# Mod to disable low pass filter on amp??



## ZAKOH

I feel like in general you never learn what things to avoid unless you own one them. I have a Jensen Power5500 5-channel amp. One thing I don't like about it is that you can't disable the low pass filter on the sub channel. There is only a rotating knob for LPF setting going from 160 down to 30Hz or so. There is also knob that sets subsonic filter. Neither can be disabled. I'd rather prefer to set the crossover frequency on the HU. I am thinking of replacing it with a better 4-channel amp. However, before I spend the cash, heck why not try to mod it? Any tips about this idea? Should I just forget it?


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## hottcakes

why not just turn the LPF on that amp all the way up? 
the signal it will be receiving is already crossed over so i'm sure at the highest setting it will have less of an effect on anything. there is a way to use them in conjunction with one another for optimum sound, but that gets somewhat complicated. 
as for the subsonic, is it that important to really need to turn off? why not just put that to its' lowest frequency?


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## ZAKOH

hottcakes, indeed, I can LPF at the head unit and also set the slope, and then set the amp LPF to the max 160Hz. I think this may work, but it's not a perfect setup. I don't know how this will sound because my new head unit hasn't arrived yet.


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## hottcakes

perfect, no. will it work rather nicely, yes. say you low pass your sub at 80hz with a 12db slope at the head unit and the sub for the 160hz at whatever slope that is. the signal your amp Should get would be one that is -12db at that 160hz level already so that filter on the amp theoretically will have little effect on the signal at all. 
do you see a problem that i don't? i currently have my sub amp set with the LPF all the way up (320hz?) since i cannot defeat it either while being bridged.


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## ZAKOH

hottcakes said:


> do you see a problem that i don't? i currently have my sub amp set with the LPF all the way up (320hz?) since i cannot defeat it either while being bridged.


I suspect this should work ok. Wouldn't efficiency be a problem though? I believe every time you apply a passive crossover, it does reduce efficiency. This port 2ohm stable, providing 250Watts. Perhaps if there was not an amp crossover on the way, the RMS rating would go up. That's just my guess that's not backed by any real understanding of how amps work.


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## davidebender

Active crossovers do not reduce amp output, they work on the signal stage not on the output stage like a passive.
Passives reduce as much power as the impedance increase (0.5 ohm for joe the average inductor).
Active XOs actually give much more dynamic output to the amp due to the fact that the amp isnt playing a part of the music at all, rather then playing it first then sending it back to "minus pole".

Back to the OP, if you use LPF to 160hz and 80hz from the HU you are having a 12dB slope that turns out to a 24dB slope at 160hz, sounds good on paper.


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## hottcakes

i was always under the assumption that the crossovers on amps were active-ish. why would an amp amplify a signal then cross it over? it would make more sense the other way, crossing then amping. i'm not too into the technicalities, but there may be some sort of phase issue with the two crossovers as well however that may not be that important to you.


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