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> How To Connect Amp Heads With Cabinets, Ohm values, Watts and other eletronic experts - help!
Ivan Milenkovic
post Nov 26 2007, 06:58 PM
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I have a request for all the electronic experts on the forum. Now I can write this myself but i think that someone who is really in this field can explain it mch better than me. Besides, i don`t understand it fully myself smile.gif

So the request is this:

Please write how do you know what ohm values go with what, and why and how does this affect the watts and other important stuff regarding this matter. Thank you in advance! smile.gif


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Hardtail
post Nov 28 2007, 03:43 AM
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The Ohm is a measure of electrical resistance. Also known as Impedance (do not confuse with "I" below which is the Amperage).

One Ohm = Power / Amperage^2

Or one Ohm is related to the amount of Amps flowing through a circuit at a given Power (watts). Remember that Amps is the "amount of electric charge per second". This is better explained using examples.

Example 1:

100 watt Amp Head with 8 Ohm speaker puts out how many Amps?

Answer:
8 Ohm = 100 watts / (I)^2
Solve for I. I = (100/8)^.5 = 3.5 Amps


Example 2:

100 watt Amp Head with 4 Ohm speaker puts out how many Amps?

Answer:
4 Ohm = 100 watts / (I)^2
Solve for I. I = (100/4)^.5 = 5 Amps


From the above examples we can see that a 4 Ohm jack puts out 5 Amps which is more than the 3.5 Amps from the 8 Ohm jack on a 100 watt Amp head. In fact at 100 watts 4 Ohms puts out 43% more electrical current than the 8 Ohms. This is why you should NEVER plug a 16 Ohm speaker cabinet into an 8 or 4 Ohm jack on your Amp head. But plugging an 8 Ohm speaker cabinet into the 16 Ohm output of your Amp head may not always cause damage right away if at all.

The moral of the story is to make sure your speaker Impedance (Ohms) match your Power Amp Impedance.

If you would like more info on wiring a cabinet for proper Impedance let me know. I used to use a 16 Ohm cabinet that I rewired to be 8 Ohm. Must have 2 or 4 speakers in the cabinet to rewire 8 Ohm speakers to do this though mind you.

Hardtail


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Hardtail
post Nov 28 2007, 04:09 AM
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Now, on to wattage.

Let's say you've now got the most important thing down. Let's say you correctly have your Impedances matching on both the Amp Head and the Cabinet (Say 16 Ohms jack on both).

Let's say it's a 4x12 cabinet rated at 100 watts. Note, it won't matter how you wire the cabinet up. Whether it's wired for 8 ohm or 16 ohm or is full parallel, full series, or partially parallel and series. The watts simply add up to give you your cabinet wattage. I used to use 4 Celestion Greenbacks rated at 25 watts each. My cabinet was 100 watts.

So you have your 4x12 100 watt cabinet and aren't sure if your 120 watt ENGL Savage 120 will destroy the cabinet. Some things to remember with wattage output on Amp Heads. It is directly (note I did not say linearly) related to the output volume knob. If you crank the Savage at max volume all the time then YES you will eventually wear out the cabinet and damage the speakers. If you are a sane person and play at around the 2 to 5 volume level then you are not putting out nearly 100 watts of juice and the cabinet will last you a good long time.

Please note, some experts have claimed that using a 100 or so watt head on say a 350 watts cabinet will never get the cabinet speakers to break in properly or allow them to breath because they are so underpowered. They claim that this under-use of the cabinet speakers can cause them to become stiff and brittle over time thus reducing their life as well. This makes sense but I have yet to personally witness this.




My final note on this topic is speaker quanitities.

Quick quiz.

1. What is louder to the human ear? 100 watts through (1) 12" speaker or 50 watts through (4) 12" speakers?

Answer: More than likely the 50 watts will be louder. This is due to several reasons. First is the decibel rating achieved by each speaker. Second is the fact that each speaker "pushes" a certain volume of air which is limited based on the physics of the speaker and the power of the magnet.

This gets fuzzy later on:

2. What is louder to the human ear? 50 watts through (1) 15" speaker or 50 watts through (2) 10" speakers?

Answer: As far as loudness, most likely the 2x10's will be louder. BUT the 15" will reach certain frequencies the 10's can't and will produce a louder sound at those frequencies.

Hope that helps!

Hardtail

This post has been edited by Hardtail: Nov 28 2007, 04:10 AM


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Ivan Milenkovic
post Nov 28 2007, 08:56 PM
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It really helps a lot man, thank you very much.

And some more questions if i`m not too boring.... smile.gif

I wanna buy some marshall cabs for my AVT50X combo. It has a 4Ohm speaker output. What can i choose? Can i put in there like some 4x12" 4omh cabinet? What power should the speakers have (since the amp is 50W i guess that there isn`t much purpose to connect some 100W cabs right- or wrong?? ). Next, can i buy 2x12" cab? what power should that cab be rated to match my amp the best?

Thanks smile.gif

This post has been edited by Milenkovic Ivan: Nov 28 2007, 08:57 PM


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ZakkWylde
post Nov 28 2007, 09:14 PM
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Ivan I will do the opposite of what youre doing; I will run a 100watt tube head through the speaker of my avt50x combo^^. I'll have to do this because when I buy the head I will be broke so that I can't effort a cab for it anymore...

As far as I know you can only use an 4Ohm speaker for the combo, cab size does not matter (go for 4x12 if you find a 4Ohm 4x12 cab)

This post has been edited by ZakkWylde: Nov 28 2007, 09:14 PM


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Ivan Milenkovic
post Nov 28 2007, 11:14 PM
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thats okay, this little amps speaker is very nicely built. Good luck in getting your 4x12 soon smile.gif

I will try to get an 4x12, but only have themoney for 2x12 unfortunately sad.gif


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Hardtail
post Dec 1 2007, 03:50 AM
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Ivan,

You will notice a nice volume increase going from the 1x12 in your combo to even a 2x12 cabinet. The sound will be fuller. It will sound crisper at maxed out volumes. And the sound will be less uni-directional (in other words it will spread out and fill the room better).

You CAN run the combo into a 4x12 cabinet if you like. Let me know what your goals are. Personally a 1x12 should fill most bar venues up to around 200 people. If your using a PA system then you can mike the combo and you won't need any cabinets.

Just remember... whatever cab you get make sure it is 4 ohm.

Hardtail


--------------------

Beginner's Amp & Guitar Gear Guide
My Keeley Blues Driver BD-2 Review
My Line6 Flextone II Review

Guitars: Martin D-16RGT with B-Band UST & '99 Fender "Big Apple" Hardtail HH Strat
Amps: Fender Blues Junior Special Edition & Fender Studio 85 (Simultaneous... yummy)
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Sircraigery
post Mar 26 2008, 07:30 PM
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FOR THE 1x12" VS 4x12" :

Also, the impedance (aka ohms) of a speaker changes with how the cone moves, ex: cone moving more changes the impedance more. When the cone moves out more (away from rest position, x=0) the impedance is the least. Kinda think of it like a spring, the more you push the cone, the more resistance to flow it has.


Therefore, with more speakers, the power gets divided up, and the impedance is the smaller than if the one speaker is moving 4x as much. Lower impedance from speakers = more power output biggrin.gif


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Ivan Milenkovic
post Mar 27 2008, 01:24 AM
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I really don't understand that electronic mathematic mumbo jumbo smile.gif but there is all well explain don celestion's site. I've been there recently and found everything nicely explained.


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Marcus Siepen
post Mar 27 2008, 04:01 PM
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Interesting topic, even though it reminds me a bit of my physics class in school tongue.gif


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