What is the practical impact of too high an impedance

[Rootus]

I have a very simple setup -- one amplifier, an Infinity 7541a. Two door speakers, Polk db6500's. And a Kicker Solo-Baric S8L54 subwoofer. It is not yet installed, my intention is to create a custom sealed box for it and put it under the passenger seat.

The issue -- the subwoofer is the 4-ohm version (DVC), but my amp is a 2-ohm amp. The manual for the 7541a says minimum impedance when bridging two channels is 4-ohm, so I've got a choice between running the subwoofer at twice the recommended impedance, or half of it.

First question -- what is the real, practical effect of using an 8-ohm subwoofer with an amp which is rated at 278W RMS for 4-ohms? I suspect it is not so simple as "half."

Second question -- would it even work if I wired it at 2-ohms? Someone suggested that to me, "wire it as 2-ohm, don't crank the gain up." But I'm thinking the amp has probably got a protection feature that would detect the low impedance.

Third question -- How much is enough? //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

Okay, that was a loaded question. With a bit more detail, perhaps I can get some good guesses. I am looking for enough bass to match the output of the db6500's. I do not crank the db6500's up until my ears bleed, although they seem to be able to do it -- all I want is something that can overpower road noise at 75 mph (which can be considerable in an STI), so that I can enjoy the music. I do not desire to share my music with my neighbors or anyone else, and I like my hearing. Given this, and the fact that I'm going to put the subwoofer inside the passenger compartment instead of in the trunk, it has been suggested to me that even if I only got 139W of clean, honest power, I would likely be more than satisfied. There is a lot of subjectivity there, but does anybody have any thoughts on this?

I am trying to improve my understanding of how this all works. It seems to me that an amp doesn't have any inherent limit on how much power it can produce -- more like it has a practical limit if you don't want to overheat it or get unclean output that will destroy speakers. If that supposition is anywhere near correct, then it seems to me that just doubling the impedance shouldn't automatically mean that the amp can only drive the speaker to 139W. A bit of a loss due to inefficiency, but shouldn't it still be able to drive nearly the same load? Say ~220W instead of 278W? Or am I totally, completely wrong? //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

Thanks for your help. I'm going to continue searching, to try and answer some of what I just asked. It's really the first question I'm having the most trouble finding a satisfactory answer to, so that is why I posted here.

Thanks,

Dave

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[JimJ 5,000+ posts]

First of all, that's one of the best first posts I've seen around here in a while...

Secondly, you're right that there's more to it than just halving the power. But for all intents and purposes, that's the primary thing that you're going to deal with. Technically, damping factor changes, but for SS amps it's really not that important anyway.

 

[Rsx_03 10+ year member]

Question 1: well i have an idea ive had for a while but nobody will confirm if its safe ...

for the front two channels on your amp hook up your front speakers

for the rear two channels plug each channel to a voice coil (since your sub is a dual voice coil running at 4 ohms)

Question 2 : this way your amp runs on the same ohm (damages your amp if each channel is runnin on a different ohm or below the mininum impedance)

REPEAT: make sure hooking each channel from the same amp to each voice coil of your subwoofer is safe before you do this

Question 3: sound deaden around you door speakers ... it will give better clarity and sound louder against road noise

if you really want to .. sound deaden your whole car but its a lil costly =/

 

[Johnny Drama 5,000+ posts]

Question 1: well i have an idea ive had for a while but nobody will confirm if its safe ...
for the front two channels on your amp hook up your front speakers

for the rear two channels plug each channel to a voice coil (since your sub is a dual voice coil running at 4 ohms)

there is no point in this. When you bridge an amp at 8ohms each channel is seeing a 4ohm load even though the amp is in mono.

 

[Raven 10+ year member]

Bravo on the first post. That was excellently laid out and asked.

My only sugguestion is to do some experimenting. Wire the sub up at 8 ohm to the amp, and drive the car around like normal for a day or two. If it is loud enough for you, then there is no problem. =)

Also, all technical stuff aside; on some woofers each impedance will offer a different sound. So try 2 ohm as well. If you're not going to absurd volumes you won't have to worry about your amp overheating from the higher current.

