Custom low-pass filter

I was planning on creating a custom low-pass filter for my rear speakers. If I wanted to filter before the amplifier, would I need to use an active filter design? What negative effects would occur if I implement a passive filter design instead? Are there any resources for designing an active crossover or filter? Are there any specific op-amps that you may recommend? I'll just go with an off-the-shelf solution if I'm just getting in over my head. I'm considering Harrison F-MODs or a seperate active crossover from another company. While sound quality is important, it does not have to be perfect. The car is just a daily commuter. Thanks in advance!

They're just a pair of Alpine Type-S 6x9s (SPS-690A). It's one of the few pairs that could fit in my rear deck without modifications. I wanted a custom filter so I could play around with the corner frequency to see what would sound best to me. With the F-MODs, I would just have a limited selection to choose from and changing frequencies would require a new one. What frequency would you guys personally pick?

I think you are looking for a HIGH-PASS solution... you are trying to prevent them from playing too much bass and flapping, right? That's a high pass crossover. Low-pass would be used in a woofer or subwoofer application, where you dont want any high frequencies getting to the drivers. As far as a frequency, I would start with 80Hz, but have a 100 and a 60 available as well... too much lower than that, and you'll probably beat your speakers up... Also, op-amps will require a bi-polar power supply, something that will be difficult at best to acheive in a car... bi-polar power supplies don't come easy from +12 volts! They will be complicated to build, or expensive to buy. The last time I looked for a 1A ±15v bi-polar (the required power for most op-amps) power supply with a +12v to +15v input, it was about 70 bucks... By the time you spend all of that money, time and energy trying to build such an animal, you might as well buy a pre-made crossover that meets your specifications... Just a suggestion. If you still want to attempt the crossover section, here is what I came up with in a quick search on Yahoo... most of those links will give you some pretty decent information, with various orders and op-amp options. Good luck!It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

I was actually going to low-pass the rear-speakers to create a better front soundstage. I was getting too much midrange and highs from behind me for my tastes. I'm not quite sure where I want to start cutting off though. I'll eventually add a high-pass filter at 80 Hz to it when I get a subwoofer. I still need the trunk space for another year or so. I didn't know those op-amps required so much power! I'll definately look into it though. I remember I used a TI op-amp powered from a 2W DC-DC converter (12V in, +-12V out) once for a power electronic project. The power supply was about $20 or so, I believe. I'm not sure if that op-amp is suitable for what I want though. The op-amp itself was about $4. While it may not be a feasible option, it may be a fun side project for a rainy day. :) I wouldn't be surprised if it didn't turn out well though! I may just have to use a passive filter after the amplifier or buy an electronic crossover. Thanks for the help though!

Hmm, sounds to me that what you really need is level control for front to rear balance, and maybe EQ.  And definitely to add a subwoofer if what you're trying to accomplish is low bass augmentation using your rear deck speakers.  There are 6X9 subwoofers, by the way, that you can use in place of your full-range speakers.Support the12volt.com

My goal is to get a nice clean full sound. I want something that can go to a comfortable listening level without distorting. I don't need to have a perfect sound stage where I can spatially pinpoint the instruments, but I don't want to hear instruments from behind me where they don't belong. I've looked at other 6x9 speakers, but very few fit in there correctly due to the shallow mounting depth. I definately need to get a subwoofer at some point, but I'll leave that for later. With rear speakers, I figured I would just bandpass it the midbass frequencies or so. I'm just working with what I have for now. I may just disconnect the rears once I get a subwoofer and bridge my 4-channel amp (Alpine MRP-F240) to power only the front speakers (Infinity Reference 6000cs). I'll just have to play with the setup when I get there until I get something I like. I hope the clarifies what my goals are. Thanks again!