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Post subject: Stage 100 DSP head wiring question
Posted: Sat Jan 24, 2009 6:55 pm
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This may seem like a pretty easy questions or somewhat childish, but never the less. i have a stage 100 DSP head that i am working on making a new cabinet for. when i sat down and really thought about the wiring aspect i came to only one question i could not answer. hopefully you guys can help me out.

I have 2 12" 8 ohm speakers, running them in a twin cab in parallel would make the over all load 4 ohms, which the head can do, with both 1/4" cables plugged in. Now, if i were to wire the speakers up in parallel, and have only 1 1/4" jack for both speakers, could i use a 1/4" Y cable to connect the head to the cab. in theory this seems to work to me, but maybe im missing something.

Any input on this would be fantastic.

Thanks in advance,
Jay


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Posted: Sat Jan 24, 2009 11:31 pm
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yes you can, the wiring is still in parallel, the speakers just happen to "join-up" at the "Y". if you do it this way, do NOT try to wire your speakers in parallel in the cabinet. Why? because you might short "hot" to "ground" and that would blow your amp's output stage. in any case, be carefull and double check with ohm meter before you plug into amp and power up. you must have a reading of several ohms across jack (the part that is going to plug into the amp). If ohms are zero, you have a miss wire - a short. so check before you plug in and power up.

johnny


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Posted: Sun Jan 25, 2009 1:52 pm
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ahhh, this is were im having my problem, i was going to wire the cab in parallel to make the over all load requirement to be 4 ohm. if i wire the cab in in any other fashion it will make the over all load 16 ohm which my head doesn't support.

So, how would i wire 2 12" 8 ohm speakers with one shared 1/4" connector, and still use the 4 ohm option with the head by using both 1/4" outs on the back?

My brain still tells me that a 2x1 1/4" Y should work?


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Posted: Sun Jan 25, 2009 6:43 pm
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"ahhh, this is were im having my problem, i was going to wire the cab in parallel to make the over all load requirement to be 4 ohm."

Yes you can do that. The cab load will then be 4 ohms only if you wire 8 ohm speakers in parallel. If you wire 8 ohm speakers in series, that will be 16 ohms. You can check what you did with a ohm meter. You will need an ohm meter to be sure unless you are carefull and are technically savy. I always check - you never know if you have a blown or damaged speaker.

"if i wire the cab in in any other fashion it will make the over all load 16 ohm which my head doesn't support. "

There are only two ways to wire 2 speakers: series or parallel (assuming you don't open circuit or short circuit wire the cab - this happens so check).

So, how would i wire 2 12" 8 ohm speakers with one shared 1/4" connector, and still use the 4 ohm option with the head by using both 1/4" outs on the back?

Well, here's the rub: if you parallel wire two 8 ohm loads, the overall load will be 4 ohms (two 8 ohm cabinets in parallel will be an overall 4 ohms). To get a cab to be 8 ohms overall, you will need to wire two 16 ohm speakers in parallel in one cabinet to get 8 ohms overall, same recipe for the other cabinet, and two 8 ohm cabinets wired in parallel will result in a 4 ohm overall load.

If you wire two 8 ohm speakers, you will get a 4 ohm cabinet. If you then wire a 4 ohm cabinent to an 8 ohm cabinet in parallel, you will get a 2.6 ohm load on the amp. That will strain the amp, possibly overload and burn out the output stage transistors.

If you wire two 8 ohm speaker in series to make a 16 ohm cabinet and then parallel wire a 16 ohm cab with a 8 ohm cab, the overall load will be 5.33 ohms.

Unlike tube amps, transistor amps will blow when the load shorts out. Unlike transistor amps, tube amps blow when the load goes open circuit. In a live situation, speakers tend to burn and go open circuit. In a wiring situation where you don't check, short circuits happen. There is a way to prevent this for both tube and transistor amps, but no manufacturer does it because it adds a lot of weight and expense. It's assumed common sense prevails!


"My brain still tells me that a 2x1 1/4" Y should work?"
The formula for loads in parallel is: 1/R_1 + 1/R_2 +....=1/R_load. In your case:
case 1: 1/4 + 1/8 = 1/2.66, the load is 2.66 ohms
case 2: 1/8 + 1/8 = 1/4, the load is 4 ohms
case 3: 1/8 + 1/16 = 1/5.33, the load is 5.33 ohms.

If you try to wire-up one 4-ohm cab with a 8-ohm cab in parallel, you risk blowing the amp because overall load will be 2.66ohms. If you wire one 8-ohm cab with another 8-ohm cab, you will get a 4-ohm load, the amp should be able to take that. If you wire a 8 ohm cab with a 16 ohm cab, overall load will be 5.33 ohms, the amp can take this load as well.

No matter what you do, always check final load to amp with an ohm meter.

ciao & good luck,
johnny.


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Posted: Sun Jan 25, 2009 7:39 pm
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this is where i wish i could do a video responce, since it sounds like everyone is telling me what i already know. so ill go over this one more time with all sorts of adj.

The head i have supports 8 and 4 ohm, to make the head 4 ohm you must plug in both 1/4 inch jacks. if you just plug in one, you get 8 ohm.
So, i have 2 12" speakers that if i wire (in cab) to parallel, making the load on the 1/4 inch jack for the cab 4 ohm. correct thus far? So i would have a 2 12" cab, wired for 4 ohm?

then taking a 2x1 Y 1/4 Inch cable two jacks into the head (making the head work in 4ohm), one going to the 4 ohm cab, that should work, yes?


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