[rxter]

I am thinking about adding a pair of 8 ohm speakers in parallel to the 4 ohms that I have in the back of the cab.



I also have the standard door mid and tweeter set up in the front - I presume that these also combine for 4 ohm.



Based on my research it appears that the resistance would be 2.67 for each channel of the rear set up. This is where stuff gets fuzzy for me.



My headunit/amp manual says the following.



Maximum power output ....... 50 W × 4

50 W × 2/4 ohm + 70 W × 1/2 ohm (for subwoofer)



Continuous power output ... 22 W × 4 (50 Hz to 15 000

Hz, 5% THD, 4 ohm load, both channels driven)



Load impedance ...................... 4 ohm to 8 ohm × 4

4 ohm to 8 ohm × 2 + 2 ohm × 1



So what are the chances that I melt my head unit if I run a set up like this? I tested it and it ran fine for 20 minutes at a pretty good volume, but as I recall running a lower impedance can heat things up, and there ain't much airflow where that head unit is stuffed.

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[flash]

pretty good chance of damage



what are you trying to achieve?

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[rxter]

Trying to get the sound to withstand 70 mph with the top down. Same old problem. I know that you solved it with the smallest of tolerance in the door pocket. I am trying something a bit simpler, if less elegant by adding to the read of the cockpit - essentially on the parcel shelf. But I like the spacing that the current side pod rears offer so I was hoping to keep those. If I can find 8 ohm 4" speakers that would probably work right? (2) 8-ohm in parallel = 4 ohm.

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[bombfactory]

Maybe run the door speakers off the head unit, and add an external amp in the trunk to run the rear speakers?

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[rxter]

Good idea Austin. Of course I am looking for the cheapest and easiest way to do it. Problem with my plan though is that it sounds bad then blows up.

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[flash]

dynamat first - then tune - then decide about gear

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[rxter]

Pulling the door panels off and I find this.



http://www.jlaudio.com/tr525-cxi-car-aud...tems-91030



So far so good.



Then I find this.



http://www.jlaudio.com/product/54216.104...Crossovers



The 525 is 4-ohm and the tweeter is 8-ohm. And of course they are wired in parallel. Arghhhh......



This is not a good set up due to the resistance mismash, right?



Looks like a trip to fry's is in order.

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[DaveN]

Right! You need matched components and a proper crossover.

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[rxter]

well now is the time as the doors are apart. The Xover is integrated into the tweeter connections, but it is designed to work with a 6.5" 8-ohm woofer, not a 5.25" 4-ohm coax. Damn these car audio installation guys. The <acronym title='previous owner'>PO</acronym> had this stuff put in. Speakers are nice, but it looks like I'll end up removing all of them at this point....

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[DaveN]

At least the tweeter had a built in cross-over. That means your amp never saw 2.6 ohms it only saw 4 ohms on the low side and 8 ohms on the higher frequencies. We are dealing with reactance here Z and not resistance R. Your highs would have been crappy but ok if you compensated with the equalizer in your deck. I don't know what SPL rating they had at 1 watt so they may have been ok to listen too if the tweeter was more efficient.

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[rxter]

huh?



So clearly I missed this course in school. I thought that the resistance was a constant thing. This is from the spec sheet for the tweeter.

Continuous Power Handling (RMS)* 50 W Peak Music Power* 125 W Recommended RMS Amplifier Power (per Ch.)* 2 - 50 W System Efficiency 91.0 dB @ 1 W / 1 m System Nominal Impedance 8 Ω System Frequency Response 4 Hz - 22 KHz ± 3 dB



This is for the Coax 5.25.

Continuous Power Handling (RMS) 50 W Peak Music Power 225 W Recommended RMS Amplifier Power (per Ch.) 10 - 75 W System Efficiency 89.0 dB @ 1 W / 1 m System Nominal Impedance 4 Ω System Frequency Response 63 Hz - 22 KHz ± 3 dB



Frankly they sounded OK but I did not want to hurt the head unit so I was planning to pull them and replace with a nominal 4-ohm component setup.



