Guitar Amps and Cabinets

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Guitar Amps and Cabinets

Postby Mathieu Benoit » Wed Mar 05, 2008 11:25 am

OK... I'm finally going to admit some that I'm kind of ashamed of. I don't understand how guitar amps work in terms of "X ohm load" and such... can someone give a tutorial of what is the difference between 4 ohm load and 8 ohm load, ect... With my luck I'll blow up a guitar amp without proper guidance. I need to understand this in terms of power amps and PA speakers and stuff anyways, so I might as well ask the dumb question so I can move on with my life... thanks.
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Postby Jef » Wed Mar 05, 2008 11:52 am

The load that they are referring to is the resistance rating of the speaker cabinet that you will be using with that particular guitar amp.
If the impedance of the cabinet is not printed on it, you can use an ohmeter to measure it.
The input jacks on speaker cabinets for guitar amps are usually 1/4" so all you need is a 1/4" (TS) plug with a short length of cable that has the wire ends bare. Just take a reading across the + and - wires.
The cabinet may have more than one speaker in it and in many cases they may be wired in a combination of parallel and series configuration. This is why you need to check the impedance of the cabinet and not the individual speakers.
For instance, if there are 2 speakers in a cabinet and each speaker is rated at 8Ω, if they are wired parallel, your cabinet will have a total impedance of 4Ω. If they are wired in series, it will be 16Ω.
Many guitar amps have a switchable speaker output to match the speaker load.
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Postby Mathieu Benoit » Wed Mar 05, 2008 12:00 pm

Thank you! Two questions arise. What happens if the amp output and the load aren't matched for impedance. Also what difference am I getting tangibly for these various impedances?
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Postby Jef » Wed Mar 05, 2008 12:11 pm

Obviously, the greater the impedance, the less output gain you will get.
Usually amps have a minimum speaker load rating... (sometimes 2Ω).
If you don't know what the minimum is, you would be safer to not load the amp down any less than 4Ω or you may risk damage to the amp.
The higher impedance will not hurt it, it will just reduce your output volume (kind of how a power soak works).
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Postby Mathieu Benoit » Wed Mar 05, 2008 12:39 pm

Makes good sense... Thanks!

(Don't tell my brothers that I asked this question, let's keep this between us... :roll: )
"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 Jef » Wed Mar 05, 2008 12:45 pm

...mums the word... :-D
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