TalkBass Forums Ohms explained? I'm new here

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#1
07-21-2003, 11:01 AM
 Registered User Join Date: Jul 2003 Location: Columbia, South Carolina
Ohms explained? I'm new here

I am trying to get an explanation of how Ohms work. Really just how to not blow voice coils or overheat amps. It seems like everyone I talk to thinks they understand it all but when they try to explain they can't. Thanks.
#2
07-21-2003, 12:07 PM
 Registered User Join Date: Jan 2003
The following is a gross simplification but then again so is Newtonian physics and it is still useful for most of us ;-) :

The things we really care about are heat=watts=power and ohms.

Almost all watts used end up as heat. Very little is converted into sound energy. Heat is the enemy, it makes things burn up! Cabs in particular have watt=power=heat ratings that refer to how much they can take before something cooks.

About twice as many watts go into a everyday amp as come out. The amp has to have enough cooling to get rid of the difference. The cooling capacity of the amp determines it's maximum power out. Stereo amps are really two separate amps as each side usually has it's own cooling. There is a bit of interaction though, as there is usually one shared power supply and the cooling is somewhat shared, so if only one channel is used it can put out a bit more than what it can if the other channel is also used. The rated ohms of an amp is based on some scary stuff we don't really care about, we just need to know what it is so we can choose cabs that will do what we want.

Ohms is a measure of how agreeable the cab is to taking the available watts from an amp. A cab rated at 8 ohms will only take half the amp's rated power from an amp rated at 4 ohms. Multiple cabs each take their power independantly so two 8 ohm cabs take the same total power out of the amp as one 4 ohm cab (and two 4's = 2) - but again, each only sees half the heat.

Bridging is a way to twice double the agreeability of a cab to taking power. The first doubling comes from each channel of the amp seeing the cab as two cabs, one real and one a reflection created by the two channels fighting each other. The second doubling occurs because there are in fact two channels so the amp ends up seeing four cabs and you end up with in reality the single cab taking four times the power (and heat!) it would if it was plugged into just one channel.
#3
07-21-2003, 12:23 PM
 Registered User Join Date: Oct 2002 Location: Maple Grove, MN
Ohms are the little people inside a speaker. when they get fed they push and pull on your speaker cone from the inside to make sound. They can push really fast to get high notes and really slow to get low notes. (some ohms can push slower and faster than others) Usually there are small families of 4 or large families of 8. They get hungry so you have to feed them with your amp. The amp has to pump juice into your cabinet to feed the little ohms.

Their juice is called watts. The big families have more kids so they need more juice (watts) to feed them. The little families need less juice to make the same amount of noise.

Now when two families live side by side most of them get together and goof off so it seems like there are fewer ohms to feed. so instead of two families of eight needing twice the juice (watts) you only need to feed four of them and they do more work to make up for the twelve that are goofing off.

So I hope this helps. the important thing to remember is that more is less. 8+8 = 4. nuff said.

Now really, ohms measure how much resistance your speaker has (oops impedance really) and when you hook up two in parallel, through a daisy chain on the cab or connect with a Y cable, you get half the impedance. think of it as two paths to get to ground instead of one. So if you had a perfect amp, it would be able to give you twice the power. But it has limits, so usually it is less than twice.

When you hook up more than two 8 ohm speakers together you get less than 4 ohms and some amps don't like this, so they try to send out more current then they are capable of and get hot and eventually blow up. (there are other factors too)

Make sure you don't get too many of those ohm people goofing off.

OTOH, if you connect too big of an amp to your speaker it will put too much power into the speaker and the voice coil will heat up causing it to melt or "blow" (kinda like the transmission in my van did yesterday). Clipping your amp is bad too - the speaker sees it as a really high power signal (hard to explain, but it's like hooking up a huge battery to your speaker - it causes the cone to stop moving momentarily and all that juice drowns the little ohms - then they can't push the speaker).

So, laugh, cry or flame me for being confusing. But don't hook up too many speakers. Ohms have rights too you know.

Who's explanation do you like better?
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#4
07-21-2003, 02:59 PM
 Registered User Join Date: Jul 2003 Location: Columbia, South Carolina
I think between those two explanations I can get by. Thanks for the info, you have helped alot.

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