ohms

[Bassplayer58]

what is an ohms? I have wondered for a while and finaly got around to asking. I have herd of 2, 4, 8 are there others, what do they mean.

[Addix Metzatricity]

An ohm is a unit of measurement. It measures resistance.

[rumpelstiltskin.]

to really understand what an ohm is, you'd have to wrap your brain around some pretty abstract notions.

 

in very basic terms, ohms are a very important way of describing a cabinet -- more important to your amplifier than what size the speakers or how many there are. every amplifier is rated to deliver its power into different loads, which are specified in ohms. it is vitally important that you never place a smaller load on your amplifier than its rating. (for example: 300W into 4 ohms means you should never connect fewer than 4 ohms to your amplifier.)

 

so how do you know what ohms your speakers are? i'm glad you asked. the math is a little convoluted, so unless you want the detailed explanation, take me on faith. in almost every case you will ever run into, if you're plugging in two 4 ohm cabinets, the total load is 2 ohms. two 8 ohm cabinets make a total load of 4 ohms. an 8 ohm cabinet and a 4 ohm cabinet make a total load of 2.67 ohms.

 

make sure you never connect fewer ohms to your amplifier than its rating! if you're ever confused, ask someone at a store. they should know, and if they don't, stop going to that store. if you're really lost, buy a combo amp or a setup advertised by a single manufacturer (like a full stack package from carvin, ampeg, eden, etc.)

 

let me know if you want more details or if this was too technical for you.

 

robb.

[Addix Metzatricity]

I get too wrapped up in semantics... robb is actually talking about impedance matching. Which, though I understand, I'm unable to communicate the idea well enough.

 

He is correct though. That's probably the only place you'll ever have to worry about ohms. Unless you start getting involved with electronics.

[GeorgeR]

"Ohms" are a simple artificial mathematical unit which serves to describe the relationship any electrical device exhibits when electricity is applied to it.

 

Put simply, given a device's fixed rating in "Ohms" the current passing through the device will change linearly with the applied voltage.

 

This relationship does not change whether Direct current or Alternating current is used, nor if the device under test is reactive (capacitive or inductive) or purely resistive.

 

The mechanics of the mathematics change dramatically, becoming quite complex when working with highly reactive loads, because of many additional parameters to calculate instantaneous voltage and currents - at times both differential and integral Calculus may be employed as well.

 

But the relationship remains entirely predictable based on this relationship Mr. Ohm outlined.

 

"Ohm" is called such after the German mathematician, Georg Simon Ohm , who in the mid 1820s observed, deduced and recorded on paper the relationship between the newly illustrated concepts of voltage and current.

 

Until the late 1700s to early 1800s, the phenomena and study of electricity was as much a phantom force as anything science fiction might create today and therefore subject to much conjecture and superstition.

 

Not bad work, considering they had no accurate voltage or current measuring devices as we know them today!

[Addix Metzatricity]

Hence... Ohm's Law: I=E*R (I think... some ET I am).

 

I= Current

E= Voltage

R= Resistance

[GeorgeR]

Addix - close but no cigar.

 

It's:

 

E = I * R

 

E= Electromotive force in Volts

I= Intensity, current, in Amperes

R= Resistance, in Ohms

 

For calculating power it's:

 

P = E * I

 

as well as:

 

P = (I^2) *R

 

and

 

P = (E^2)/R

[Addix Metzatricity]

Damnit! 2 years in school for electronics and I always had to have that written down. I can never remember it.

 

Oh, well. You get the idea.

 

Thanks for correcting me.

[fantasticsound]

Let's go back to a practical, simplified explanation of ohms as they relate to amps and speakers.

 

The rating, in ohms, of a speaker or speaker cabinet describes the amount of resistance the amplifier is pushing against. The amp, as was noted, will be rated for a certain number of watts at different loads. Usually 8, 4, or 2 ohms, but guitar amps often include 16 ohm load ratings because 4x12 cabs are rated for 16 ohm operation.

 

As the ohm rating (resistance) of the speaker is reduced, the more actual power an amp can provide to that load. A Mackie 1400i amp is rated to provide;

 

300 watts @ 8 ohms

500 watts @ 4 ohms

700 watts @ 2 ohms

 

It doesn't matter how many speakers are involved. Whatever the entire system of cabs presents to the amplifier is all that matters. Just remember that two cabinets that are rated for 300 watts (irregardless of the ohm rating) will crave a total of 600 watts from the amp. With the above amp, you'll have 500 watts, which should work fine.

 

So how do ohm loads add up?

 

In parallel wiring, which resembles a "Y" adapter (as most instrument cabs are wired), combining identical loads (ohms) halves the total load of both speakers/cabinets. 8 ohms + 8 ohms = 4 ohms. 4 ohms + 4 ohms = 2 ohms.

