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> Help ... Ohm, Impendance, "As My Brain Gently Bleeds"
post Mar 6 2009, 11:11 PM
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Hey Guys I recently bought a Crate Flexwave 120 half-stack problem is the had to go to repair before I get to bring it home haha ... so I have a FW412A cab and they lent me a spider III HD75 until they fix the head. Now I only have a little small corolla to get my drummers place each weekend so I don't want to bring the freaking cab each saturday ... I bought a much cheaper crate cab as a spare I'll leave at his place the problem is ... It's not as ohm versatile friendly as the flexwave cab ...

The second cab is a G412ST and I would really appreciate if anyone can help me out as to how I should plug the Spider III with the 4 ohm speaker outlet to the Impendance: 8 ohm on the G412ST ... I usually read the Owner's Manual thoroughly ... but it's a old product and they don't have the manual online ... So If anyone can tell me how to connect the head to the G412ST I would really appreciate it.

Haha I'm doing my Electricity & Magnetism class right now and we are doing exactly this ... so if somebody explains how to practically put it together it be great. I understand the theory ... just don't know what to use between the head and the cab ... I can't touch the resistance on the head since it's borrowed ... and I don't really want to void any warranty so if a transformer is a possible alternative it'd be great to know what I'm talking about haha.

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post Mar 6 2009, 11:35 PM
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You got a head 4 Ohm. And a cabinet at 8 Ohm.

For these occacions I think of Andrew's 1'st impedance rule that says "Speak to high and your head will fry"
Meaning in this case that if the cabinet you connect has an impedance higher than the head impedance, things could cook.

So with reference to Andrews first impedance rule I'd say your idea is nothing to recommend.

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Ivan Milenkovic
post Mar 6 2009, 11:51 PM
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I'm afraid it cannot be done mate. You can possibly try to alter the speakers wiring inside the cab, so you connect them in different way so you match the speakers impendance with the head. If you want to do some soldering, tell use what speakers are in the cab, and what is the labeled impendance on them, and we will come up with the wiring.

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Bogdan Radovic
post Mar 7 2009, 12:16 AM
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Hmm I thought you could connect higher impedance cabs to lower heads no problem but not vice versa... ???

I have 4 ohm bass "head in a combo" and inside is the 8ohm speaker...Since its 8ohm speaker , the head is not working on its 250watts and 4 ohm but lower something like 170watts.In manual it says that the extension speaker out is wired in series so when I add more cabs impedance goes even higher to 16ohm etc...They wired it in series because of safety reasons (higher impedance can't hurt the amp they say)...From what I found by research online when you connect higher impedance speaker (8ohm) to 4 ohm head , power drops but amp runs cooler.

This rule is only for straight HEAD to CAB CONNECTION!

If you are connection multiple cabs or have a combo amp you need to properly match impedance depending on internal wiring of extension speaker jack in order not to hit lower impedance (bellow 4ohm) which amp can't handle!

This post has been edited by Bogdan Radovic: Mar 7 2009, 12:18 AM

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post Mar 7 2009, 04:55 AM
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Me either, I believe that connecting a 4 ohm output to a 8 ohm cab, is not a problem, that is because, the current load on amp would be divided by two, so it wont be dangerous for the equipement, its just not efficient. But if you are really obsessed to have 4 ohm impedence, you may try to connect a high wattage capable resistor of 8 ohm in paralel with your speakers, that makes 8 ohm speakers with 8 ohm resistor in paralel will make a 4 ohm system. Still, your cabs will run inefficiently, but that solution will overcome the impendence incompatibility. It is imperative that you use a high wattage (like 150W or more) resistor. If you attempt to use a regular resistor, that can even cause a fire biggrin.gif

Hope this helps smile.gif



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