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post #1 of 7 (permalink) Old 10-18-2010, 01:12 PM Thread Starter
 
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which ohm setting to use?

I've got a simple (and most likely stupid) question.

My head is has a plug on the back for 4 ohm speakers and 1 for 8 ohm speakers.

my cab is a 2x12 with 4 ohm speakers.

Im thinking I gotta run from the 4 ohm plug on the head, but I just want to be sure.
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post #2 of 7 (permalink) Old 10-18-2010, 04:14 PM
 
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Re: which ohm setting to use?

Yes, it is pretty obvious isn't it?

You have a 4ohm cab, plug it in the 4ohm plug. If in doubt consult your amps manual.
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post #3 of 7 (permalink) Old 10-18-2010, 05:33 PM
 
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Re: which ohm setting to use?

It's not a stupid question at all...

If the cab is clearly labeled as a 4 ohm cabinet then easy enough - connect it to the 4 ohm plug on your amp...but if the cabinet isn't labeled and you can see that the speakers are labeled as 4 ohm speakers then they are likely wired in series, which would create 8 ohms of resistance and you could use the 8 ohm plug.

Here is my understanding of speaker wiring and matching impedance: two 4 ohm speakers wired in series (likely in this scenario) would create 8 ohms of resistance but two 4 ohm speakers wired in parallel (less likely in this scenario) would create less than 4 ohms of resistance. A saying that helps to remember is "low into high will fly but high into low won't go" meaning that you could run a 4 ohm head into an 8 ohm cab without any problems but running a 8 ohm head into a 4 ohm cab would be inviting problems. So in your case if your aren't sure of the wiring and resistance of your speaker cab then the 4 ohm setting will likely be the safer option but if you can confirm that the two 4 ohm speakers are wired in series then I would use the 8 ohm plug.

The safest thing to do is to check with a voltage meter (most voltage meters have an ohm setting to measure resistance). The voltage meter likely won't read exactly 4 or 8 ohms so check the reading and go with the closest lower ohm setting on your amp.

This is all based on my understanding of speaker wiring & impedance but perhaps someone else can chime in and confirm the info in my post to give you some added piece of mind...
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post #4 of 7 (permalink) Old 10-18-2010, 05:41 PM Thread Starter
 
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Re: which ohm setting to use?

yeah, its two 4 ohm speakers in wired in series, thats where my apprehension came from
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post #5 of 7 (permalink) Old 10-18-2010, 05:53 PM
 
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Re: which ohm setting to use?

Quote:
Originally Posted by RI_RGplayer View Post
yeah, its two 4 ohm speakers in wired in series, thats where my apprehension came from
If you know that the speakers are wired in series then my understanding is that they should create 8 ohms of resistance and I would go with the 8 ohm setting...

BUT if you are unsure and/or uncomfortable going with the 8 ohm plug then go with the 4 ohm plug because you are safer running low into high than you are running high into low...

I made an error with impedance matching once and it fried the tubes in my amp (they actually lit up like christmas lights before frying) and I'm lucky I didn't do more damage than that and catch my amp on fire...

maybe someone else can confirm to give you some added piece of mind...I know I always like to get a couple of opinioins before I do something I'm unsure of ;o)
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post #6 of 7 (permalink) Old 10-18-2010, 07:33 PM
 
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Re: which ohm setting to use?

Sorry I mis-read your post, I thought you were saying the cab said 4 ohms. Something should be listed on the back of cab telling you what it should be plugged in to though.
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post #7 of 7 (permalink) Old 10-22-2010, 01:50 AM
 
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Re: which ohm setting to use?

Quote:
Originally Posted by doriangrey View Post
If you know that the speakers are wired in series then my understanding is that they should create 8 ohms of resistance and I would go with the 8 ohm setting...

BUT if you are unsure and/or uncomfortable going with the 8 ohm plug then go with the 4 ohm plug because you are safer running low into high than you are running high into low...

I made an error with impedance matching once and it fried the tubes in my amp (they actually lit up like christmas lights before frying) and I'm lucky I didn't do more damage than that and catch my amp on fire...

maybe someone else can confirm to give you some added piece of mind...I know I always like to get a couple of opinioins before I do something I'm unsure of ;o)
doriangray is correct. Just to help clarify whether your speakers are setup series or parallel, look at how they are wired. For series, they would be wired from each speaker's positive to the other's negative. If they are wired from positive to positive and negative to negative, they are parallel. Just a reminder that if the speakers are mounted with the terminals facing each other (set to the middle of the cabinet) the wires would cross if they were wired in parallel, go straight across from terminal to terminal if in series.

Check out Celestion's website, click on Ask Dr. Decibel, select Speaker Wiring Diagrams under Dr. Decibels secrets and it will show you the different options for wiring.
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