Ohms questions?

tonyw1

tonyw1

New member
Hi guys
sorry if i'm coming across as stupid but i still don't really understand ohms?
at the moment i have a fender fm100h and the matching 4x12 cab
the cab is 4ohms as far as i know was wondering if i bought a new amp head either 8/16 ohms would it blow the speakers?
many thanks
Tony
 
It wouldn't blow the speakers, it would blow the amp.

But you could wire the cabinet for 16 ohms and use it with the new amp.
 
Increasing the impedance (ohms: resistance to current in ac or dc circuit) will not damage your speakers or your amp. Decreasing the impedance may damage your amp and or speakers by sending a DC current though the speaker. An increase in impedance may cause a decrease in volume of your speakers. (that depends on the design of the speaker.)
 
First off why not get an amp to match? Most valve amps are switchable and will match your cab, most transister amps wont care as long as the resistance is above 2 ohms.

Try if you can to match the impedence think of it as pushing water through a pipe - as the bore decreces the water finds it more difficult to pass through and will, if pushed generate heat.

2 Ohms greater resistance and usually as low as you can go 2 x 4 ohm cabs in series = 2 ohms
4 ohms less resistance
8 ohms lower resistance

When you look at amp specs they will ususlly state something like 100watts at 2 ohms, 75 watts at 4 ohms, 50 watts at 8 ohms. in reality most wouldn't notice a difference in performance if you swapped an 8 ohm cab for a 4 ohm cab.
Golden rule try before you buy, there is a lot more to it than what it says on the box.. test out the amp with your cab or better still do an A B test let you ears decide.
 
It's a good idea to look at the manual for the amp to see what the minimum speaker impedance the amp can handle is. Some can go as low as 2 but not all. Many amps can't go lower than 4 ohms. Some have a switch. A lot of amps can handle 4 ohms but again, it's a good idea to check.

Less ohms from the speaker will trigger the amp to put out more power. If it can't it will still try, which is what blows the amp if it doesn't have protection circuitry.


If you're handy with a multimeter and soldering iron you can change the impedance of the cab. Wiring the speakers in parallel will decrease the impedance. Wiring the speakers in series will increase the impedance. You can usually combine series and parallel in a bank of speakers to get the impedance value you're looking for.

What head are you looking to get?
 
bruiser1964 said:
2 Ohms greater resistance and usually as low as you can go 2 x 4 ohm cabs in series = 2 ohms
4 ohms less resistance
8 ohms lower resistance
Click to expand...

This is backwards. In a DC circuit "ohms" is a value of resistance. More ohms is greater resistance.

2 x 4 ohm cabs in series = 8 ohms. Most amps have the speaker jacks wired in parallel so that usually if you were to hook up two 4 ohm cabs it would drop to 2 ohms load.
 
A higher resistance (8 ohms compared to 4 ohms) cabinet will typically give you a lower volume - I say typically because the efficiency of the speakers also matters.
A solid state amp (non-tube) should not have a lower resistance cabinet plugged into it than it is designed for, it will cause the output circuit to overwork, heat up and eventually die.
 
I have not seen any modern tube head without a switch so it shouldn't be an issue.

If it's something old you might have a problem but it's usually not an issue. My 70s bassman for example is 4ohm only, there is no switch.
 
snow lizard said:
This is backwards. In a DC circuit "ohms" is a value of resistance. More ohms is greater resistance.

2 x 4 ohm cabs in series = 8 ohms. Most amps have the speaker jacks wired in parallel so that usually if you were to hook up two 4 ohm cabs it would drop to 2 ohms load.
Click to expand...

Yeah sorry - good job I'm not a sparkie :) I thought tho when you wire cabs in series i.e. cab - cab the resistance goes down 4ohm cab + 4 ohm cab = 2 ohm? no?
 
Usually that's what happens but it isn't called "series".

If you have two 4 ohm speakers and you wire them together so that the positive terminals are connected to each other and the negatives are connected to each other the load will drop to 2 ohms. This is usually what's going to happen if you have an amp with 2 speaker jacks or if you daisy chain 2 cabs together. This wiring scheme is called parallel, not series.

