What does 8 or 4 Ohms mean?

Mapleg4

Senior member
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491
I've been looking at various bass amps online, and what does "8 or 4 Ohms" mean? What is the most common to use?
 

JaySwear

Senior member
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3,006
i think 8 ohms is the most common.

beyond that, i don't really know much. i think it's mainly for matching a head to a cabinet, and if you have a head thats more powerful than the cabinet it could cause some problems. anyway i didn't have to worry about it much, because i found a head to match a cabinet i had sitting around, both 8 ohms.

having said all that, i know bass set ups are almost always more complicated than guitar set ups!
 

Patrick from Davis

Senior member
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2,197
It is "Impedance"  This is measurement of the opposition to alternating current.  Sort of like Resistance for AC.  Since audio waves are AC it measures that.  Lower impedance = higher wattage for the same rig.  If you are going to use one cabinet, 8 ohms is common.  Two 4 ohm cabs in series makes an 8 ohm load.  It is hard to recommend a 4 or 8 ohm cab, you should try them both out to see.  Most amps can handle several impedances, you set a switch, or plug the cab into a specific jack.  Sorry if I didn't help, but it is an odd question to answer.
Patrick

 

mayfly

Senior member
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8,299
If you are comparing amps, then ones that say "4 or 8 ohms" are more flexible and will allow a wider choice of cabs than ones that just say "8 ohms".  Even better are the ones that say "4, 8 or 16 ohms".  If you can't decide between two essentially equivalent amps, take the one with the more flexible output impedance.

Regarding bass rigs, a 4x10 bass cab is usually 8 ohms.  However, if you have a head that gives you the option of "4 or 8 ohms" it will allow you to run TWO of those 4x10 cabs at the same time - or just one of them for quieter gigs.

On a final note, most transistor bass amps will handle 4, 8 or 16 ohms right out of the box.  It's the tube bass amps that might not have all the options.
 

AndyG

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562
Patrick from Davis said:
It is "Impedance"  This is measurement of the opposition to alternating current.  Sort of like Resistance for AC.  Since audio waves are AC it measures that.  Lower impedance = higher wattage for the same rig.  If you are going to use one cabinet, 8 ohms is common.  Two 4 ohm cabs in series makes an 8 ohm load.  It is hard to recommend a 4 or 8 ohm cab, you should try them both out to see.  Most amps can handle several impedances, you set a switch, or plug the cab into a specific jack.  Sorry if I didn't help, but it is an odd question to answer.
Patrick

Actually, with most amps you do not have the option of running the cabs in series (if the amp has 2 speaker jacks they will be wired in parallel), therefore, your two 4 ohm cabs would create a 2 ohm load for the amp, and probably fry it.

8 ohm cabinets seem to be the most common.  Running 2 of these from the same amp will create a 4 ohm load, which is the maximum most amps are designed to handle.
 

Patrick from Davis

Senior member
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2,197
Actually, quite a number of bass amps will run at 2 ohms, and not that many run at 16 ohms.  Guitar amps and bass amps have an impedance frame shift there.  Still, the series parallel thing is something that can be a problem.  There is no way to quickly see which is occuring.  Get a multimeter and check the resistance between the leads going to the amp.  In general the resistance times 1.6 should equal close to the impedance at rest.  So 8 ohms impedance is about 5 ohms resistance at rest, 4 Ohms impedance is about 2.5 Ohms resistance...  It is not exact, but it will tell you if it is in series or parallel.  Good luck.
Patrick

 

mayfly

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8,299
Volitions Advocate said:
Patrick from Davis said:
  Two 4 ohm cabs in series makes an 8 ohm load 2 ohm load. 

TYPO! Don't confuse the poor guy.

Actually Patrick is right - two 4 ohm cabs in series DO make 8 ohms.  In series the impedances add.

two 4 ohm cabs in parallel make 2 ohms, since this is governed by the equation

1/L1 + 1/L2 = 1/L3
where L1 and L2 are the two cabs, and L3 is the resulting impedance.
 

