Whats the deal with ohms?

Steoob · 19-05-2005 7:09pm #1

hey im gonna be buyin a new bass cab soon... i was lookin at peavey cabs a 4X10 theres two of them and they both cost 499 the only difference between the two is one is 4 ohms and one is 8 ohms... i haven't a clue of the deal with ohms and what they do for me. can anyone help me on this?

thanks

Zapp Brannigan · 19-05-2005 7:13pm

actually id like to know aswell. it has something to do with the amp i think.
Like an amp can put out say 400 watts at 8 ohms but 500 at 4 ohms, or something like that. but thats all i (think) i know!

feylya · 19-05-2005 7:16pm

I can never remember the deal with them but the best thing you can do is get a cab that matches your heads output.

themole · 19-05-2005 7:23pm

ohms is a resistance value (R)

R=V/I, (resistance =voltage/current)

wattage = voltage*current

the voltage will stay the same, but the current will change depending on the resistance.

so an amp will have a higher "wattage" value for lower resistance loads.

the amp may work best with a partiular ohm value speakers, you should lookinto this.

Steoob · 19-05-2005 7:26pm

yea i knew about the whole resistance thing but what does it do for me? the head im gettin outputs 2 4 and 8 ohms which is best?

John · 19-05-2005 7:26pm

I always wanted to talk about ohms but I've always been met with resistance

ZING!

Steoob · 19-05-2005 7:29pm

John2 wrote:

I always wanted to talk about ohms but I've always been met with resistance

ZING!

o dear

It wasn't me! · 19-05-2005 7:33pm

Dear God john... *shakes head*

We may have to reconsider your presence on the dabhoys raid if you come up with more material like that. Then again, you do have the van...

Ohms are the unit of resistance. As far as I know, a cab with lower ohms will deal with a higher wattage, or put out a higher wattage, or something like that. I don't do physics. :cool:

Doctor J · 19-05-2005 8:12pm

Read this for an in depth discussion on this. Basically, the greater the ohm rating, the harder your amp has to work to push out the goods. So, you think you want the lowest ohm rating your amp is designed for, sometimes 2, often 4 on better quality and frequently 8 on cheaper heads. My Ashdown is rated at 300W output at 4 ohms and 240W at 8 ohms. I have one cab which is 8 ohms, so despite my head being 300W I've only ever run it at 240W or thereabouts. I plan on getting another cab, also rated at 8 ohms which, when wired in parallel will have a total resistance of 4 ohms which means my amp will finally put out 300W. Confused? Good.

When you run cabs in series (ie output of one cone to the input of another) you increase the resistance, so 2 8 ohm cabs run in series will be 16 ohms, whihc means your head will have a harder time making things happen. When you run them parallel it halves the ohm rating, they rate at 4 ohms. So you need to plan your head and cab purchases to ensure you're not pissing your money away.

The major downside to low resistance is more power means more heat and if your amp isn't built to take it, it will probably burn out at some stage.

Ancient1 · 19-05-2005 10:33pm

It's high time we all did a refresher course

Good topic, steoob - why didnt anyone think of posting some physics before?

feylya · 19-05-2005 10:37pm

Like natural string harmonics, why scale lengths are different, tensions, string thicknesses? Hmmm, yes...

Ancient1 · 19-05-2005 10:38pm

Yeah that could be interesting.

Although it would require far more brainpower than we currently use

(Fey, where did your sig go?)

John · 19-05-2005 10:40pm

Ohm my god not physics, what a Bohr

(I'm so so so so so sorry, I had to get it out of my system!)

feylya · 19-05-2005 10:43pm

Ancient1, members.boards.ie has gone down for some reason. I keep my sig there.

I've been meaning to write a few posts alright, one about basic "this is a guitar, this is how you look after it and set it up", one about different makes and model numbers, another about guitar construction (more advanced than the other one, maybe some fret placement theory, tempered tunings, stuff like that). Loadsa fun!

Ancient1 · 19-05-2005 10:44pm

2-nil for John

Edit - sounds good Fey, maybe we can sticky-up some real nerdy links.
John2 would go nuts

John · 19-05-2005 10:48pm

Hehe, I actually really like all the theory behind music, I just don't know much about it. I really just wanted to make a bad pun.

I really want to find out more about psychoacoustics but that's for after my exams.

Ancient1 · 19-05-2005 10:53pm

I thought you were messing at first, but then i googled for "psychoacoustics" (never heard that term before)...........

You're of some use anyways, we'll keep ya

feylya · 19-05-2005 10:54pm

I'll get onto those posts after my exams. And after I format my computer. And find a job. And record my album. And find a band. And have some food.

John · 19-05-2005 10:55pm

Yeah I thought it was some made up term the first time I heard of it. It's really quite fascinating. I've a couple of cd's that make use of some of the principles, one I've only been able to listen to once because the frequencies make me feel dizzy and weird.

Ancient1 · 19-05-2005 10:57pm

the frequencies make me feel dizzy and weird

Now that's freaky - "sounds" like something straight out of H.P. Lovecraft !

John · 19-05-2005 11:00pm

There's a book called Tape Delay, I forget the author, about the UK underground experimental scene in the late 70's/early 80's. Pretty much all the artists were experimenting with unusual frequencies. Throbbing Gristle famously used to make people vomit at their shows (and at another show one woman claimed to have an orgasm). Whether it's all true or not is a different story.

Whats the deal with ohms?

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