If you have two 4 ohm speakers in a cab and you wire the positive terminal of one speaker to the positive of the jack, and the negative of the other speaker to the negative of the jack, and the remaining positive and negative terminals of the speakers to each other, this is a series circuit. The ohms go up, not down.
 
bruiser1964 said:
Yeah sorry - good job I'm not a sparkie :) I thought tho when you wire cabs in series i.e. cab - cab the resistance goes down 4ohm cab + 4 ohm cab = 2 ohm? no?
Click to expand...
No, that's when you wire two cabs in parallel. Most guitar amps (possibly all of them) have the speaker outputs wired in parallel.
 
Last edited:
To the original question. Yes try to get the head that matches. Most cabs are 4 or 8, and some 2 or 16. Most modern heads are 4 and 8 so either will work. Ill bet your cab is 8 so you should be good.

If you find a head you love that won't match your cab, find a tech who will rewire your cab (or ask here). It'll probably cost less than $100. So get the head you want and 90% it'll work with your cab.
 
I've fried speakers before and it was due to Ohm mismatching...like a car audio speaker hooked up to a home amp (trying make some car speakers work for monitor/mix/master checks.) It worked for awhile, but the fyring heat was slow.. the paper at the edges actually became so brittle it fell apart in a week.
2ohm speaker to a 100watt 8ohm amp... the amp survived luckily! but I wasnt driving it loud, but obviously there was some heat going on...the speaker cone was so brittle as if it had been near a fire, and cracked and brittle is what it was.

Be careful and use the right cables too, guitar cables dont take well to high current either.
Speaker cables handle the higher current.

its often safe to just follow the basics and match things, thats my vote too.

DIY heads can do it easily, ..but if you were one of them you wouldnt be asking the question,..
 
Yup, going with a 2 ohm load, your amp was capable of pumping 400 watts into those speakers. You are lucky your amp didn't fry, not all amps can handle a 2 ohm load without smoking.
 
let's rock said:
Increasing the impedance (ohms: resistance to current in ac or dc circuit) will not damage your speakers or your amp.
Click to expand...
There is no guarantee of that at all.


bruiser1964 said:
think of it as pushing water through a pipe - as the bore decreces the water finds it more difficult to pass through and will, if pushed generate heat.

2 Ohms greater resistance and usually as low as you can go 2 x 4 ohm cabs in series = 2 ohms
4 ohms less resistance
8 ohms lower resistance

When you look at amp specs they will ususlly state something like 100watts at 2 ohms, 75 watts at 4 ohms, 50 watts at 8 ohms. in reality most wouldn't notice a difference in performance if you swapped an 8 ohm cab for a 4 ohm cab.
Click to expand...

Your ohm's law has already been corrected. The water in the pipe analogy is good, but incomplete. If the pipe gets smaller (more ohms) and the pressure stays the same (voltage from the output transformer) then eventually the water pressure (voltage) will push back into the supply (back into the transformer) and cause a pipe to burst inside the house (inside the amp). In an amp, this concept is called flyback voltage, and it can fry things inside the amp.

You can't just make a blanket statement that it's always safe to use a higher impedance with any amp. Not true at all.

Yes, in theory a lower impedance cab could blow the speakers. If you have a 100 watt 8 ohm cab and a 50 watt 4 ohm cab, your amp puts out 50 watts into an 8 ohm load or 100 watts into a 4 ohm load, you mightblow those speakers when you plug into the 4 ohm cab.
 
There's a lot of confusion and misinformation in this thread.

Just match the ohm rating of your amp and cab. Simple. A 4 ohm cab connected to a head rated for 8 or 16 will probably end badly.

Maybe you can rewire the cab to be 8 or 16 ohms to match your head.
 
Greg_L said:
There's a lot of confusion and misinformation in this thread.

Just match the ohm rating of your amp and cab. Simple. A 4 ohm cab connected to a head rated for 8 or 16 will probably end badly.

Maybe you can rewire the cab to be 8 or 16 ohms to match your head.
Click to expand...

There you go! Two options:

1) Match

2) Rewire to match
 
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