# Amp subsonic or infrasonic filter necessary?



## LittleJoe (Feb 16, 2011)

for a sub? If I am using a 4 channel bridged amp to feed 350-400 watts to a JL 8w7 does does not have a subsonic or infrasonic filter will it damage anything? I just remember back in the day I ran tons of power to multiple amps that did not have those filters. no harm done.
I am not sure if this box will be sealed or ported. I am wanting sealed simply for space.


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## TREETOP (Feb 11, 2009)

An infrasonic filter is generally only used with a ported enclosure, to help prevent the sub from unloading below tuning frequency. I normally set it at 1/3 octave below tuning frequency.
For sealed, don't even sweat it.


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## LittleJoe (Feb 16, 2011)

well originally i was going to go sealed but I only have about 350 watts available. That lead me to thinking I should go ported but I don't have the filters on my amp. Therefore I may be back to using a sealed box. I'm just worried that the sub will not give that much output.


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

Reducing the subsonic info, leaves more power for "other" things..


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## LittleJoe (Feb 16, 2011)

Aaron'z 2.5RS/WRX said:


> Reducing the subsonic info, leaves more power for "other" things..


I'm not sure what that means. 
Will a amp without filters be ok to push a 8w7 sealed using 350-400 watts?


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## The Baron Groog (Mar 15, 2010)

LittleJoe said:


> I'm not sure what that means.
> Will a amp without filters be ok to push a 8w7 sealed using 350-400 watts?


He means that by removing in-audible content below 25Hz (or so) you are INCREASING the power available to the sub in the freq ranges you can hear. To play a freq of 20Hz to the same volume of a 40Hz freq would require 4x the power, so even if you are running a sealed enclosure an infrasonic filter will benefit your system's efficiency/loudness in the audible range.


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

Thank you Baron


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## The Baron Groog (Mar 15, 2010)

^ no worries


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## legend94 (Mar 15, 2006)

i keep searching for information on subsonic filters and this is why I want to add one even though i am using a sealed box. sadly my hu does not have one built in(not that many do). what would be the cheapest solution to filter out the frequencies that the sub can't play? 




The Baron Groog said:


> He means that by removing in-audible content below 25Hz (or so) you are INCREASING the power available to the sub in the freq ranges you can hear. To play a freq of 20Hz to the same volume of a 40Hz freq would require 4x the power, so even if you are running a sealed enclosure an infrasonic filter will benefit your system's efficiency/loudness in the audible range.


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## minbari (Mar 3, 2011)

The Baron Groog said:


> He means that by removing in-audible content below 25Hz (or so) you are INCREASING the power available to the sub in the freq ranges you can hear. To play a freq of 20Hz to the same volume of a 40Hz freq would require 4x the power, so even if you are running a sealed enclosure an infrasonic filter will benefit your system's efficiency/loudness in the audible range.


x12! a SSF is absolutely necessary for a ported enclosure, for the reasons stated. but can be usefull for sealed boxes as well.


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## BuickGN (May 29, 2009)

Does your sub amp have one?


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

BuickGN said:


> Does your sub amp have one?


 
Did you get my PM?


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## legend94 (Mar 15, 2006)

BuickGN said:


> Does your sub amp have one?


no, but i could always buy one that does! i know i dont want to go the rca route because i think that degrades the signal going into the amp way too much. the HU i will be using is an alpine 9835 but it doesnt have one either.


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## minbari (Mar 3, 2011)

legend94 said:


> no, but i could always buy one that does! i know i dont want to go the rca route because i think that degrades the signal going into the amp way too much. the HU i will be using is an alpine 9835 but it doesnt have one either.


not picking on you or anything. but how do you think the amplifiers that have them built in do it? its a HPF in the preamp section. no different than what you could put on the RCAs. either via a x-over or Fmod.

you could do it passively to protect the woofer, but it would negate the power saving properties of doing it in the preamp.


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## legend94 (Mar 15, 2006)

minbari said:


> not picking on you or anything. but how do you think the amplifiers that have them built in do it? its a HPF in the preamp section. no different than what you could put on the RCAs. either via a x-over or Fmod.
> 
> you could do it passively to protect the woofer, but it would negate the power saving properties of doing it in the preamp.



no pick away, i am here to learn. what you say makes sense i have just had bad results with the harrison lab rca products. it could be that the decade ago that i used them not many head units produced a high enough signal.

my goal is to protect the woofer and i will be using an arc series 10 inch sub with dual 2ohm voice coils in a sealed box. i will be using 400 watts from and audio art amp and if that is not enough i will use the 1k from my rf 25 to life 150!


