ohm question

[hellrider77]

o.k. so as most of you know i am electronically challenged. i have a question about ohms on amps and speaker cabinets. my crate excaliber head says on the back of the amp, "90 watts rms at 2 ohms, 2 ohms minimum" what will happen if i run it through an 8 ohm cabinet or a 16 ohm cabinet. also if someone doesn't mind, could you explain the differences in ohms. i'm completely lost and dont want to screw up my amp or cab.

 

[Quencho092]

Re: ohm question

The amp likes to see 2 ohms at the cab for max efficiency-90 watts rms.

Rule of thumb with ohms-try to never go more than double up or half down. For 2 ohm amps, don't go any lower in impedence, but try not to go above 4.

When you have multiple speakers or cabinets and you want to achieve a lower impedence (your case, you have 8 ohms and 16 ohm cabs), pair them up in parallel.

Both of your cabs will equal 5.3 ohms when paired together in parallel (both plugged into the amp's speaker jacks directly.) which is fairly safe for a 2 ohm amp, but I'd try to stick to 2-4 ohms.

How many speakers are there in the cabinets you just mentioned, how many ohms for each of the speakers? Depending on how many speakers there are in the cabs you mentioned, you could re-wire them to get closer to your amp's optimal ohm preference and run it safer.

DONT RUN THE AMP THROUGH THE 16 ohm cab by itself!!!!! It could fry the output transformer when you're that off.

 

[hellrider77]

Re: ohm question

i was planning on buying kacs carvin 2x12 cab and its rated at 8 ohms. if i buy it what should i do to it? and how much money would i be looking at to have it changed to 2 or 4 and what would be the best? also, if i run it at 8 ohms, what could happen?

 

[Quencho092]

Re: ohm question

Each speaker is probably 16 ohms, both of them in parallel to give you 8 total. You couldn't get it lower than 8 ohms unless you change speakers.

If you run it at 8 ohms, the amp will seem to have alot less volume, and the output transformer will be taking a bit of a beating. If it were a 4 ohm amp this wouldn't be a problem, as it's still within range of the 'rule of thumb'(double up or half down), but once you have to quadruple impedence ratings for the speaker load, things won't be running close to 100%.

It seems like a pretty solid cab, if you want to buy it but want to run it so that your amp is getting full efficiency or at least 4 ohms so it's close to your rating, go for new speakers. I'd recommend a set of 8 ohm Texas Heats by Eminence-great speakers for a cross between american and british tones, 150 watts each of power handling, and they could be had for well under 70 bucks each. At 4 ohms total, you could still throw your 16 ohm cab in for 3.2 ohms which is perfect.

If you have that 16 ohm cab, you could run it with the 8 ohm carvin cab stock and you'll be safe.

 

[hellrider77]

Re: ohm question

nah, i don't have a cab right now. thats why im so desperate to find one. to tell the truth i don't think i've even seen a 2 ohm cab. i would be happy to find a 4 ohm though. is the excaliber just a bastard head? also, if i ran a 4x12 with 8 ohm speakers would that make it a 2 ohm cab? the more i learn about electrical stuff the more it confuses me. lol

 

[Mr 9finger]

Re: ohm question

You have been learned in the yahoo school of electronics, now carry on....

 

[hellrider77]

Re: ohm question

ok i got it figured out thanks to 9. the head is meant to run 2 cabs period. if i run a 4x12 cab. i will have to have 2 jacks inputs and with 16ohm speakers i will run 2 to each jack in parrallel. this will be 8 ohms each, which after both sides are plugged in it will cut the total impedence down to 4 ohms. sound right? if i run 8 ohm speakers it will cut it down to 2 ohms.

 

[Pink Unicorn Horsey]

Re: ohm question

9finger probably explained this to you off the forum since I don't see his post, but here's how you figure out equivalent resistance for multiple loads:

Assume you've got two speakers, R1 and R2. If you wire them in series the equivalent resistance is R1 + R2. If you wire them in parallel the equivalent resistance is (R1 * R2)/(R1 + R2).

As an example, consider two speakers with a resistance of 8 ohms each. Wiring them in series gives an equivalent resistance of 8 ohms + 8 ohms = 16 ohms; wiring them in parallel gives an equivalent resistance of (8 ohms * 8 ohms)/(8 ohms + 8 ohms) = 4 ohms.

When R1 and R2 are the same value (i.e., R1 = R2), placing them in parallel makes the formula much simpler: R1 / 2. I don't know of any cases in which speakers of different load values are mixed when loading amplifiers, so it's easier just to think of wiring two speakers in series as doubling the load and wiring two speakers in parallel as cutting the load in half.

Make sense?

- Keith

 

[JacksonMIA]

Re: ohm question

If you wire them in parallel the equivalent resistance is (R1 * R2)/(R1 + R2).

Just keep in mind that this only works with two speakers. It gets a bit more complicated with three or more.

 

[Pink Unicorn Horsey]

Re: ohm question

Just keep in mind that this only works with two speakers. It gets a bit more complicated with three or more.

Not really. If you've got three speakers -- R1, R2 and R3 -- do the formula for R1 and R2 and make that result R4. Then you've got R3 and R4 and you can use the formula the same way. It's always a matter of reducing group loads into equivalent resistances.

