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Running 3 Ohm 6.5s In Parallel With 4 Ohm Tweeters

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Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
Printed Date: September†26, 2022 at 2:53 PM

Topic: Running 3 Ohm 6.5s In Parallel With 4 Ohm Tweeters

Posted By: kenwood_nut
Subject: Running 3 Ohm 6.5s In Parallel With 4 Ohm Tweeters
Date Posted: April†01, 2021 at 11:32 AM

I'm getting ready to do some rocket science calculating, so I need help! I just bougth 4 new JBL GTO full-range speakers, the GTO930 6x9's for the back and the GTO629 6.5's for the front doors. The 6x9's are 2.5 ohms and the 6.5's are 3 ohms. Nice! Anyway, I'm also going to keep my present setup of running my tweeters in parallel off the front channel of my Eclipse 4-channel amp. My present Nakamichi 4-channel amp is doing okay, but not good enough. I'm running the 4 ohm Cerwin-Vega tweeters in parallel with the front Pioneer 4 ohm door speakers. So my amp should be running the fronts at 2 ohms if I'm not mistaken. Sounds 10 times better than the rears at 4 ohms, that's for sure! But now I'm worried because I'm trying to figure out how I'm going to run 3 ohm speakers and brand new Cadence 4 ohm tweeters off an amp that's only 2 ohm stable. Apparently I can't run them in parallel. If half of 4 ohms is 2 ohms, and half of 3 ohms is 1-1/5 ohms, it looks like Iíll be forced to run them in series. Is that correct? Or maybe series-parallel? The last thing I want to do is run the tweeters off the head unit, always a waste of time and not a good idea anyway. So would I add 4 to 3 and come up with 7 ohms, which would be 3-1/2 ohms in parallel to each channel? If thatís right, it seems I would be fine. But Iím guessing it canít be that easy. Iíve never owned 3 ohm full range speakers before so this equation is new to me. Can anyone offer and ideas here? I do NOT want to fry my new Eclipse amp, so series connection would work if I have no other choice! One final note unrelated to my front stage dilemmaÖ Iíve decided to run the rear 6x9ís in mono instead of stereo. Makes for a better rear fill than stereo. This will be the most Iíve EVER spent on speakers in my life, and itís only thanks to that stimulus check! Hey, if Iím going to stimulate the economy, it might as well be the car audio economy! LOL!

The way I figure it:
4 ohms plus 3 ohms is 7 ohms. Running THIS in parallel would be 3.5 ohms. Is that right? But don't they say your amp actually SEES half of that? So my amp would SEE it as 1.75 ohms? Dang this is complicated. Apparently JBL speakers weren't designed to be run in parallel. They boast their ability to sound good even if driven of a head unit with no amp, which is believable. But I sure can't/won't run my tweeters off the deck.

So my only real question that I had to make more complicated than it really needed to be is this:
Am I safe running a pair of 4 ohm and a pair of 3 ohm speakers in parallel off an amp that's only 2 ohm stable?
I'm guess not. I'd have to run them in series, right?

About the only thing I know FOR SURE is that this is the most I've spent on full range speakers in my life! $90 for the 6.5's and $172 for the 6x9's, plus tax!


Posted By: the12volt
Date Posted: April†01, 2021 at 12:16 PM
Wiring one 3 ohm speaker and one 4 ohm speaker in parallel would have a nominal impedance of 1.71429 ohms, however impedance is relative to frequency. If you are using 2-way passive crossovers between the 6.5" speakers and your tweeters, then the lowest nominal impedance would be 3 ohms below the crossover frequency and 4 ohms above it*. If you are only using passive crossovers on your tweeters and not on the 6.5" speakers, then everything above the crossover frequency will have a nominal impedance of 1.71429 ohms and everything below the crossover frequency would have a nominal impedance of 3 ohms*.

*Not counting any changes in impedance per speaker at different frequencies. Example: If my memory serves correctly, Rockford Fosgate 5" and/or 6" midrange drivers back in the '80s had a natural rolloff at and above 5KHz and did not require low pass filters unless you wanted to limit frequencies below 5KHz. At 5KHz and above the speaker's impedance rose sharply and greatly.

posted_image the12volt ē Support

Posted By: kenwood_nut
Date Posted: April†01, 2021 at 10:54 PM
I removed my defective Cerwin-Vega active crossovers and now run 5.5k bass blockers plus 20-watt 8-ohm resistors. Does this help? The Cadence tweeters I just ordered only have passive crossovers.


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