Sorry, need some ohms help

[Misanthrope189]

I have a dilemma. I have the option of getting a cab with 8 ohm or 4 ohm speakers. Since I have a stereo power amp, if I get the 8 ohm 412 in stereo, that makes it a 4 ohm load, correct? If I bought a second 8ohm 412 in stereo, trying to combine that with the first 412 would make it a 2 ohm load, which is impossible on my poweramp. My poweramp has 2 8 ohm jacks, and 4, 4 ohm jacks. If I bought an 8ohm 412 in stereo, and ran that as a 4 ohm load...could I buy a second 412 as an 8ohm mono cab, and then run one cabinet in stereo--using the two 4ohm jacks on one side of the poweramp, and then run the mono 8ohm cabinet as a 4 ohm load, using one 4 ohm jack on the other side? This confused the hell out of me trying to think if it worked...so I'm hoping to employ some of your expertise on the issue.

Does this make sense with ohms?

Cheers,
Nick

[MountainDog]

First off, using one stereo 412 cabinet with a 4 ohm load on each side (which I think you are saying) and then one mono 412 w/8 ohm load, may work with the power amp, but the 8 ohm load cabinet will not be as loud as the two 4 ohm sections of the first cabinet.

Help me figure out your options:
First the stereo power amp. You say each side has one 8 ohm output jack and two 4 ohm jacks, yet you say your amp won't go to 2 ohms. If you were to plug a 4 ohm load into each of the 4 ohm jacks on one side of the power amp, then that side is indeed working at 2 amps. I am wondering if the power amp really means you can plug a 4 ohm load in the 4 ohm jack. To me it doesn't make sense to have 1 jack accepting an 8 ohm load and 2 jacks accepting 4 ohm loads. What I'm thinking it might mean is if you have a single 8 ohm cab, plug into the single 8 ohm jack. If you have two 8 ohm cabs, then plug one into the first 4 ohm jack, the second cab into the other 4 ohm jack. Plugging two 8 ohm cabinets into parallel jacks on an amp would equal a 4 ohm load. You mention the power amp not going to 2 ohms, to my sceanario makes sense. Do you have a picture of these jacks? or what power amp is it? Is there an internet link to the manual?

Second, let me go into cabinet options. You said you can get a stereo cab with either 8 ohm speakers or 4 ohm speakers.
If you go with 8 ohm speakers, each side of the cabinet can be wired as two 8 ohm speakers in parallel wiring (giving 4 ohm load) or wired as two 8 ohm speakers in series wiring (giving 16 ohm load).
If you go with 4 ohm speakers, each side of the cabinet can be wired as two 4 ohm speakers in parallel wiring (giving 2 ohm load) or wired as two 4 ohm speakers in series wiring (giving 8 ohm load).

If it turns out your power amp outputs really mean either one 8 ohm load into the 8 ohm jack or two 8 ohm loads into the two jacks labeled 4 ohms, then getting the cabinet with 4 ohm speakers and having the two on each side wired in series to give you 8 ohm load per side makes the most sense. You can run just one stereo cab, left side of cab into the power amp left side 8 ohm jack, right side of cab into the power amp right side 8 ohm jack. Then you can add second identical cabinet. one cabinet left side going to 1st 4ohm jack of left side of amp, right side of that cabinet going to the 2nd 4 ohm jack of the left side of amp, and doing similar with the 2nd cabinet. (or if you want to stack the cabinets, have the left side of each cabinet go to the left side set of 4 ohm jacks and the right side of each cabinet going to the right side set of 4 ohm jacks).

So on the above, I am trying to guess on your set up by guessing at what the jacks on the amp really signify and assuming you would run each cabinet in stereo, ie, input jack on the cabinet for the speaker pair on left and input jack on cabinet for the speaker pair on right.

Let me know if you understand any of this!
Oh, and I didn't stay at a Holiday Inn Express last night...I really work for a speaker company

[Misanthrope189]

Well...my power amp is a Mesa 50/50, which might soon be a Mesa 2:90. So you're saying, if I use all 4 4ohm inputs, that equals a 2ohm load? And if I understand you, running a stereo cab is the parallel wiring, with two jacks on the back of the cab: one going to the two left speakers, and one going to the two right. That would make sense, because right now, with 4 16ohm V30s in my cabinet, I run it as an 8ohm stereo cabinet, so that sounds like parallel wiring.