 

[ErikH]

Re: ohm question

This amp is all solid state. The same rules apply BUT, you can run a 16 ohm cab with it. That note on the back says you'll get the full 90 watts of power by using a 2 ohm cab, but you don't have to. "as low as" isn't the same as "must be run at".

This is straight from the manual:

21. SPEAKERS: Connects the amp to
your speaker cabinet(s). Uses speaker
cables with mono 1/4” plugs. These
jacks are wired in parallel. Make sure that
your speaker combination never totals
less than 2 ohms! Use the chart below to
help determine parallel loads. If in doubt,
ask your dealer.

Here's a link to it. Save the manual or print it off.
http://www.crateamps.com/product/manuals/GX900H_2200H.pdf

Since it is solid state, I don't think you'll hurt your transformer at all. You should be able to run a 8 ohm cab without a problem.

Now if it were tubes, then that's a different story. Things get a little more strict there.

 

[aleclee]

Re: ohm question

This amp is all solid state. The same rules apply BUT, you can run a 16 ohm cab with it. That note on the back says you'll get the full 90 watts of power by using a 2 ohm cab, but you don't have to. "as low as" isn't the same as "must be run at".

Since it is solid state, I don't think you'll hurt your transformer at all. You should be able to run a 8 ohm cab without a problem.

Now if it were tubes, then that's a different story. Things get a little more strict there.

+1. With the type of impedance mismatch we're talking about here, the only "problem" is reduced power.

SS amps don't have output transformers and, if they did, there wouldn't be the issues with flyback voltage that causes problems in tube power sections.

 

[Kac]

Re: ohm question

nah, i don't have a cab right now. thats why im so desperate to find one. to tell the truth i don't think i've even seen a 2 ohm cab. i would be happy to find a 4 ohm though.

My Mesa vertical 2X12" cab is 4 ohms, find yourself something like that, cuz it is absolutly awesome!

Rock On! :headbang:

 

[ErikH]

Re: ohm question

One other thing. The speaker jacks are in parellel on that amp so if you have two 8 ohm cabs, that's a 4 ohm load, two 16 ohm cabs = 8 ohm load, two 4 ohm cabs = 2 ohm load (hey, the amp minimum). Basically, if you have two cabs, take the total impedeance of both cabs and divide it in half.

 

[JacksonMIA]

Re: ohm question

Not really. If you've got three speakers -- R1, R2 and R3 -- do the formula for R1 and R2 and make that result R4. Then you've got R3 and R4 and you can use the formula the same way. It's always a matter of reducing group loads into equivalent resistances.

True, but the more loads you add, the more equations you'll have to do. I'd rather just group them all together and use one. I guess it's all just personal preference.

 

[hellrider77]

Re: ohm question

so is the equation i mentioned earlier right?

a 4x12 cab with 16 ohm speakers, 2 jack imputs for the cab, 2 speakers ran parralell to each jack. when both jacks hooked up, it would be running at 4 ohms?

also, if i was to run a 16 ohm cab with the model head that it is, would i run the risk of burning up the power transformer like quencho said?

 

[Pink Unicorn Horsey]

Re: ohm question

Well if you assume all the speakers have the same load rating (R) the formula boils down to R / n, where n = the number of speakers wired in parallel. If your load ratings and wiring schema vary you have to either break down the circuit piecewise or create one larger formula.

Either way works. It's six of one, sqrt. 36 of the other.

 

[Pink Unicorn Horsey]

Re: ohm question

so is the equation i mentioned earlier right?

a 4x12 cab with 16 ohm speakers, 2 jack imputs for the cab, 2 speakers ran parralell to each jack. when both jacks hooked up, it would be running at 4 ohms?

also, if i was to run a 16 ohm cab with the model head that it is, would i run the risk of burning up the power transformer like quencho said?

If the cabs are 16 ohms each and the jacks are wired in parallel as well (which I'm guessing they are) then yes, the equivalent load is 4 ohms:

R = 16
n = 4

16 / 4 = 4 ohms

 

[JacksonMIA]

Re: ohm question

Either way works. It's six of one, sqrt. 36 of the other.

Yeah, and how many times do you come across three or more non-equivalent loads wired all in parallel?

Hellrider, I think you'll be ok, but you'll probably want to eventually get cabs that match the impedance, so you can get the most out of your amp.

 

[ErikH]

Re: ohm question

so is the equation i mentioned earlier right?

a 4x12 cab with 16 ohm speakers, 2 jack imputs for the cab, 2 speakers ran parralell to each jack. when both jacks hooked up, it would be running at 4 ohms?

also, if i was to run a 16 ohm cab with the model head that it is, would i run the risk of burning up the power transformer like quencho said?

As much as this info is helpful, your amp has no output transformer, it is solid state so you can use a 16 ohm cab and be fine. That @ 2 ohms is "minimum". You can use a higher impedeance cab with no problems.

And yes, using a 4X12 like you outlined above will make a 4 ohm load, as long as both pairs of speakers are hooked up. If only one pair is hooked up, then the amp sees an 8 ohm load.