 

In serial wiring, the + from the amp goes to the first speaker's +, the - from that speaker hooks to + on the next speaker, and finally, + on the second speaker connects to the - terminal on the amp, like a big, electrical ring. (Think of a batteries in a circuit, usually wired + to - to + to -, etc.) Ohm loads that are combined will add together. 4 ohms + 4 ohms = 8 ohms. 8 ohms + 8 ohms = 16 ohms.

 

So why does this matter?

 

First, if you run at a higher ohm load, you get less power, which usually means less perceived loudness. Run at a lower load, and you get more power and more percieved loudness. But that's not all.

 

If you attempt to run an amp below it's rated load limit, its power supply will attempt, but fail, to keep pace with the input voltage. Imagine running down a hill that get's too steep. Eventually gravity makes you move faster than your legs can keep up and you fall on your face. In the case of your amp, the power supply will burn up. Not pretty, but very expensive to fix!

 

There are other threads on these forums with more math intensive explanations of matching ohm loads and there are LOTS of resources on the internet. Use the search function to find ohms on the SSS, GM's forum, the Lowdown (here) or the guitar forum.

 

Some advantages of using multiple speakers at a lower rating vs. a single speaker at the higher rating (remember, most cabs are wired for parallel operation in tandem with another cabinet) are coverage and air movement. It's easier for smaller speakers to move air than larger ones, so multiple small speakers can actually move more air than a single large speaker. There are also other considerations that have nothing to do with load. I'll leave that for another conversation.

[GeorgeR]

It's ok. You remember the triangle? That always worked for me!

 

http://www.mitedu.freeserve.co.uk/Theory/images/vir.gif

[MartinJ]

...or is it plural for a chanting/humming sound?

 

(Actually, I just wanted to see if the "avatar" I choose is working...and I don't even know what the hell "avatar" means. Hmm. Ooohhmmmmm.)

[Addix Metzatricity]

Originally posted by GeorgeR:

It's ok. You remember the triangle? That always worked for me!

 

http://www.mitedu.freeserve.co.uk/Theory/images/vir.gif

Well, we had a circle (which also included P), but all I could ever remember was the circle itself... Never where anything went.

 

It doesn't really matter anymore. In practical applications I very rarely have to call on any equations.

[Prague]

The basics:

 

1 volt ® can deliver

1 amp (I) of current through

1 ohm ® which dissipates

1 Watt (P).

[Addix Metzatricity]

Originally posted by paostby:

The basics:

 

1 volt ® can deliver

1 amp (I) of current through

1 ohm ® which dissipates

1 Watt (P).

Only one problem there...

 

1 Volt (V or E)..

[Bisgit]

Ohms are houses that families live in. ohm sweet ohm.

[Tom Capasso]

Originally posted by Addix Metzatricity:

Originally posted by paostby:

The basics:

 

1 volt ® can deliver

1 amp (I) of current through

1 ohm ® which dissipates

1 Watt (P).

Only one problem there...

 

1 Volt (V or E)..

Voltage is expressed as the letter "V" when practical people are talking about volts.

Voltage is expressed as "E" by scientific types because it stands for "electromotive force". That's because voltage is the force of electron movement (as opposed to the "amount" of electrons - that amps).

 

Hope this helps.

Tom

[The Bear Jew]

Wow... I'm impressed.

 

I was expecting Tom to say that "Ohms" were something that helped you relax during meditation. Wocka wocka.

 

Hey, bite me.

[Addix Metzatricity]

Well... I'm a practical person, but I use 'E' anyway, 'cause that's what the Navy taught me to use. And I was a good little sailor.

 

Not really.

 

Anyway, I was just showing that they were interchangeable. I'd hate for anyone to get hung up on that.

[Tom Capasso]

HA !! CMDN learns that I'm more than just a (un)funny bassist !! Ohm-y goodness !! (masticate on that old boy).

 

I meant to say the there is almost never a clear delineation between "practical types" and "scientific types", especially when it comes to electricity. Everybody dives in !!

 

Tom

[Prague]

Originally posted by paostby:

The basics:

 

1 volt ® can deliver

1 amp (I) of current through

1 ohm ® which dissipates

1 Watt (P).

Hey, dumbass. Chek fr tyyypos.

[Talisman]

When dealing with speakers and amplifiers, you can get just about as in-depth with the concepts as you want. The bottom line is: (example) If you have an amp that is rated at eight ohms, don't use a cabinet that is rated at less than eight ohms. Also, don't connect two eight ohm cabs to it (because that makes it four ohms).

 

Some amps (most, I think) have two channels rated at eight ohms. One eight ohm cab per channel. If you connect a lower impedance cab (4 ohm) to a higher impedance amp (8 ohm) you make the amp work too hard and it'll fry if you turn it up.

[Bassplayer58]

thanks it helped alot

 

-Dan