I notice a lot of discussion about ohms, etc. Well, I've worked in sound reinforcement quite a bit, so I'll give you some helpful formulas here, and you can E-mail me your questions ( NOSPAMbryanjs@bellsouth.net ).

I'll try to update it regularly, I'll cover impedance, and all that jazz, but also some more in depth issues, like slew rate, bridging, impedance curves, crosstalk, intermodulation, pink noise, etc. I have 3 electrical engineers and myself to answer your questions and whatnot...

Let me start by explaining, that very few people have posted a good formula for impedance. Impedance is a complex form of resistance. Resistance is measured in ohms, named after Ohm's law, which not surprisingly, was discovered by a man named Georg Simon Ohm (1789-1854), a German from Bavaria. Ohm published his book Die galvanische Kette, mathematisch bearbeitet in 1827, which contained the law.

Ok, so that all sounds dandy, but when it comes to ohms, practically speaking, we are interested in “Nominal Impedance,� which is important, for this is the load the amp “sees.� Nominal impedance is roughly the lowest impedance within the speaker’s frequency response (this is open to debate, one company’s 8 ohm speaker is another’s 4)

8-ohm speaker
[IMG] http://www.theloudspeakerkit.com/ima..._impedance.gif [/IMG]

Read:
http://www.gamry.com/App_Notes/EIS_P...EIS_Primer.htm

All you need to know are the following equations:

Z=Impedance in ohms

Z = 1/[(1/Z)+(1/Z) etc.]

So if we had 2 8-ohm speakers:

Z = 1/[(1/8)+(1/8)]
Z = 1/(1/4)
Z = 4 ohms

Or say, a 16-ohm, an 8-ohm, and a 6-ohm speaker:

Z = 1/[(1/16)+(1/8)+(1/6)]
Z = 1/[(1/16)+(2/16)+(1/6)]
Z = 1/[(3/48)+(6/48)+(8/48)]
Z = 1/(17/48)
Z = 1/.34516
Z = 2.8972070923629621045312318924557 ohms…
Parallel involves connecting the amp’s + terminal to the speakers’ + terminals, and the amp’s – terminal to the speakers’ – terminals.

Series involves connecting the amp’s + terminal to the speaker’s + terminal, the speaker’s – terminal to the next speaker’s + terminal, and that speakers – terminal to the amp’s – terminal. The equation for this is simple:

Z = Z1+Z2+Z3 etc.

So 2 4-ohm speakers in series become 8 ohms, and a 6, 4, 8 and 16 in series become 34 ohms.

More to come later.

Some rules of impedance and amps:

1. A "Multimeter" will not give you an accurate reading on speaker impedance, never ever use one to measure speaker impedance, because they measure resistance, not impedance.

2. Never run 2 mismatched loads on an amp, like a 4 and 8 ohm cab, even if it can handle 2 ohms. This results in a mismatch in power given to each cab. You will be pushing the lower impedance cab harder.

3. Never run a large number of speakers or cabs in series. Series is rarely used in sound reinforcement for anything more than 2 loads. The reason is if one speaker or cab fails, then the whole chain will fail.

4. Never, ever, ever, ever touch the inside of a tube amp when it's on, even after it's off. Tube amps can hold charge for long periods of time. If you need to touch the inside of a tube amp soon after it's off, do 2 things, make sure the tubes have cooled down, and always keep 1 hand behind your back (this will keep you from creating a circut through your heart, and possibly killing you!) When in doubt, take it to a professional!