Impedance : the basics, please !

[Jonath]

Hi you all warwicked Warwickers,

What should I take care of, when I have a 400W/4 Ohms head, if I want to buy a cab ? Is it dangerous to have, for instance, a 8 Ohms cab which power is greater than 200W ? In which way shall I calculate that ?

What are the risks ? Are marks compatible ?

Thx to all, CU around !
Jonathan

 

[Benjamin]

Not dangerous at all.
A head that runs 400w at 4 ohms will put out 400w only if the total impendance of the cabs reaches 4 ohms. You can get this with a single 4 ohm cab or two 8 ohm cabs.
Say you have a head that runs 1000 watts at 2 ohms. 800 watts at 4 ohms and 500 watts at 8 ohms. You get a single 8 ohm cabinet for it - then you're pulling maximum 500 watts from the head.
Later on you get a second 8 ohm cab for your rig. You're now pulling 800 watts from your head.
If you wanna upgrade from here, you've got two basic options to reach 2 ohms. You can either get two more 8 ohm cabinets, or one more 4 ohm cabinet. - Single 4 ohm cabinets are louder than (Same model) 8 ohm cabinets.
Hope this helps, first time ever I try help out with technical stuff

 

[Benjamin]

Oh, here's the fancy pants smart guy explanation

And, I'm very confused about the whole Ohm deal with the cabs and heads. Could anyone put me in on this?

iirc, cabs are run in parallel. The Ohm rule, when you have two 'resistors' R1 and R2, in parrallel, the overall resistance is (R1 * R2) / (R1 + R2).

EDIT : To be more accurate, 1 / R = 1 / R(1) + 1 / R(2) + .... 1 / R(n) for n cabs.

So in the end, with two 4 ohms, you run your output through 2 ohms. That in turns, draws more current (think shortcuts, the lower the resistance, the closer to a short circuit you get).

So, the lower the resistance, the higher the stress on the head. Hence some heads having the ability to drive cabs with a 2 ohms combined cab resistance, or 4 ohms, or 8 ohms. and The lower the cabs' resitance, the better the signal to noise ratio in general.

... Did I pass?

Welcome to the forum, Jonah!

 

[Jonath]

Hey thank you Ben, that are great stuff, that you gave me there !

And is there any trick to calculate the power @ a given impedance ?

 

[Benjamin]

Not that I know of

 

[Callum]

Hey thank you Ben, that are great stuff, that you gave me there !

And is there any trick to calculate the power @ a given impedance ?

I guess you could apply circuit theory to it..but this adds more variables and tbh its not worth working out...

Its about 30% loss in power ie a 500 watt 4 ohm head plugged into a 8 ohm cab will put out about 300watt ish....give or take.

 

[Florin]

Hey Jonath, welcome to the forum man
Please check this topic

 

[Vinnie]

That thread.... showed me how much I've learned from this forum. :lol:

 

[Gusts]

Hi,

Florin's post pretty much explains it all

try this website: www.tunemybass.com look for bass amp setup guide or smth. like that.
good and simple

peace!

=g=