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How to create a high pass filter for my door speakers

Old 01-24-2015, 06:53 PM
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How to create a high pass filter for my door speakers

I'm designing a basic sound system to replace old stuff in my 1990 Miata. All the mids and highs will be handled by one pair of door speakers. Plans include a pair of 8" subwoofers behind driver and passenger.

I'm wondering if I wouldn't get smoother, better sound from the 3-way door speakers if I ran a high pass signal of perhaps 80hz to these speakers, rather than run them full range; i.e., let the subwoofers handle the lows. The Infinity Kappa 6.5" 3-ways are rated for frequency response down to 45 hz., but I bet they would sound better with a high pass incoming signal crossed in around 80hz at 12db/octave. Any thoughts on this?

The door speakers already come with a remote crossover that splits the signal at 3500 hz. to two sets of terminals on the speakers.

So how would I rig a high pass into the door speakers. Just use another passive crossover in the speaker line ahead of the other crossover. CAn I do that?

The way I WISH I could do it is take the line level signal from the head unit into the subwoofer amp first, and have the line level pass-through in that sub amp send only 80hz. and above over to the main speakers amp. I don't think the sub amp I'm looking at will do that, nor, I think, will others I've looked at. Ideas?

I already have:
1. Kenwood KDC-158U head unit
2. Infinity Kappa series 63.5i 6.5" 3-way door speakers, 75w rms max
3. Rockford Fosgate R150x2 (2 x 50 watts rms for the door speakers

Considering:
1. Rockford Fosgate R250X-1 (250 watts rms into 2 ohms sub amp)
2. Pair of (undecided) 8" subwoofers in the rear deck, wired to 2 ohms for the pair
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Old 02-03-2015, 09:08 PM
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Are you using the KDC 158U in this car then? It has a preamp outputs. Why use line level?
But to answer your question you have several options
1: a high pass is just an inductor.
for typical 4 ohm speakers 8.2mH will give you 80Hz high pass.

2: poor man's solution: set your deck bass control down, but boost your sub amp to compensate!

3: Get a deck that has a high pass and sub out RCAs
4: use the pre amp out, use an electronic crossover, or an amp that has high pass settings.
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Old 02-04-2015, 09:07 PM
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Originally Posted by zoomer View Post
Are you using the KDC 158U in this car then? It has a preamp outputs. Why use line level?
Good question, and thanks for your reply!

The most limiting factor in my installation seemed to be the single set of pre-outs on the KDC 158U head unit. After receiving my amplifiers and looking at them, I think I have figured this out. I will send the single, left and right pre-amp signals to the main speakers amp. This amp has a line level, adjustable-frequency crossover that is switchable to high pass or low pass. I will set it for about 80 hz high pass. That will send the frequencies I want to my door speakers, 80 hz and up.

This main amp also has a set of pre-amp out jacks. Assuming it passes/mirrors the full range, incoming signal, what I want to do should work. I will take this full range pre-amp signal output from the main amp, run it through a Harrison Labs "Bass Blocker" 37hz, 18db/octave high pass filter (designed for pre-amp signals), then into the subwoofer amp. The subwoofer amp also has a switchable, adjustable, line level crossover that will allow me to set it for about 80 hz. low pass. So my subwoofer will be crossed in/out at 37hz/80hz.

I'm limiting the sub-bass frequencies because I am forced to do a free air (open baffle) installation of the two, 8" subs; there is no room for an enclosure in this tiny car.

So again, I think the only potential hiccup would be if the main amp cannot provide a high pass filter while simultaneously passing the full range pre-amp signal to the pre-amp "out" jacks. I will inquire with Rockford Fosgate to clarify.
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Old 02-04-2015, 09:33 PM
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looks like you found the solution !
RF has great web site for manuals. you can probably find your answer there.
I have a few of their amps.. they are the same company that made the Hafler home audio amps.. same internal circuitry... at least in the old amps I have. great sound!
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Old 02-08-2015, 10:16 AM
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Should anyone find this thread while searching for an answer to the same question I had, here it is.

When you supply a line level input signal to the RCA inputs of a Rockford Prime series amp, it passes that full range signal through to its RCA out ports. It passes the signal full range regardless of whether you select low pass, all pass or high pass on the amplifier, and regardless of where you set the crossover adjustment on the amp.

So in practical application, I'll run the RCA line level signal to amp #1, select high pass at about 80 - 100 hz, then patch the RCA outputs of amp #1 to the inputs of amp #2, setting that amp for 80 hz low pass and powering my subwoofers. Problem solved.

Nice but not mandatory, this also allows me to put a Harrison Labs 18db/octave, line level subsonic filter between the two amps which is very helpful when running a free air (infinite baffle) subwoofer setup.

Last edited by Sunshine Guy; 02-08-2015 at 10:20 AM.
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