I also thought that if you put two stereo cabinets together that ran parallel with 8 ohm loads, you'd have a total load of 4 ohms, and you'd plug the two cabinets into the 4 ohm slots. My worry is that if I replace the V30s with 8ohm speakers, in stereo the cabinet will already run at 4ohms. So trying to put a second stereo cabinet with it would create a 2ohm load, which I didn't think made sense with my power amp.

Cheers,
Nick

[MountainDog]

Thanks for the Mesa 50-50 info.
First off, when I said if an amp had two 4 ohm labeled jacks next to each other and they really meant that you could put a 4 ohm load on each, since jacks next to each other like that are usually wired in parallel, putting a 4 ohm load on each would mean the amp would see 2 ohms.

HOWEVER, in reading the Mesa 50-50 manual, I guessed completely correct when I said it didn't make sense that the two jacks labeled 4 ohms would really accept a 4 ohm load. My statement "If you have two 8 ohm cabs, then plug one into the first 4 ohm jack, the second cab into the other 4 ohm jack" was correct. Let me quote from Mesa 50-50 manual: "When using a single 8 ohm speaker or 8 ohm cabinet on a channel, connect the speaker or cabinet to the 8 ohm jack. If using two 8 ohm speakers (assuming they are wired in parallel) or cabinets on a channel, plug them both into the 4 ohm jacks. (The logic here is that two 8 ohm loads connected in parallel add up to a 4 ohm load.)"

http://www.mesaboogie.com/manuals/50-50%20Stereo.pdf

So each side of your Stereo 412 cabinets NEED TO BE 8 ohms. Your current cabinet with four 16 ohm speakers will give you 8 ohms on the two speakers on the right side when wired in parallel and 8 ohms on the two speakers on the left side when wired in parallel.

You are correct when you say "I also thought that if you put two stereo cabinets together that ran parallel with 8 ohm loads, you'd have a total load of 4 ohms, and you'd plug the two cabinets into the 4 ohm slots"

You have one cabinet in your possession already with perfect 8 ohm per side loads, so to work with the Mesa, you need the second cabinet to be the same 8 ohm load per side. You should duplicate your first cabinet, ie, go for four 16 ohm V30 drivers. (As mentioned above, you can wire up a four 4 ohm speaker cabinet as series between two speakers to give you 8 ohms per side, but I wouldn't mix one cabinet with four 16 ohm speakers wired as 8 ohm stereo and 2nd cabinet with four 4 ohm speakers wired as 8 ohm stereo as the reactance of each set would be different and might be sonically different sounding to your ears).

If you run just one cabinet, then the 8 ohm load of one side of cabinet goes into the 8 ohm jack on one side of amp, and same with other side. 2 cables needed. When adding a 2nd cabinet, then you have 4 cables, which should all go into the four 4 ohm jacks on the amp.

There ya have it!

[Misanthrope189]

That all makes sense But, my V30s are blown, and I was going to replace them with something else, but the speakers I'm looking at only have the option of 8ohm speakers or 4 ohm speakers. I don't know how to deal with this in order to allow for the possibility of two cabinets.

Thanks for the very clear and very helpful info!

Cheers,
Nick

[MountainDog]

Ahhh...I was wondering why your original email mentioned only 8 ohm and 4 ohm speakers as a purchase option.

As I mentioned in my first post, if you got the 8 ohm speakers and wired in stereo, you'd have a choice of parallel wiring of two to give you 4 ohm load or series wiring to give you 16 ohm load, but these options won't fit your needs.

Now from all we've learned, you need an 8 ohm load per side of speakers, so you will have to purchase the 4 ohm speaker option and wire each side of the stereo cabinet (two speakers) as series wiring to give you 8 ohms per side.
Here's what series wiring looks like:

http://www.usspeaker.com/images/speaker-wiring1-size255.gif

So get the cabinet you were speaking of with the 4 ohm speakers and purchase four of the 4 ohm speakers to replace your blown V30s. I'm assuming the new cabinet is Stereo? Then verify if the new cabinet is wired up as series for the 8 ohm load per side you need. If its not, resolder the wires so it is. Then install the four 4 ohm speakers into your old cabinet and wire up as series. Then get 4 cables, plug them all into the four jacks labeled 4 ohms and you are good to go!

[Misanthrope189]

Thanks so much for your help!

Cheers,
Nick