Ohm math

Mosfed

Ok - can someone help me understand ohm math?  And post a formula?

If I am putting both a 4 ohm and 8 ohm cabs into an amp - what is the ideal ohm setting for the amp that has both 2 and 4 ohm options?

I would guess 4, but?

Paul_C

not sure it's recommended, but it depends on how you're doing it - a 4ohm cab and an 8 ohm cab in series is 12ohms, in parallel it's 2.66.

bertie

MATHS

I now have to shoot you

Mosfed

Paul_C said:

not sure it's recommended, but it depends on how you're doing it - a 4ohm cab and an 8 ohm cab in series is 12ohms, in parallel it's 2.66.

I would do it in parallel.  So you are saying put the SVT on 2 ohms?

Mosfed

Paul - I would really appreciate you posting the formula.  I would love to understand it better.

Paul_C

I'm not saying anything other than doing the sums for you

I'd get two 8ohm cabs and set it on 4

but as bertie's going to kill you I don't suppose it matters.

bertie

Paul_C

http://www.sengpielaudio.com/calculator-paralresist.htm

Mosfed

I guess my yank education is shiiiinin' through!

In series, I get it.  4+8=12

In parallel, I don't. 

Mosfed

GOT IT!  Thanks Paul.

That's the beauty of the internet - you don't really need to understand.

What am I missing?  Why would running 2.66 into 2 be a major issue?  Would it be better to run it into 4?

GuitarMonkey

It's the reciprocal of the sum of each element's reciprocal.

i.e. One divided by (one eighth plus one quarter)

Paul_C

      1
__________  = 2

1/4 + 1/4

       1
__________  = 2.66

1/4 + 1/8

Mosfed

Many thanks.

Paul_C

IIRC, but don't quote me, the difference in the two cabs will mean that you may well find one will be quieter than the other.

Mosfed

Hadn't thought of that.  Very likely indeed.

ICBM

Mosfed said:

Why would running 2.66 into 2 be a major issue?

It's not. Although it's 2 into 2.66, just to be pedantic . (The amp is an *output*, the cab is an *input* and hence the amp goes into the cab and not the other way round.)

You can usually mismatch by up to about a factor of two in either direction with no harm, to a valve amp - and if anything, a low mismatch (cab impedance lower than amp impedance) is less likely to harm a valve amp than a high mismatch. But 2.66 is close enough to 2 that I would use 2 ohms.

(NB, for a solid state amp, never run a low mismatch - you'll probably blow the power transistors.)

Paul_C said:

IIRC, but don't quote me, the difference in the two cabs will mean that you may well find one will be quieter than the other.

But not by very much. In some cases with different cabs it can actually improve the balance, if the less sensitive cab is then given more of the power. The power division is 1/3 to the 8-ohm cab and 2/3 to the 4-ohm cab, so make sure the 4-ohm cab can handle at least 2/3 of the total power of the amp, not half.