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Old 5th August 2008, 03:42 PM   #1
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Default Guitar Cab - Need help with Ohms

Hey guys,
I'm building a 4x12 guitar cab... and I need help figuring out what ohms of speakers to buy.

I'd like to buy 4 of these:
(8ohms 100 watt)
http://www.musiciansfriend.com/produ...00w?sku=660186

but the power amp I want to use with them (only because I have it sitting around not doing anything) is this:
(2x1,200W @ 2 ohms; 2,400W @ 4 ohms bridged)
http://www.musiciansfriend.com/produ...Amp?sku=480697

And for the life of me, I can't figure out what would be the best speaker ohms / wiring setup.

Any help would be greatly appreciated... thanks!
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Old 5th August 2008, 03:56 PM   #2
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Resistances in parallel divide, in series they add up. So, your 4 speakers of 8 ohms paralleled give 8/4 = 2 ohms.

If you put two in series, that's 16 ohms. Then, if you parallel two of those series sets of 16 ohms, the whole thing has again 8 ohms. That is probably the best solution. What's the amp's power in 8 ohms?

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Old 5th August 2008, 04:55 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

You could go 4 x 4 ohm http://www.musiciansfriend.com/produ...00w?sku=660185

However see Section 3 of the amplifiers manual :

It states you should use 8 ohm speakers per channel for optimum
operation, which is very, very different to using 2 ohms per channel.
(In bridged mode this implies 16 ohms compared to 4 ohms).

In reality it needs to be able to drive 8 ohms bridged which implies
it also must be able to drive 4 ohms per channel when in stereo.

It also states they should be 8 ohm and > 450 watts per channel.
Which those drivers at 100W each they do not match up to.

You could wire up the 8 ohm drivers as 16 ohm per channel
(stereo cabinet) for the least chance of blowing them up.

You could get the 4 ohm version of the drivers, wired as 8 ohms per channel.

Or go for 200W drivers. 4 x 8 ohm. Either way of wiring them will
not matter. 2 x 4 ohm per channel (stereo parallel pairs) or
bridged into 8 ohms series / parallel wiring.

/sreten.
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Old 5th August 2008, 08:30 PM   #4
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"What's the amp's power in 8 ohms?"

I have no idea :P

The spec sheet only lists 2ohms or 4 ohms. Is there a way to find that out?


Also just to make sure I'm understanding correctly...

If I have 4 8ohm speakers.

I wire speaker 1's positive to the terminal positive... speaker 1's negative to speaker 2's positive, speaker 2's negative to the terminal negative...

Then..
I wire speaker 3's positive to the terminal positive... speaker 3's negative to speaker 4's positive, speaker 4's negative to the terminal negative...

right?
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Old 5th August 2008, 08:30 PM   #5
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after some quick googling, I believe at my amp is 2x500 watts @
8ohms

and the total wattage for the cab should be around 400. (with those speakers)
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Old 6th August 2008, 01:42 PM   #6
sreten is offline sreten  United Kingdom
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Location: Brighton UK
Hi,

Yes that will give you a 8ohm 400W mono cabinet.
Which should be used with up to ~ 200W for reliability.

As it is mono you need to bridge the amplifier.
You can expect ~ 1.5kW in bridged mode 8 ohms.
That is ~ 3 times the per channel rating into 8 ohms.
You will blow up the speaker at some point.

Your safest bet is 2 x 16 ohm stereo unbridged.
(wire each side 2 drivers in series)
You will get ~ 250W a side.

If your rig is not stereo use parallel mode.

see here for stereo cabinet wiring

stereo/mono WIRING

but you need to change it so its 16 ohms a side / 8 ohms mono.

/sreten.
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Old 7th August 2008, 10:12 AM   #7
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Quote:
Originally posted by sreten
Hi,

You could go 4 x 4 ohm http://www.musiciansfriend.com/produ...00w?sku=660185

However see Section 3 of the amplifiers manual :

It states you should use 8 ohm speakers per channel for optimum
operation, which is very, very different to using 2 ohms per channel.
(In bridged mode this implies 16 ohms compared to 4 ohms).

In reality it needs to be able to drive 8 ohms bridged which implies
it also must be able to drive 4 ohms per channel when in stereo.

It also states they should be 8 ohm and > 450 watts per channel.
Which those drivers at 100W each they do not match up to.

You could wire up the 8 ohm drivers as 16 ohm per channel
(stereo cabinet) for the least chance of blowing them up.

You could get the 4 ohm version of the drivers, wired as 8 ohms per channel.

Or go for 200W drivers. 4 x 8 ohm. Either way of wiring them will
not matter. 2 x 4 ohm per channel (stereo parallel pairs) or
bridged into 8 ohms series / parallel wiring.

/sreten.
hi anakinjay,

This is the perfect answer for you.
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