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Old 24th August 2007, 06:36 PM   #1
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Default A question on low pass filters

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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Old 24th August 2007, 06:47 PM   #2
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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.
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Old 24th August 2007, 06:55 PM   #3
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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 ?

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Old 24th August 2007, 07:27 PM   #4
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What you propose is about right, but you may find 80Hz on the sub a bit too high if your main speakers are floorstanders.
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Old 24th August 2007, 07:37 PM   #5
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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 !

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Old 24th August 2007, 07:53 PM   #6
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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.
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Old 24th August 2007, 07:55 PM   #7
JRace is offline JRace  Canada
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Quote:
Originally posted by Vaughan
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)
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Old 24th August 2007, 08:03 PM   #8
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Quote:
JRace
Quote:
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 ?

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Old 24th August 2007, 08:29 PM   #9
JRace is offline JRace  Canada
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LFE can either be bass from the mains or a completely different signal, depends on how the disc was mastered. I have found best results sending both LFE and <80Hz to the sub, this way you don't miss any info that may be present on the LFE channel.

http://www.hometheaterhifi.com/volum...pril-2000.html

explains it much better than I could.
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Old 24th August 2007, 08:31 PM   #10
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Quote:
Originally posted by Vaughan
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.
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