# Defeating a High-Pass Filter



## Josuah (Apr 26, 2006)

I'm curious, but would anyone have any knowledge or opinions on defeating the high-pass filter built into a subwoofer's plate amp? I realize it would then no longer be protecting the subwoofer from over-excursion, but I'd still like to know if anyone's got any information on doing this. The reason I ask is because I've got a subwoofer that has a high-pass at 25Hz which really sucks. I don't want to push it hard, I just want to be able to get 20Hz out of it.


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## Mike P. (Apr 6, 2007)

You can change the filter on some plate amps by changing 2 resistors.


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## Josuah (Apr 26, 2006)

I figured it might be something like that. I suspect I'll just have to find a diagram of a high-pass filter and see if I can figure out what part of the amp matches it, and then do something about it.


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## Mike P. (Apr 6, 2007)

There are known and documented changes for specific plate amps. What make and model do you have?


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## brent_s (Feb 26, 2007)

Make/model are very important.

Considering this isn't a DIY forum, I'm guessing Johsuah has a commercial sub of some sort. Chances are the manufacturer put a high pass filter there for a very good reason. I know you said you didn't want to push it hard, but chances are with that high a filter, removing the filter will result in you having less usable output overall b/c you never know when the death stroke 20hz signal is going to come along if you try playing at your pre mod levels.

BTW, how did you determine the filter was at 25hz? Just as any other xover, the filter is not a brickwall...most are designed to be -3db at the specified cutoff frequency with a 12db/octave slope below that. 

-Brent


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## brent_s (Feb 26, 2007)

BTW, here is page with lots of plate amp info and a spreadsheet for calculating frequency and filter Q.


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## Josuah (Apr 26, 2006)

I'm talking specifically about the AV123 Rocket UFW-12. Audioholics IIRC listed the 10th order high-pass setting. I've also measured it there; my measurements match those as taken by Secrets.

I don't care so much about risking bottoming out the sub. I don't and won't push it hard; this is for my video game setup.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Piece of cake. With an equalizer like the BFD you could use a filter to essentially over-ride the HPF.

Regards,
Wayne


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## Josuah (Apr 26, 2006)

I don't think I want to stick a 10th order gain filter in front of the subwoofer amp...somehow it seems wrong. Not to mention it would probably play havoc with the phase and delay (although I guess I could try to correct that in the unit as well).


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## Wayne A. Pflughaupt (Apr 13, 2006)

Functionally it would be no different than bypassing the filter. Either way, you’re flattening response that is now rolled out. 

Phase is only an issue when have something like a crossover between a sub and main speakers when the two aren’t physically co-located.

That said, I doubt the equalizer could fully overcome a 10th order filter, but it should easily be able to get the goods down to 20 Hz for you. Besides, you did say you won't be driving it hard... 

Regards,
Wayne


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## Josuah (Apr 26, 2006)

Hm. Well, I suppose I could try modelling it in WinISD to see what it thinks the effects of adding a filter would be, theoretically. But it should add an additional delay due to signal processing which would affect the phase (relative to other frequencies produced by the subwoofer), unless I don't know what I'm talking about. Even without crossovers, the phase of a single driver isn't the same everywhere.


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## F1 fan (Jul 6, 2006)

That HPF maybe of the underdamped variety and is actually adding a bit of boost before it starts to roll off the response.Defeating it may actually decrease your bass extension but it depends on the Q of the filter.Also phase is affected in that if you run the signal through a 12db filter, phase will rotate 180 degrees.Therfore the phase switch (or if it is a variable phase control) will need to be readjusted if the filter is bypassed.


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## F1 fan (Jul 6, 2006)

OK I'm guilty of posting before reading the entire thread and checking the links so didn't pick up on the very unusual and steep 60db per octave filter incorporated into that unit.The majority of plate amps(at least the ones used by DIY'ers) that have integral HP filters are 12 db per octave types and my comments were based on that.
However the extreme rate of roll off and the increased group delay will also likely result in the sub having a different phase relationship with the mains if the HPF was to be removed thus requiring adjustment.

I can't find any info in regards to whether the filter is also adding boost before it starts it's job of attenuating?.


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## brent_s (Feb 26, 2007)

As F1 highlighted, the 2 resistor mod for HPF's mentioned earlier assumes the typical Sallen-Key 2nd order filter used in stock plate amps designs. The UFW-12 appears to have some serious modifications to get a 10th order (60db/oct) filter. From Ed's review at Secrets, there's another 24db/oct coming from the driver for a total of 84 db/oct (*14th* order) below 23hz. That 24 db/oct is worth noting since this is a sealed alignment which _should_ have something approximating 12db/oct naturally. This is especially noteworthy given how big the cabinet is for a 12" sealed...I guestimate 4 ft^3 gross internal. 

Knowing the reputation of the engineering talent that MLS likes to use on AV123 designs, if they're working that hard to essentially brickwall the UFW-12 at 23hz, I'd pay attention...they've got to have their reasons. Considering the apparently substantial EQ already being applied, it should have been a fairly trivial exercise to add appropriate frequency specific power limiters and shift the HPF lower if there was any usable output below 23hz, IMO.

As a relatively simple experiment, at the risk of "*not covered by warranty*" damage to your driver, you could pick up something like the BASH 300 or SA240 from PE. Their default HPF is set at 18hz. Add a set of binding posts somewhere inconspicuous on the cabinet and connect the driver/substitute amp to these posts for testing. Actually, the same could be accomplished with any spare amp/receiver you have on hand.

-Brent


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## Josuah (Apr 26, 2006)

I actually just asked Mark Seaton about trying a different amp, because the Dayton 1000W amp sold by PartsExpress is is the same dimensions, same enclosure, but different internals. He discouraged attempting to use it with this particular driver because the extension would be lost; probably something like F1 described is going on here. And yeah, it's about 4ft^3 inside, because you take about 1ft^3 away for the amp enclosure and driver.

I suspect that the driver will bottom out of the HPF is removed and you run in hard, based on current discussions about having to replace both the amp and driver, rather than just the amp. But since I'm not running it hard, I'm not as worried about bottoming out.


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