Question 2 : this way your amp runs on the same ohm (damages your amp if each channel is runnin on a different ohm or below the mininum impedance)

I can't remark about high volumes/output; but in my tests, I've wired two channels of a two channel amp to unique ohm loads (8 and 2, respectively) without any problems. I never drove the amps past more than 10% of their rated power so take this with a grain of salt.

Question 3: sound deaden around you door speakers ... it will give better clarity and sound louder against road noise

Yes, but not entirely. Drive around with the radio off. Listen for where the road noise is loudest coming into your vehicle. These are your trouble spots. For example, mine are the door seals and firewall. Sound deadening is deadening the sound of the road outside, which comes from everywhere, not just around your door speakers.

 

[Rootus]

First of all, that's one of the best first posts I've seen around here in a while...

Thank you //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

Secondly, you're right that there's more to it than just halving the power. But for all intents and purposes, that's the primary thing that you're going to deal with. Technically, damping factor changes, but for SS amps it's really not that important anyway.

Perhaps I should make a real effort to get the correct subwoofer, rather than compromise. It has been difficult locating a 2-ohm version locally, but I think I can special order it.

well i have an idea ive had for a while but nobody will confirm if its safe ...
for the front two channels on your amp hook up your front speakers

for the rear two channels plug each channel to a voice coil (since your sub is a dual voice coil running at 4 ohms)

I have heard that this would work. The caveats that I am aware of -- it would be stereo (on my amp, at least), so the possibility exists for the coils to interfere with each other instead of working together. At the least it could cause some distortion.... at worst, I don't know?
But it wouldn't really solve the issue -- the amp is stable to 2 ohms per channel, so with each voice coil wired to a separate channel, I'd still be doubling the impedance.

sound deaden around you door speakers ... it will give better clarity and sound louder against road noise
if you really want to .. sound deaden your whole car but its a lil costly =/

This is something I'm working on //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif. I've done the front doors and trunk with a layer of RAAMmat and Ensolite foam, and behind the speakers I put an extra layer of mat. I'm still getting a lot of noise though, some from the windows (the glass is a bit thin on this car) and undoubtedly some from through the floorpan. Once the temperature comes back up a bit, I'm going to pull out the carpet and mat the entire floorpan.
Thank you very much for your input, this helps a lot. I am quickly coming to the conclusion that I should just stop right now and wait until I can get the correct speaker, rather than try to move forward with a mismatched setup. My gut says it would work out fine, but in the end the cost is the same, the drawback is just some extra waiting.

Dave

 

[helotaxi 5,000+ posts]

Yep, your best bet is to get the D2 version of the sub. I don't know the specs of that amp off the top of my head, but as far as power to the sub running the rear two channels bridged into 8 ohms (the best configuration for what you currntly have) it should put a total of the sum of the 4 ohm stereo rated power of the two rear channels. Basically each channel is driving half of the 8 ohm load, so it will produce its 4 ohm power and the power from each channel is additive.

I wouldn't suggest wiring it up to 2 ohms even just to try it out. Best case the amp will shut down with the overheat/overcurrent from the load. Worst case the amp dies a horrible death and you have to replace it.

 

[Rsx_03 10+ year member]

oh yea ... i dont know why it didnt click to me just run the sub at 8 ohms lol ...

well runnin amps under there minimum impedance also depends on the model amp ... the kx600 ive seen people run at 1 ohm pretty fine but my mtx amp i cant run at 2 ohms bridged for more than a couple seconds at high volume ... but at 3 ohms it runs fine

btw ... nice car rootus

 

[Rootus]

I don't know the specs of that amp off the top of my head, but as far as power to the sub running the rear two channels bridged into 8 ohms (the best configuration for what you currntly have) it should put a total of the sum of the 4 ohm stereo rated power of the two rear channels.

Ahhh. So the answer is that instead of 278W RMS, it would be 222W. I believe I would be perfectly happy with that, but I will make an effort anyway to get the 2-ohm Solo-Baric instead.

Worst case the amp dies a horrible death and you have to replace it.

Yes, that would **** //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif. I definitely want to keep this amp alive & happy for a long time. In my last car I had an Infinity Basslink... for about two days, then the amplifier took a dump. I hope their standalone amps are better //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif.

btw ... nice car rootus

Thanks! //content.invisioncic.com/y282845/emoticons/cool.gif.3bcaf8f141236c00f8044d07150e34f7.gif
And thank you everyone else who replied for the helpful input!

Dave