Patrick from Davis

Senior member
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2,197
Those equations tend to get me all mixed up as well, so while I laugh a bit at the irony, I understand.  I prefer 8 ohm cabs because just about anything will run them.  I also put the quick and dirty way to get the impedance off of the multi cab setup because like the equations it is easy to get confused about these things.  The fact of the matter is, you really do have to play with a couple and see what clicks for you.  Good luck finding a bass cab.
Patrick

 
Messages
8,318
In series, they add normally.  Two 8 ohm = 16 ohm, etc.etc.  Parallel is more common and practical because there are no special chords or wiring, but the formula is more confusing.
This is just a little different from Mayfly's, but the formula is actually 1/((1/L1)+(1/L2)) = L total.  When adding in parallel, the total ohm load is always smaller than either of the separate values.  Two 8 ohms is 4.  4 is smaller than 8.  An 8 ohm and 4ohm = 2.666', which is smaller than 4.

The easiest way is to just put two of the same value together and divide one of them by two.  Two 8s is 4, two 16s are 8, two 4s are 2, etc.

 

dbw

Senior member
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4,531
Rick said:
I always thought ohms were resistance.

They are, but when dealing with AC (like the signal your amp sends to the cab), it's useful to talk about the "effective resistance" for certain frequencies.  That's what is meant by an "impedance" of 8 ohms.  If you measure the resistance of your cab with a simple multimeter you will probably get a low reading, but the speaker responds differently to your guitar's AC signal than the DC one you use to measure resistance.
 
Messages
8,318
Ohms are a unit of resistance.  Impedance refers to everything resistive in the circuit, and it too is measured in Ohms.  Just think of an ounce.  It can measure a liquid volume or mass of an object.  Horsepower technically has a watt equivalent, but we think of horsepower as mechanical energy, so amps and lightbulbs tend not have a HP rating.
 

Superlizard

Senior member
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2,514
Basic Impedance Rule O' Thumb For Those Who Don't Want To Do The Math:

Match the value of the amp to the speaker cab.

8 ohm amp - 8 ohm speaker cab
16 ohm amp - 16 ohm speaker cab
etc...

You *will* be safe doing this.
 

Rick

Senior member
Messages
4,494
http://www.guitarplayer.com/article/how-to-replace/Aug-06/22534

How To Replace Speakers
By Josh Workman


Aside from swapping out a blown or damaged speaker, why would you bother replacing the speakers in your guitar rig? Well, by considering different magnet types, power ratings, and impedances, you may improve your amp’s (or speaker cab’s) tone and performance, and tailor its sound to better deliver all the nuances of your playing. It’s really not difficult to replace your own speakers and experiment with different models until you find your perfect match. Here’s a step-by-step guide.




Most speaker manufacturers have pretty solid Web sites loaded with info on how their different models are voiced, so start there. You can also seek out info on your favorite players’ rigs and see which speakers they load their cabs with, although other factors (such as amp, signal path, etc.) will color the sound you’re digging. Local players and guitar-oriented forums can provide excellent references, as well.

Once you zero in on a few options, you should consider the speaker magnet. There are various kinds of magnets for different applications, but, for guitarists, it usually boils down to alnico or ceramic. Alnico (aluminum-nickel-cobalt) magnets have a lot of allure for blues and roots players, because as you play louder, the voice coil’s own magnetic field begins to temporarily demagnetize the alnico structure, resulting in decreased speaker movement, but also smooth compression and sweet sustain. Ceramic (strontium ferrite) magnets are far less susceptible to compression and demagnetization, so, at high volumes, the speaker simply goes as far as it can, and then breaks up with a hard, bright crunch that’s perfect for edgy rock or metal.

A speaker’s power rating is another critical factor, as you’ll definitely need the speaker to handle the power your amp is pouring into it. As a rule of thumb, if your amp is rated at 60 watts, you shouldn’t use a speaker that’s rated at less than 60 watts. While cool sonic things can occur when speakers are pushed hard—as in a 30-watt Vox AC30 equipped with two 15-watt Celestion Blues—it’s probably wise to use a speaker (or combination of speakers) that can handle about twice the rated power of your amp.