If this looks OK I'll leave them in - no sense in doing more work than I have to.



Thanks for the advise Dave!

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[DaveN]

Yes confusing. DC resistance is constant but impedance (or reactance) is AC and varies with respect to frequency. You can have 4 ohms at 50 Hz and 5000 ohms at 5 Khz in an AC cct. You should try microwave frequencies where the signal travels only on the outside skin of the conductor and ignores the rest of the core. <img src="/forum/images/smilies/968/wacko.png" class="smilie" alt="" />

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[rxter]

Do the specs give you any pause for concern? Like I said it sounded OK. Mid-lows out of the coax 5.25 sucks, but that's what the dynamat is for, and I am working up a solution for the rear that should deliver better mid-lows as well.

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[flash]

specs look dubious. i have never seen a speaker that goes to 4hz. not even in the studio. you sure it isn't 40? (keep in mind that the real world response of any speaker is determined by the enclosure or lack thereof)



as for stacking up speakers, ohms are ohms. add speakers and you will cut ohms. cut them too far and the amp will go. this is because as you decrease ohms you increase watts. 100 watts into an 8 ohm speaker will be 200 watts into a 4 ohm speaker. that can get dicey.



this might help:

http://www.marktaw.com/recording/Electro...akers.html



you might try wiring them in series instead of parallel. while it might get phasy sounding, you would maintain the resistance.



http://www.termpro.com/asp/pubs.asp?ID=124

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[rxter]

the specs were straight from the website. And the 4 Hz was for the tweeter. Can't be right huh. We can't even hear 4 Hz. They must have meant 4 kHz.



Anyway, I put the specs up so that Dave could look at the SPL rating.



So we seem to have differing opinions from 2 pretty smart guys on the subject. Dave says the amp will see between 4 and 8 ohms with the crossover and I think that flash is saying that the amp is likely seeing less than 4-ohm from this pairing.



I have an 8-ohm tweeter wired in parallel to a 4-ohm coax 5.25. From everything I've read that makes for a 2.67-ohm load on the amp. Its been in there for 2-years with another pair of 4-ohm coaxes in the rear of the car. So, should I stop worrying and put the doors back on or tear these speakers out and get the front speakers to work at 4-ohm (I can buy a component system that runs at 4-ohm as a system)?



What say you audio engineer types?

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[flash]

lol - i say ring me up so i can try to figure out what the heck you're doing. mixing and matching ohm loads will change how much power goes where.

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[rxter]

Will do.

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[DaveN]

When the spectrum content is in the cross-over region your impedance will drop to 2.67 ohms, this is where your amp can get overloaded. Your crossover freq will also move as you are mismatching the driver to the design characteristics of the cross over so you may be expecting a 3K crossover and infact be getting 1.5K or 5k (guesses) which will screw your sound as well.

Best keep apple to apples with these things unless your last name ends in Kreisel <img src="/forum/images/smilies/968/wink.png" class="smilie" alt="" />



Good links Flash and I would never debate your sound skills but for the new guy the link states:

"Alternating Current makes things even more complex, but I'm not going to address that here. These calculations are based on DC, but serve as good averages for AC as well."

Remeber that DC never passes throught a driver in a perfect world and the only time your amp sees 8 ohms (really about 6ish in the DC world) is at 0 Hz. Even though speakers are rated at a certain "resistance" (i.e. 8 Ohms), the actual impedance varies with frequency (speakers have inductance) and is usually measure at 1K Hz.

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[flash]

yeah - i always worry about posting stuff like that, because somebody might misinterpret or misapply the information, but i thought the series/parallel stuff might help

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[rxter]

No chance of me either misinterpreting or misappyling, since I have no idea what you are talking about. In fact given this discussion and the things that I have done to car stereos in the past I'm surprised there's never been a fire <img src="/forum/images/smilies/968/wink.png" class="smilie" alt="" />

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