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## minbari (Mar 3, 2011)

If the sub doesnt have a SSF built in, then you can, as suggested, get an amplifier that does or get an external method.

for my money, I would get something like a miniDSP 2x4. it would add SSF, x-over, eq, ta for both the sub and midrange amplifier and its only $100.


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## legend94 (Mar 15, 2006)

minbari said:


> If the sub doesnt have a SSF built in, then you can, as suggested, get an amplifier that does or get an external method.
> 
> for my money, I would get something like a miniDSP 2x4. it would add SSF, x-over, eq, ta for both the sub and midrange amplifier and its only $100.


wow that mini dsp has some features!i was hoping for a cheaper way but thank you so much for your time. i guess i will shop around for an amp that has one built in.


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

Bass Cube Phoenix Gold Bass Cube - Black - Constant Usable Bass Enhancement - | eBay

WoofersEtc.com - Bass Cube - Phoenix Gold Bass Maximizer


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## legend94 (Mar 15, 2006)

thanks, i am looking into that if i cant find another amp that i like with subsonic filter. any amps come to mind that you have used with one built in?



Aaron'z 2.5RS/WRX said:


> Bass Cube Phoenix Gold Bass Cube - Black - Constant Usable Bass Enhancement - | eBay
> 
> WoofersEtc.com - Bass Cube - Phoenix Gold Bass Maximizer


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

legend94 said:


> thanks, i am looking into that if i cant find another amp that i like with subsonic filter. any amps come to mind that you have used with one built in?


My SS Continuum does...


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## legend94 (Mar 15, 2006)

Aaron'z 2.5RS/WRX said:


> My SS Continuum does...


it has everything!

i actually had a 10.0 many years ago and i think it had one but of course i didnt know what the heck it was.


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## nosaj122081 (Apr 7, 2012)

I did a lot of reading when I put together my ported box as my sub amp doesn't have a subsonic filter... 

http://www.diymobileaudio.com/forum/474394-post7.html

I don't have one (on my sub amp, my front speakers have a 5Hz filter via my EQ ) and don't have any problems as a result, don't have one in the house either, also in a big ported box tuned to the mid-20's.

My SS Rubicon 5.800 had one.


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## avanti1960 (Sep 24, 2011)

My amp came with a 30hz infrasonic filter (JL HD900/5, 500 watts to the sub channel). 

At first running a ported sub I did not know if I should use it or not- the overwhelming recommendation was to definitely use it for a ported sub to protect the woofer from unwanted travel when subjected to ultra low frequency rumble. 
Since I like the deep ultra low frequencies (or thought I did) I ran the 12" ported with the infrasonic off. Then I enabled the filter. The bass tightness, quickness and overall sound quality improved noticeably. It was definitely a keeper for ported enclosures. 
Then due to cargo space constraints I built a sealed enclosure for the 12" which was much lower in profile and smaller in enclosure volume. 
Because the box was now sealed, I switched off the infrasonic filter. This lasted for about 10 minutes- I could sense the woofer was pulsing on subsonic input and that it was kind of like turntable rumble from the days of vinyl- not what you want to hear. The bass was OK but I expected a much tighter sound because of the sealed enclosure. 
So I switched the filter back on. Boom- the SQ even in a sealed enclosure was much more enjoyable, musical, responsive and just better with the filter activated. Maybe it has to do with this particular amp and the specifics of the frequency and slope- I don't really know- but JL is working some magic with this amp and infrasonic filter and I will not run without it ever again- sealed or ported.


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## nosaj122081 (Apr 7, 2012)

avanti1960 said:


> My amp came with a 30hz infrasonic filter (JL HD900/5, 500 watts to the sub channel).
> 
> At first running a ported sub I did not know if I should use it or not- the overwhelming recommendation was to definitely use it for a ported sub to protect the woofer from unwanted travel when subjected to ultra low frequency rumble.
> Since I like the deep ultra low frequencies (or thought I did) I ran the 12" ported with the infrasonic off. Then I enabled the filter. The bass tightness, quickness and overall sound quality improved noticeably. It was definitely a keeper for ported enclosures.
> ...


You got me thinkin' I might have to try it now... My P300-1 has an input impedance of 20kΩ (so says the manual), by my math a 20Hz 1st order HPF would need a .4μF cap, I happen to have a .33μF mylar cap sitting right in front of me, some number crunching says that would make for a 24Hz HPF (pretty damn perfect for my tuned frequency). Dunno if I'll get to it tonight, but I'll wire this cap up and come back with results.

I don't have any inductors, otherwise I'd go 2nd, so 1st order is my only option (for free).


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