# can i set a hpf with dsp1124



## apilon (May 18, 2006)

Good evening all ,

I am building a new sub and need to set a hpf at 20Hz. Can i do that with the dsp1124 or Anthem ARC calibration 


Alain


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## gazoink (Apr 17, 2013)

apilon said:


> Good evening all ,
> 
> I am building a new sub and need to set a hpf at 20Hz. Can i do that with the dsp1124 or Anthem ARC calibration
> 
> ...


Why do you want to drive a sub from a 20Hz HPF? You could do it with the dsp1124 (not the Anthem), but can't imagine why you would.


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## apilon (May 18, 2006)

gazoink said:


> Why do you want to drive a sub from a 20Hz HPF? You could do it with the dsp1124 (not the Anthem), but can't imagine why you would.



Because the sub will be vented and i was told that with a vented sub it was a good idea to add a hpf the sub will be tuned at 20 hz and was advise to set the hpf at 20 hz


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## gazoink (Apr 17, 2013)

apilon said:


> Because the sub will be vented and i was told that with a vented sub it was a good idea to add a hpf the sub will be tuned at 20 hz and was advise to set the hpf at 20 hz


A properly designed and tuned vented sub not only wouldn't need the hpf, but shouldn't have one. I might see the point for a sealed sub as some sort of excursion limit, but there are better ways to do that. The challenge with every sub is to get extended bass response. It's hard enough to get it to do 20Hz at reference without fighting with an hpf too. A sub tuned for 20Hz would be 3dB down at 20hz at very least. Put a 20Hz second order lpf there too and you'll be 6dB down at 20Hz with the filter affecting 30Hz as well to a lesser degree.


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

apilon said:


> I am building a new sub and need to set a hpf at 20Hz. Can i do that with the dsp1124


Not really. By definition EQ filters are band pass filters, not high- or low-pass. Many stand-alone pro audio equalizers do have on-board adjustable high and low pass filters, but the BFD doesn’t. That said, some attempt to wrangle HP or LP characteristics by stacking filters, but it’s really a poor substitute for an actual HP or LP filter. See here for additional relevant info.

Regards,
Wayne


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## jtalden (Mar 12, 2009)

apilon, 

It all depends!

I have 2 very old JBL woofers (2235H) with an Xmax of 8mm in 5 cu ft boxes tuned to 20Hz. These drivers would be easily over driven with any strong signal significantly below the Fc of 20Hz where they are unloaded. I do use a HPF for protection. The higher the Fc, the louder the SPL and the lower Vd of the vented SW the more important it is to provide an HPF.

I looked up your SW driver and it is designed for SW use specifically. The Xmax is spec'ed at 25mm?! With this type of SW driver it is unlikely it will be damaged. I suspect the manufacturer knows there are cases where it will be overdriven and have probably designed it accordingly. I therefore agree with gazoink that it is unlikely that a HPF is needed if you are a typical user.

The best way to add confidence is to model the max output in WinISD or similar software and see how much SPL is available in the operating range before max excursion is reached below Fc. For example, if your intended design/usage is for the SW to be capable of 115dB 20-100Hz, you may find that it may or may not be reaching its excursion limit below the Fc. If it is then that suggests that there may higher distortion in the 20-100Hz range *if* there is an equally strong signal at say 6-12Hz causing the excursion limit to be reached. 

That said, I would expect it to be impossible to detect that distortion is higher than normal during an explosion, canon fire, or other high signal level below 20Hz. When there is no significant signal below 20Hz then the SW will be able to provide the full output with its normal level of distortion.

The short version is; if you expect to exceed the SW excursion limits below Fc in your application then a HPF is a good idea.

If you use an HPF it is easy to compensate for SPL response impact with EQ. 

I took a quick look at the dsp1124 manual and saw no indication of a true HPF capability. You may want to look at the MiniDSP or the DCX2496LE if you want this feature.


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## gazoink (Apr 17, 2013)

Applying a HPF to prevent over excursion is a rather linear solution to a non-linear problem. The HPF is not needed or desired at any time until the driver approaches XMax, and then it is. Yet, if the HPF is present all the time and the low frequency extension is affected all the time, even though XMax is rarely reached. The result is the HPF compromises performance just to help reduce the rare chance of over excursion.

The far better non-linear solution is a dynamics processor with pre and post EQ set to a curve that tracks the power vs frequency curve where the driver would hit XMax, or rather just before that. The dynamics processor is essentially a frequency selective limiter that only acts to prevent over excursion, and otherwise is passive. A fair approximation of the ideal function can be found in the Dynamic EQ feature in a DEQ2496, utilizing the BP mode and careful frequency, Q and threshold adjustment for a specific sub. You're attempting to "model" the frequency vs excursion curve of the sub, then apply a peak limiter to it. It takes a bit of playing and calibrating but it can be done, though the DEQ2496 is a bit hobbled by the "Mode" function. You really need a bit more control to nail it, but it's still way better and more effective at XMax limiting than a fixed HPF. 

Sub excursion limiters were found in early THX AVRs, but the feature was dropped somewhere in the early to mid 2000s.


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## jtalden (Mar 12, 2009)

gazoink said:


> Applying a HPF to prevent over excursion is a rather linear solution to a non-linear problem. The HPF is not needed or desired at any time until the driver approaches XMax, and then it is. Yet, if the HPF is present all the time and the low frequency extension is affected all the time, even though XMax is rarely reached. The result is the HPF compromises performance just to help reduce the rare chance of over excursion.


I'm curious as to what HPF performance compromise you are referring to; just the 3 dB lower output at 20Hz?


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## apilon (May 18, 2006)

jtalden said:


> apilon,
> 
> 
> Thank you very much for that explanation


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## gazoink (Apr 17, 2013)

jtalden said:


> I'm curious as to what HPF performance compromise you are referring to; just the 3 dB lower output at 20Hz?


Can't say exactly without knowing the specifics of the sub and HPF. IF the two were in complete aligned and rolling off at precisely the same rate, it would be 6dB total at 20Hz, 3dB at 40Hz.


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## gazoink (Apr 17, 2013)

Sorry, that's not correct, 6dB at 20Hz is right, probably not 3dB at 40, but perfect alignment between the HPF and driver is also unlikely. The question becomes, do you really want 1/4 power at 20Hz?


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## jtalden (Mar 12, 2009)

Okay. I was just curious if you were concerned for other reasons. I agree the SPL is impacted as you state. If that is a concern for the application then it is a consideration.


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## gazoink (Apr 17, 2013)

jtalden said:


> Okay. I was just curious if you were concerned for other reasons. I agree the SPL is impacted as you state. If that is a concern for the application then it is a consideration.


SPL vs frequency is the biggie anyway. There's not much else to be concerned with. Perhaps phase shift, which may matter if you're running multiple dissimilar subs, one with an HPF, one without, etc. The phase response of the driver plus filter in cutoff region will not be linear.


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