Still not getting the whole "ohms" thing!

[Inazone]

OK, someone cure me of my inability to get the whole "ohm" thing, please! I've got a couple of scenarios here that I'm not grasping.

1) I have a Jackson Apogee 50 tube combo. It has two 8-ohm Celestion V30 12" speakers. I have an 8-ohm Peavey 212M cab (two 12" Sheffields) plugged into the external speaker jack. The amp has a 4/8/16 ohm switch. What is the correct setting?

2) I'm looking at an old Peavey Triumph 60 tube head. I already have a pair of Peavey 212ST cabs, each marked 4 ohms. The Triumph head has three speaker jacks with minimum ratings for each: 4, 8, and 16 ohms. I would think that I could only use one of the 212ST cabs, since they are 4 ohms each, but would it seem logical that I could have cabs of different ratings connected to each output jack, so long as each cab met the minimum for the corresponding jack?

[img]/images/graemlins/help.gif[/img]

[Dreamland_Rebel]

Re: Still not getting the whole \"ohms\" thing!

read the rectifier manuel on boogies site. it has good info on impedence and cab matching.

[siggy14]

Re: Still not getting the whole \"ohms\" thing!

yeah goto www.mesaboogie.com and click on manuals and download the rec manual, will give you a good discription.

[hippietim]

Re: Still not getting the whole \"ohms\" thing!

I believe that Old MacDonald had an Ohm. And on that Ohm he had some resistance. Ee-Eye-Ee-Eye-D'oh.

[metallicarocks]

Re: Still not getting the whole \"ohms\" thing!

I'll try to find my Onstage article and scan it for you, it was really helpful in understanding it.

Remember that it depends on how you connect the speakers. With guitar amps, you probably won't do much daisy chaining or anything like that. If you start combining different speakers in weird configurations, it will start to get interesting.

If you're wiring in series circuits, just add up the omage rating for each speaker. This is not a practical method for wiring many speakers, as the omage gets really low and you waste power.

I'll have to get the article for wiring in parallel. This is a more efficient option, but you'd best know what you're doing. This article was more aimed at setting up a PA, but it is all relevant.

[thurston]

Re: Still not getting the whole \"ohms\" thing!

If the speakers are in series the Impedance will be the sum of all speakers attached. EG: Total Restistance = R1+R2+R3....

If they are wired in Parallel the equation to calculate the total looks like this

1/Total Resistance = 1/R1+1/R2+1/R3
So if you have 2 8 Ohm speakers:

Series: 8 + 8 = 16
Parallel: 1/8 + 1/8 = 2/8 = 1/4 = 4 Ohms total load.

If you know how the jacks are wired, you can use the same equations just use the total cabinent load.

Ideally you want to set your amp to the impedance that matches your total load. This will keep the amp running efficiently. In a pinch you can connect a higher impedance speaker than the amp is rated for (i.e. 16 Ohm cab, amp set at 4 ohm), but you NEVER want to do the opposite (4 Ohm cab, amp set at 16).

[Inazone]

Re: Still not getting the whole \"ohms\" thing!

OK, thanks folks. I'll check out the Mesa thing.

[thebisch]

Re: Still not getting the whole \"ohms\" thing!

[ QUOTE ]
I believe that Old MacDonald had an Ohm. And on that Ohm he had some resistance. Ee-Eye-Ee-Eye-D'oh.

[/ QUOTE ]

Actually, since we are talking the output of an amp, which is AC, it should read "...And on that Ohm he had some impedance..." [img]/images/graemlins/poke.gif[/img]

[metallicarocks]

Re: Still not getting the whole \"ohms\" thing!

Remeber that it's not how much power the amp is putting into the speakers, it's how much power the speakers are sucking out of the amp. The speakers will be okay if you miscalculate the omage (for the most part anyway) but the amp could be a different story.

Remember that less is more. 2 ohms draws much more power than 16 does.

Not to be confused with wattage as many people lose amps this way. Wattage is how much the speakers can hold from the amp (concerning output power). Screw that one up and it will be the speakers that take it in the arse.

On that note remember that output wattage is almost never the same as input wattage. This is important information when you're checking your power requirements for a gig. A marshall amp may put out 100 watts, but it actually draws about 800! Most outlets are rated in amps, not watts. The conversion is easy. Take your watts and divide it by 120, and that tells you how many amps you're drawing. Most household circuits run on a 20 amp breaker. That Marshall that we were talking about earlier draws about 6.6 amps (800/120). So you could get away with plugging three of them into one circuit. Input wattage means that it will draw this much power when the amp is up to full output.

Sorry to sprawl through topics, but one thing always leads to the next when you're messing around with this stuff.

HTH