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  1. #1

    Member Sales Rating: (0)

    Join Date
    Jan 2003

    Default crossover modification

    I am setting up a stereo system in my bedroom-not my main system. I have a Harman Kardon HK680i which is a very good sounding receiver. I have a pair of Polk Monitor 7b's. I want more low end bass and I'm going with a Optimus Pro SW12 sub. Now at this point I know a lot of you will say it's not woth messing with, but I'm gonna anyway. I have practically nothing in it. What I want to do is if feasable is to modify the crossover from it's current 200-250Hz down to around 80-100Hz. Keep in mind this is a passive sub and it only has in's and out's. I'd like to take some low-end stress off my 7b's. but I want it crossing over lower than 200-250Hz that for sure. Or Is there a way to cut out the lowest end of the bass on the 7b's. Yes I play music sometimes at very loud levels and the polks talk back to me and it scares me when I hear them bottom out or whatever actually takes place on strong low bass notes. I just want to cut out the lowest to the Polk's and I hope there's a way to modify the SW12 and hook up normal-output from receiver to input on the sub and then filter just enough of the bass out and then to the Polk's.
    Thanks to anyone who can help me with this, but I really want to work with this sub so please don't say buy a real sub. I've had one or two of these in the past and they worked ok-especially when they were on sale. That'll work just fine in my bedroom. Just read on some forum that by adding more acoustic stuffing lengthening the port tube or closing it up altogether would help out. I know how to use a screwdriver and soldering iron and am familiar with basic electronics so I know a little about capacitors, inductors and resistors and what the values mean and how to substitute different components. I wound my own coils for x-over's about 15 years ago, so I've gotten my feet wet in electronics and gadgets and I like to tinker. As a matter of fact today I'm replacing a burned out resistor in my 7b's. Got to noticing my high's were gone, so I took the speaker out (SL1000 or similar) and I guess I pulled a wire loose and thought that was the problem. I checked the tweeter with my VOM and it read ok (5-6 ohms). Put it back in the cabinet hooked up and tweeter still not working. So I was very confused and pissed. It ain't fun luging 7's around and doing all the screwing in and out this many time(at least not with speakers-HaHa). took tweeter back out and wire was attached checked it again and same readings. So I knew it had to be in the x-over or connections. Pulled out the x-over and there was a burned out resistor-visible. It was 4.5 ohm @7watts. I didn't even have anything like that on hand. So I thought I would make one. I used what I had on hand which were mainly 100's and 1K's of the one watt wirewound. I paralled several together until my meter said exactly 4.5 ohms. Don't look all that great but I think it should work fine.
    Thanks for reading this, but if anyone could give me a hint on what's the best modification to do on the sub sure would be appreciated.


  2. #2

    Member Sales Rating: (40)

    Join Date
    Jul 2001
    North Texas


    If I'm not mistaken, the 7b's midbass is highpassed at 60Hz to begin with. From that point down, the PR takes over.

    As far as the sub goes, find out the impedance, and install a simple choke (value determined by what freq you want to pass it at), and call it day. If you want greater than a 6db/oct roll off, you can buy pre-fabbed inline low-pass assemblies from Partsexpress, or make your own.

    Check your lips at the door woman. Shake your hips like battleships. Yeah, all the white girls trip when I sing at Sunday service.

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