effectively be high-passed at 60Hz.

The disadvantages:
- The "Large" setting for the mains means that a full-range signal is sent to the MTM config, with my only solution (currently in use for testing) being a passive high-pass. For testing purposes I'm using a cap on the MTM section and a coil on the W @ 225Hz, it seems to cope well.............is significant power being wasted by not having a receiving driver for all frequencies there????
- Cost, I have to build an active stereo 2-way (looking at the Rod Elliot stuff at present)

Does this sound like a worth while adventure, or a waste of time (the money is quite insignificant)? Considering I already had all the passive components, the only cost is the active 2-way and I'm sure I'd find a use for that if I don't like this new setup.
 
Well it comes down to if you want it for music or films. For music you should not use the LFE signal, just take the fullrange signal from left and right, so then you would set the appropriate crossover blend (usually lower than 80Hz) with the sub control.

In fact you can do films this way too if your amp can route the LFE channels to front left and right. That's what I do.
 
the crossover in the receiver is in the preamp ciurcut and so does not react the way you are describing with the crossover in the subwoofer. I would think it would merely increase the slope and not affect the frequency. I'm guessing though I may be wrong. And whether or not the front speakers and the sub are playing the same information is dependant on the settings on your receiver. I would assume if you set the front speakers to large and the other speakers to small the receiver would send the bass information from the LFE channel(duh) and the surround and center channel but not from the fronts. I have B&W DM220's in my front channels with two 8" woofers each so they are capable of decent bass performance and the rest of my speakers are very small but I still set the fronts to small because this take a great strain off of them and leave more power for clean and clear midrange. Not only that but the lower the excurtion of your woofer the lower your over all distortion. So if you're Sub is well placed so as not to make it obvious that the bass is not comming from the main speakers your better off setting your front speakers to small IMHO.
 
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