Guitar Heads & Cabinets *omega symbol*

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Guitar Heads & Cabinets *omega symbol*

Postby Mathieu Benoit » Thu Mar 26, 2009 9:18 am

So, I'm looking for a bit of an understanding as to why guitar heads and cabinets have 16ohm and 8ohm options and what happens if I start plugging the wrong outputs into the wrong inputs. I usually just follow the the labels. But what if I didn't? Teach me what I need to know about speakers and impedance.

Let's do a crash course on this! So as to educate the public (me).
"Volume automation takes time. You don't got that kinda time. You could be getting naked with somebody somewhere." -Slipperman

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Postby macrae11 » Thu Mar 26, 2009 10:51 am

Well I don't even really want to answer as there are certainly more qualified people than me on here, but I'll give you my small amount of knowledge. Others can add on and correct my mistakes afterwards.

First of all impedance is essentially resistance in an AC circuit. It's actually a combination of resistance, inductance and capacitance. One big difference between resistance in a DC circuit and impedance in an AC circuit, is the fact that impedance is frequency dependent.

Depending on the impedance of the drivers in the cabinet and how they're wired, series vs parallel, will give you the total impedance of the box. As you can see from amplifier specs the lower the impedance(resistance) there is the more power the amplifier will output. This is why it can get dangerous running more speakers than an amp is spec'd for. Some amps are capable of driving into a 2ohm load, but some will blow up pushing any serious amount of signal at 2 ohms. The good news if you're dealing with 8 or 16 ohm cabinets, there's not a lot to worry about unless you get stupid. Every time you double your speakers the impedance will be cut in half. 2 16ohm speakers become an 8 ohm load, 4 16ohm speakers become a 4 ohm load etc. With guitar cabinets this much simple math should keep you out of the woods. 16ohm 4x12, plug into the 16ohm output on the head. 2 4x12's, makes it 8 ohms, use the 8 ohm output.
Some cabinets also have impedance switching to go from 8-16ohms, so you can run more efficiently if using a half stack setup.


This is fairly basic, but hopefully I said things that make sense and don't confuse you more. I'll wait for Al or someone to come in and correct my mistakes.
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Postby Mathieu Benoit » Thu Mar 26, 2009 10:57 am

I'm with ya so far!
"Volume automation takes time. You don't got that kinda time. You could be getting naked with somebody somewhere." -Slipperman

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Postby macrae11 » Thu Mar 26, 2009 12:28 pm

Well that's what I can think of off the top of my head. I'm sure I could come up with a few more tidbits, but maybe you could help me out with some questions of what you're looking for.

If you're looking for an EE's look at impedance matching and a general overview of what impedance does, I'll leave that for Al or someone else to fill in those details. I could give you a bit of information, but I'd be worried that I might just muddy the waters even more.
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Postby Mathieu Benoit » Thu Mar 26, 2009 1:07 pm

Nah, for starters I just wanted to basic "here's what to do" that you provided. But I'd be interested in getting Al's response though because I do want to know more in terms of the "why"s.
"Volume automation takes time. You don't got that kinda time. You could be getting naked with somebody somewhere." -Slipperman

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Postby Geezer » Fri Mar 27, 2009 5:50 pm

With a tube-amp, the output is via xfmr, some have different 'taps' but you have to match the impedance or abuse the xfmr. With an SS amp, output is right off the transistor/FET, they'll take a wider range of impedance but my Ampegs, for instance, are made for nothing lower than 4ohm, they would run on 2ohm but I'd be askin' for it.

Hey, Happy Friday!
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Postby Mathieu Benoit » Fri Mar 27, 2009 7:13 pm

So there's a difference when dealing with a tube amp versus a solid state amp? Y'see, this is why I'm a drummer, and NOT a musician.
"Volume automation takes time. You don't got that kinda time. You could be getting naked with somebody somewhere." -Slipperman

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