# Ohm confusion?

Discussion in 'Amps and Cabs [BG]' started by jackbass182, Dec 27, 2007.

1. ### jackbass182

Apr 20, 2007
I really don't understand all this "8 Ohm" "4 Ohm"
Can someone please explain the compoatability between 4 Ohms and 8 Ohms
Thanks

Jan 18, 2007
Minneapolis
Physicist
I suggest you do a search, as this topic has been covered extensively here on TB. I'd start with "impedance" as your search term -- or read the FAQ thread

3. ### jimbilly

Apr 19, 2006
Let's see if I can cover it briefly:
Ohms just means 'resistance', or how hard it is for the electricity to travel from one end (+) to the other (-). Zero ohms is basically a dead-short (circuit), like stringing a big fat wire between your speaker + and -, most amps don't like dead shorts, or even close to it. Zero ohms means almost no resistance to the current flow. A bigger number of ohms (like 32) means it's much harder for the current to flow through that circuit. Most amps are built for 4 to 16 ohms (some are specific, some can handle anywhere in the range, 2ohms is too close to a short for some, just fine for others). A single speaker rated at 8ohms will probably measure somewhere around 6 to 7 on an ohm-meter, the speaker rating is nominal, as the resistance is not a fixed number.

You can imagine it kind of like this: you have one 8ohm speaker, you add another one in 'parallel' (side by side, wire both +'s to the red side of the cabinet jack, both -'s to the black side of the cabinet jack), the current then has twice the path to follow, = less resistance, usually appx. 1/2 the resistance, so 2 8ohm speakers wired parallel would make a 4 ohm load. Wiring them in series, the current goes through one speaker and then through the other (red on cabinet jack to + of speaker A, - of speaker A wired to + of speaker B, - of speaker B to - of cabinet jack: current goes through one speaker and THEN the other), is more resistance, roughly twice, = 16ohms, -a more resistant signal path to the flow of current.

4. ### jackbass182

Apr 20, 2007
Im still really confused
Can a 8 Ohm head amp be used with a 4 Ohm cab?

5. ### jimbilly

Apr 19, 2006
No, an '8ohm head' would require an 8ohm load, a 4ohm load would likely overheat it. If it says '8ohms minimum' on the back of the head, that's what it's safe to run.

6. ### billfitzmauriceCommercial User

Sep 15, 2004
New Hampshire
Owner, Bill Fitzmaurice Loudspeaker Design
Strickly speaking, no. This assumes that you're referring to a head with a specified minimum load impedance of 8 ohms.

7. ### jackbass182

Apr 20, 2007
So 4 Ohm is more powerful than 8 Ohm?

8. ### h00t

Jul 14, 2006
Central WI
Are there even any heads that can only handle 8 ohm? I thought even the most cheapo weak ass amps are 4 ohm stable. But of course, I haven't studied such amps too throughly.

"So 4 Ohm is more powerful than 8 Ohm?"
Ohm has nothing really to do with power, it has to do with resistance. But an amp running with 4 ohm of resistance will make more power than an amp running at 8 ohm. For example, the Ampeg B2-RE will make 250 watts at 8 ohm and 450 watts at 4 ohm. To run at 8 ohm you would use one 8 ohm cabinet. To run at 4 ohm, you would run either one 4 ohm cabinet or two 8 ohm cabinets.

9. ### J.Stone

There's a sticky with an FAQ about Ohms. If your Amp is rated at a minimum of 8 Ohms, if you go any lower you risk blowing the amp. If your amp is rated at a minimum of 4 Ohms, it can hold either 1x4ohm cab or 2x8ohm (In parallel). I highly suggest reading the FAQ.

Also, the sticky at the top of this forum may have answers to any other questions you have! So check that first.

10. ### billfitzmauriceCommercial User

Sep 15, 2004
New Hampshire
Owner, Bill Fitzmaurice Loudspeaker Design

11. ### jimbilly

Apr 19, 2006
You can also think of it this way: Next time you're at McDonalds, take a straw, drink some root beer through it, call the resistance of the root beer to your mouth 8ohms. Take a 2nd straw, have both in your root beer (parallel), one straw in one side of your mouth the 2nd straw in the other side of your mouth, drink some more root beer (both straws side by side at the same time). It's easier, it's half the resistance, or 4 ohms (slamming your face into the cup would be a short...). Now take that 2nd straw and tape it on top of the first straw making one long straw, -thats harder, more resistance, or appx. 16 ohms... I'm thirsty now...

12. ### J.Stone

That made me laugh. I just had this image of someone sitting at a table in McDonalds just smacking their face into their drink. Also, why don't we get Rootbeer here in Europe?

13. ### JimmyM

Apr 11, 2005
Apopka, FL
Endorsing: Yamaha, Ampeg, Line 6, EMG
Forget rootbeer...I want to know why you can't get ice in Germany!

I have a question...how is the impedance of a raw speaker changed? In other words, you have an 8 ohm 10" and a 32 ohm 10". What part of the build process makes their impedances different?

Jun 17, 2004
Ireland
I must say jimbilly you are quite good at explaining these things in simpler terms.

15. ### J.Stone

I'd imagine it's the voice coil material/wind count? If you've got low gauge voice coil with few winds, you may get less ohms than high gauge with loads of winds. This is just my logical deduction, could be way off.

16. ### Bob Lee (QSC)In case you missed it, I work for QSC Audio!

Jul 3, 2001
Santa Ana, Calif.
Former Technical Communications Developer, QSC Audio
Someone lost the recipe?

17. ### JimmyM

Apr 11, 2005
Apopka, FL
Endorsing: Yamaha, Ampeg, Line 6, EMG
Apparently so, Bob. I couldn't find any anywhere.

18. ### billfitzmauriceCommercial User

Sep 15, 2004
New Hampshire
Owner, Bill Fitzmaurice Loudspeaker Design
http://electronics.howstuffworks.com/speaker.htm

19. ### jackbass182

Apr 20, 2007
I found you post very helpful
Thanks

20. ### ModestCargo

Sep 21, 2005
Here's something to remember.

8 ohm solid state head: Can run 8, 16, 32, etc ohms, or with no speaker altogether (but you get the best tone and most power out of 8). Do not run less than 8 ohms, ever.

8 ohm tube head: Should be used with 8 but probably can run down to 4 with slight loss of power and maybe top end. Do not run with 16, 32, etc if you value the amp. Never flip it on with no speaker.

If you have two 8 ohm cabinets you can chain them together and the load will be 4 ohms. Two 4 ohm cabinets will be 2 ohms. Etc.

21. ### PrimaryTB Assistant

Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

Jul 24, 2021