Resistance question

1519 Views 18 Replies 9 Participants Last post by knewblewkorvett, Nov 26, 2005

KeltonDSMer

Discussion Starter · Nov 20, 2005

I have one sub rated at 8 ohms, if I bridge a two channel amp, is this running the circuit in parralell effectively putting a 4 ohm loead on the amp (this is what it is rated at)?

1 - 19 of 19 Posts

DRSully

· #2 · Nov 20, 2005

KeltonDSMer said:
I have one sub rated at 8 ohms, if I bridge a two channel amp, is this running the circuit in parralell effectively putting a 4 ohm loead on the amp (this is what it is rated at)?

In theory yes, but I would have to know exactly what you are installing to be sure. Depending on the setup it can change things. I assume you definately have a DVC sub then? Some amps are not even bridgeable etc.. Although you can wire something however you want, the equipment has to be capable to being stable under those circumstances. Why are you running an 8 ohm sub?

Take care,

David

knewblewkorvett

· #3 · Nov 20, 2005

um no.
Your amplifier does not determine the impedence, the speakers do. You can (and should) bridge your amp into one channel but your power output will be half of the amps's rated power. On the bight side, your amp should never overheat

But as DRSully said, no one can tell you for sure without know exactly what yo have for equipment

XCalibirTsi95

· #4 · Nov 20, 2005

yea, give some more info if you want correct feed back.
if it is an 8 ohm dual voice coil, then your amp will see 4 ohms total when bridged.
If you just have a single 8 ohm sub (god knows why) then your amp still sees 8ohms (total) Simply put 4 ohms on each channel,,

morer info would help tho

knewblewkorvett

· #5 · Nov 21, 2005

8 ohm woofer is the norm for home audio, maybe that is what he has

He might even have a dual 4 ohm coils on his woofer. In series, that would be 8 ohms.

pb4ugotobed

· #6 · Nov 21, 2005

there are a lot of 8ohm subs out there still. you will only have 8 ohms still bridged, not bridged, or not hooked up at all. get another sub, or take that one back and get a 4 ohm version.

KeltonDSMer

Discussion Starter · #7 · Nov 22, 2005

I bought the sub for $15, so don't tell me to buy another because I will not. I have a free 300W 2 channel amp (38W RMS each channel at 4 ohms).

The last system I built made 139Db in the front seat and I put it together for under $400. I know how these things work, I am just fuzzy on how resistance works.

It is a single voice coil Fosgate, I'm not sure about the model but it looks to have about a 1/2" excursion. It says that it is an 8ohm load on the back of the cone.

If you bridge an amp, you are running it in parallel right? A parallel circuit with an 8 ohm drain will cut the resistance in half to a 4 ohm load on each of the channels, right?

I think this should work just fine if my info is correct, is there anything I am missing?

XCalibirTsi95

· #8 · Nov 22, 2005

KeltonDSMer said:
I bought the sub for $15, so don't tell me to buy another because I will not. I have a free 300W 2 channel amp (38W RMS each channel at 4 ohms).

The last system I built made 139Db in the front seat and I put it together for under $400. I know how these things work, I am just fuzzy on how resistance works.

It is a single voice coil Fosgate, I'm not sure about the model but it looks to have about a 1/2" excursion. It says that it is an 8ohm load on the back of the cone.

If you bridge an amp, you are running it in parallel right? A parallel circuit with an 8 ohm drain will cut the resistance in half to a 4 ohm load on each of the channels, right?

I think this should work just fine if my info is correct, is there anything I am missing?

whats the matter cant afford another $15 sub.. lol
and you didnt read what was written. when you hook up one sub it is neither parrellel or series. if you bridge an 8 ohm sub to the amp IT IS STILL 8 OHMS.
you need more that one sub to wire it in parrallel. This is what i do for a living not "hobby". Did you make sure that your amp is bridgable?

Bottom line if you want lower resistance buy another sub, or go get some resistors.. lol and play around until you get it down to 4 (not really recommeded) BUt you are partally right,, if you do get another sub (8 ohm) and you wire them in parallel then you will have a 4 ohm load.

knewblewkorvett

· #9 · Nov 22, 2005

no... no matter what you do you will not get 4 ohms
You have a 8 ohm sub and that is what your amp will see.
I.E., your amp is rated at 600 watts bridged into 4 ohms, your amp will see a 8 ohm load and therefore put out 300 watts into that 8 ohm load. Is this a bad thing...no, your amp will be happy and run cool. Are you using your amp to it's fullest cabilities...no, but chances are a free amp will like that 8 ohms better than 4 ohms.
Best wishes

mystkn

· #10 · Nov 22, 2005

Word I feel this

X-Pride

· #11 · Nov 23, 2005

I thought that if an amp was bridged (if it is a quality amp) and hook it up to a 8 ohm speaker your ohms drop by half and the result is 4 ohms. The only way you are going to get the correct answer is to look in the amp manual. Let me guess you don't have one. Than go to the venders web site and look up the information. The theory above holds true for my kicker amp but not all amps.

knewblewkorvett

· #12 · Nov 23, 2005

sorry, not true, no matter what amp you have.

