Ohms - the dark art...

[Bass Of Spades]

So, I have managed to get myself in an Ohm quandary - I have a 2x10 cab rated 8 ohms, and a 2x15 cab rated 4 ohms... I have been running the amp through each cab individually, but fancy seeing what I get if I were to run through both. Knowing very little about Ohms, I thought I better ask the question. Can I run both cabs from the amp, and if so, what output do I run the amp at (would this be 2 Ohms?) 

Also, do I run an output to each cab individually (using both outputs from the amp) or do I link the cabs together using one output from the amp - just want to ask you good folk before I blow up my amp, speakers, or indeed both...

[fretmeister]

Why do people post this sort of question without saying what the amp is?

[Bass Of Spades]

People? This person simply didn’t realise it was important... Mesa TT 800, it has a switchable output 8, 4, or 2 ohms... 

[JJMotown]

Why don't people post this sort of question in the Amps & Cabs section, where they talk ohms?

[fretmeister]

1 hour ago, Bass Of Spades said:

People? This person simply didn’t realise it was important... Mesa TT 800, it has a switchable output 8, 4, or 2 ohms... 

 

That would give you 2.67ohms so set the amp to 2 ohms.

2 cables from the amp or daisy chaining will be fine.

[neepheid]

Going forward, so you can work it out for yourself in future without the danger of subjecting yourself to snippy comments - adding impedances together in parallel works like this:

 

 

So in your case:

 

1/R = 1/8 + 1/4

1/R = 3/8

R = 8/3 = 2.67 ohms

 

 

[Bass Of Spades]

@fretmeister and @neepheid really appreciate your help - I know its probably a basic question but when you dont know you dont know. Appreciate your time in helping out.  cheers

[bremen]

On 28/03/2025 at 19:04, neepheid said:

Going forward, so you can work it out for yourself in future without the danger of subjecting yourself to snippy comments - adding impedances together in parallel works like this:

 

 

So in your case:

 

1/R = 1/8 + 1/4

1/R = 3/8

R = 8/3 = 2.67 ohms

 

 

Or if you're as lazy a sod as I am:

https://www.allaboutcircuits.com/tools/parallel-resistance-calculator/

[Suburban Man]

You may have already discovered that you will get twice the power going into the 4 ohm cab as you get going into the 8 ohm cab. Sonically quite interesting since it’s the 15” speakers that will be working hardest. 

[bremen]

True, but the 8 ohm cab only needs to be 3dB more efficient than the 4 and they'll both be equally loud.

 

And they'll have response variations that might result in better, maybe worse sound.

[Suburban Man]

5 hours ago, bremen said:

True, but the 8 ohm cab only needs to be 3dB more efficient than the 4 and they'll both be equally loud.

 

And they'll have response variations that might result in better, maybe worse sound.

Agreed. I was probably being a little too oblique when I said “sonically interesting“ 🤔 

[prowla]

On 28/03/2025 at 17:37, JJMotown said:

Why don't people post this sort of question in the Amps & Cabs section, where they talk ohms?

Why don't people write Ohms correctly? 🙂

[Bass Of Spades]

At least the sun us out...

[prowla]

1 hour ago, Bass Of Spades said:

At least the sun us out...

...doo do doo-do.