Amp and speaker ohms ... help!!

  • Thread starter Thread starter noisedude
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noisedude

noisedude

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So I've got a combo amp which is 100w hybrid (essentially solid-state) which has an extension output that says 8 Ohms on it. I'm looking at an extension cab which is a 2x12 and says "4 Ohms mono / 8 Ohms per side stereo".

Can I use this amp with this cab? Will my amp blow up?

I don't really understand how impedances work tbh. :o
 
You can use one side of that cabinet. Only one of the 12's
 
Nope. That cabinet has two 8 ohm speakers in it. The only thing you could do is rewire it to be a 16 ohm cabinet.
 
You can run a 16 ohm load with 8, not ideal, but it won't croak a transformer as fast as running into a 4 ohm speaker.
 
ermghoti said:
You can run a 16 ohm load with 8, not ideal, but it won't croak a transformer as fast as running into a 4 ohm speaker.
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you'll only get half the power

but it won't hurt anything



if you can get two of those cabs

you could use them in series

to get an 8 ohm load for the amp



btw, how is the cab rated for

power handling capability (watts)?
 
In a nutshell

if you use a cab/speaker with a higher impedance rating than your amp calls for, you will get less output as far as volume goes, because you are presenting too much resistance to the amp. If you use a cab with a lower impedance than is called for, you not be presenting enough resisitance to the amp, and you run the very real risk of cooking it sooner rather than later.

Very generalized, but that's the concept.
 
The cab is rated at 130w ... it's a Marshall MC212. Cheap Celestion-loaded way of moving more air without having to sacrifice my lovely Fender clean sound!

Running one side of it would put me at risk of blowing whichever driver it used because they're rated at 65w each.

Is there something I could do, solder something in to make it work? It's just that the dimensions of this cab are absolutely perfect for the back of my car, I can't have anything bigger and this cab is a bargain. But if it won't work, it won't work.
 
What's the impedance

of your combo speakers?

You might be able to plug the other side of the cab instead of the combo speaker(s), or just plug the cab in mono instead of the combo speaker.

I gotta go. Someone else please carry on.
 
YOu could rewire the cabinet for 16 ohms. It will work and won't blow the amp.
 
Or my favorite solution.....

Replace the speakers in the cab with two 16 ohm speakers, then the mono jack would be 8 ohms and each single in stereo would be 16 ohms. There are several great speakers available rated for 100 watts + starting with the Weber Michigan $100, Eminence Swamp Thang and Tonker each about $80, Jensen C12K $80.
 
On the subject of replacing the speakers; there are two ways of doing this to get the desired result. 1, You can use two 4 ohm speakers wired in series or 2two 16 ohm speakers wired in parallel. Either way you get 8 ohms on a single input. If you can find some 8 ohm resistors rated to handle the power (good luck on that) you could add them into the wiring to balance the resistance but replacing the speakers would probably be easier than finding resistors large enough to handle the load.
 
Ok well I'll never need to use the amp in stereo so how hard is it to wire it so that it runs both and comes to 8 Ohms??

Sorry for the basic lack of electrical understanding guys .... you should see me wiring guitars .... it's not pretty!! :)
 
noisedude said:
Ok well I'll never need to use the amp in stereo so how hard is it to wire it so that it runs both and comes to 8 Ohms??
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As covered above, you'll have to replace the speakers to get to 8 ohms, period. 2x4 ohms series = 8 ohms, 2x16 parallel = 8 ohms. The actual wiring couldn't be much simpler. Series is wired just like a bunch of batteries in a radio, parallel is wired with each like pole tied together, like if you put in one battery backwards.

Series: RT = R1 + R2 Parallel: RT = (R1R2)/(R1 + R2)

No combination of 8 ohm speakers makes 8 ohms.

Series = 16 ohms Parallel = 4 ohms

Math and diagrams here: http://www.termpro.com/articles/spkrz.html
 
noisedude said:
Ok well I'll never need to use the amp in stereo so how hard is it to wire it so that it runs both and comes to 8 Ohms??

Sorry for the basic lack of electrical understanding guys .... you should see me wiring guitars .... it's not pretty!! :)
Click to expand...

Two 8-ohm speakers can not be wired in combination to yield 8 ohms. Period.
 
noisedude said:
The cab is rated at 130w ... it's a Marshall MC212. Cheap Celestion-loaded way of moving more air without having to sacrifice my lovely Fender clean sound!

Running one side of it would put me at risk of blowing whichever driver it used because they're rated at 65w each.
Click to expand...

So, the combo speaker is 8 ohms, your external speakers are 8 ohms each, and your amp is 100 watts, right? If you run one external speaker they (the internal speaker and the single external one) will split the output of the amp equally, i.e. 50 watts per. It seems to me that you will be OK, especially if you're not diming the amp.
 
Dani Pace said:
If you can find some 8 ohm resistors rated to handle the power (good luck on that) you could add them into the wiring to balance the resistance but replacing the speakers would probably be easier than finding resistors large enough to handle the load.
Click to expand...

I have always been told that this is not recommended at all. Inline resistors do not have the same reactance that a moving voice coil assembly has, it will never sound the same.....assuming you can even find a resistor that will handle the power. Those little resistors you see inside your consumer electronics are rated 1/4 watt, 1/2 watt. A 50 watt resistor could easily be the size of a large flashlight, and it could cost you more than the speaker.
 
soundchaser59 said:
I have always been told that this is not recommended at all. Inline resistors do not have the same reactance that a moving voice coil assembly has, it will never sound the same.....assuming you can even find a resistor that will handle the power. Those little resistors you see inside your consumer electronics are rated 1/4 watt, 1/2 watt. A 50 watt resistor could easily be the size of a large flashlight, and it could cost you more than the speaker.
Click to expand...

Not to mention that you would be throwing away a significant portion of your volume away as heat, and isn't getting more volume from the amp the whole point of this exercise?
 
noisedude said:
I'm looking at an extension cab which is a 2x12 and says "4 Ohms mono / 8 Ohms per side stereo".
Click to expand...
stop looking at that cab

you're either going to need an amp with 4 ohm output impedance

or speakers/cab with 8 ohms impedance



your choices with this combination have been covered:

1) use one speaker (but it won't handle all the power of the amp)


2) wire the speakers in series to give 16 ohms (easy to do)

but will only give you half the power the amp is capable of


3) there is a third choice - just connect the cab (mono - 4ohms)

to the amp - if you don't turn it up too high

you may not blow the amp :)
 
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