You’ll also need to consider impedance. For example, if the speaker in that 60-watt amp was rated at 4 ohms, then you shouldn’t replace it with anything that puts a load of less than 4 ohms on your amp’s output circuit. Less is a deceiving term, however, because when you lower the impedance, you’re actually increasing the current sent from the amp. Now, it’s one thing to blow up a speaker, but going below the impedance rating can fry the output transformer in a tube amp, or all the output transistors in a solid-state amp. Figuring total impedance for multiple speaker arrays involves Ohm’s Law and some relatively simple math (all of which can be explored in detail on the Web), but here are some common configurations:

• Two 4-ohm speakers wired in series provide a total impedance of 8 ohms.

• Two 8-ohm speakers wired in parallel provide a total impedance of 4 ohms.

• Four 16-ohm speakers wired in series/parallel yield a total impedance of 16 ohms.

What the heck is series, parallel, and series-parallel wiring? Glad you asked. Read on.


Wiring Schemes
Of course, the easiest way to replace two or more speakers in a cabinet is to rewire them exactly the way they were before you started futzing around. Take a digital photo for reference. If you want a little science behind the practical, however, here’s some knowledge.

In many cases where two speakers are involved, the pair were likely wired in parallel—meaning all the positive terminals of each speaker are connected to the positive (tip) side of the speaker jack, and all of the negative terminals are connected to the negative (ring) side of the jack (Fig. 1). An advantage of this wiring scheme is that if one speaker blows, the remaining speaker (or speakers) will continue to get current and produce sound. In addition, as more speakers are connected, the impedance load on the amp decreases, which results in more acoustic output.

In a series configuration with two speakers, the positive terminal of one speaker is connected to the positive (tip) side of the speaker jack, the negative terminal of the second speaker is connected to the negative (ring) side of the jack, and remaining positive and negative terminals on both speakers are connected together (Fig. 2). In series connections, the more speakers used, the higher the impedance, which results in a lower acoustical output.

A series/parallel wiring scheme allows you to use additional speakers to increase volume, while maintaining a total impedance that’s compatible with the amplifier (usually between 2 and 16 ohms). In a typical four-speaker setup, a single wire from the positive (tip) side of the speaker jack runs to the positive terminals of speakers A and C. Next, the negative terminals of Speakers A and C are wired to the positive terminals of Speakers B and D. Finally, a loop is created by running a single wire from the negative (ring) side of the jack and splitting it between the negative terminals of Speakers B and D (Fig. 3).


Getting Started
Lay your amp or cab facedown (so you can easily lift the speaker in and out), unbolt the speaker from the cabinet, and get ready to detach that sucker and prep a home for your new speaker. Depending on the brand and type of speaker, it may have slide-on, pushbutton, or solder-style connections that need to be detached. For slide-ons, grab the female connector on each speaker wire and carefully work it off. Pushbuttons are easy—just depress the button with one hand, and pull the wire out with the other.

To unsolder a wire from a speaker terminal, hold onto the insulated part with one hand, and apply the soldering iron with the other hand just long enough to slowly pull the wire off. Make sure you wear goggles (hot solder can splatter), and keep the window open (you’re working with a lead-based product). Now, check out the photos for help on soldering your new connections.

 

Alfang

Senior member
Messages
2,596
resistance is a value that doesn't change, it slows down current flow and creates heat.

Impedance is simply the opposition to current flow, and changes it's value or opposition based on the frequency applied,  impedance is generally measured in transformers or inductors, where expanding and collapsing magnetic fields are involved.
 

jerryjg

Senior member
Messages
506
If you have 8 Tibetian Monks sitting in a hall chanting, then thats 8 ohms. If you have four Tibetian Monks sitting in that same hall chanting , then thats 4 ohms.
Hope this explains things
 

kboman

Senior member
Messages
2,378
jerryjg said:
If you have 8 Tibetian Monks sitting in a hall chanting, then thats 8 ohms. If you have four Tibetian Monks sitting in that same hall chanting , then thats 4 ohms.
Hope this explains things

This is the only reply I've actually understood in this whole thread  :toothy12:
 
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