This is, again, an example for a quality amp (lesser amps will do less into a bridged or 2 ohm bridged mode).

Say you have a 2-channel amp rated at 150 watts per channel.
She will therefore put out:
150 watts per channel into a 4 ohm speaker for each channel
75 watts per channel into a 8 ohm speaker for each channel
300 watts per channel into a 2 ohm speaker for each channel

bridged she will put out:
600 watts into a single 4 ohm load (that would be one 4 ohm coil)
300 watts into a single 8 ohm load (our thread starters sub)
smoke

into a single 2 ohm load

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Tenacious-B

· #13 · Nov 23, 2005

knewblewkorvett said:
smoke into a single 2 ohm load

True, but if you do this then for a brief moment you can turn the volume up to 11.

pb4ugotobed

· #14 · Nov 23, 2005

KeltonDSMer said:
I bought the sub for $15, so don't tell me to buy another because I will not. I have a free 300W 2 channel amp (38W RMS each channel at 4 ohms).

38 watts per channel at 4 ohms, would be approx 75 watts rms bridged at 4 ohms, meaning you will get about 38 watts rms bridged at 8 ohms. good news, your amp will never overheat. bad news, that sub won't even move with that kind of monstrous power. i have about 5 times that amount going to my tweeters alone, let alone my mids or subs.

can you use that sub? sure. can you use that amp? sure. just don't expect a lot out of it.

KeltonDSMer

Discussion Starter · #15 · Nov 24, 2005

I found an audio site that answered my question, the 8ohm load on a bridged channel only loads the amp 4ohms on each channel just as I had thought.

Check out http://stereos.about.com/cs/theamplifierfaq/f/bridge_mono.htm

It explains it well, it also mentions how some multi-channel amps are not bridgable in the first place.

knewblewkorvett, you are right about the power outputs for your example amp, but lotts of amps are stable down to 1 ohm and you can run them that low without overheating. My bro hit 147Db with two Eclipse Ti 10" subs using a monochannel amp, so he was running only 2 ohms and never ever had a problem. Notice how the 2nd chart you posted says the impedence of the "load", the resisteance "load" is effectively halved from the sub's impedence when running that load off of a bridged channel. Check out the link.

I will be running 75W RMS through a sub rated at only 100WRMS, of course it will move and close to it's potnetial I might add (which obviously isn't too much). Rmember guys, this is a $30 sound system (amp/sub/box/ebay wiring) and I don't care if it don't make the sound pressure I'm used to. I just can't stand the attempt of the stock 6X9's to hit any sort of low note.

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knewblewkorvett

· #16 · Nov 24, 2005

you are very correct about that there are amps out there stable to 1 ohm. I own several such as the JBL 1200.1BP in my Vette. She is a mono amp that is 600 watts into 4 ohms, 1200 watts into 2 ohms, and 1200 watts into 1 ohm. In theory, it should put out 2400 watts into 1 ohm but it can't, thus it is internally regulated so it doesn't even try.
I have "old school" USAmp VLX-25s which is stable down to 1/4 of an ohm

At a demo years ago,(at a USAC event) USAmps would use that amp as a welder to cut pop cans in half

. That amp would do it too, never shutting down. Those were called "cheater amps" My VLX-25 (I still use two in my white Talon) is rated at 12.5 watts per channel

The amps each measure 26"X12"X2"...a tad big for a 25 watt amp

I'm getting 800 watts out of each

Most two (or four) channel amps do not like to be bridged into anything below 4 ohms. There are exceptions of coarse.

Good luck on your system

and here is a link to 3 of my 4 current vehicles.
http://www.cardomain.com/ride/412350

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KeltonDSMer

Discussion Starter · #17 · Nov 25, 2005

That link is amazing, I love the originality of the C5.

bbtmitsu14

· #18 · Nov 26, 2005

knewblewkorvett said:
8 ohm woofer is the norm for home audio, maybe that is what he has He might even have a dual 4 ohm coils on his woofer. In series, that would be 8 ohms.

They do make many different ohm ratings for subs. I have 3 of the JL Audio 12w312 which is a 12 ohm sub so when I bridge them they are about 4ohms

knewblewkorvett

· #19 · Nov 26, 2005

They sure do, I have six of the JL Audio 18W6s which are dual 6 ohm voice coils. Still, when it comes to home (as I clearly stated) woofers, I would guess over 90% of them are 8 ohm. Some home speakers are 6 ohm and even a few 4 ohm. In the car world, I've seen 1, 2, 3, 4, 6 ohm voice coils.

By the way, I have a bunch of 12" subwoofers in the For Sale section

Check them out.

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