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what are ohms ?

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(@steveobouttorock)
Estimable Member
Joined: 20 years ago
Posts: 87
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what are ohms or ohms ratings ?

be good at what you can do-


   
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(@gnease)
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Joined: 20 years ago
Posts: 5038
 

Probably more than you wish to know:

An ohm is the unit of electrical resistance to current flow. The simplest mathematical relationship between voltage, current and resistance is

I = V/R,

where I = current in amperes, V = voltage (actually electomotive force or EMF) in volts and R = resistance in ohms.

This means if one applies a voltage across a device with two terminals, the resulting current that flows between those terminals is the value of the voltage divided by the value of the resistance.

This formula is valid at DC and is a pretty good approximation for most audio frequencies.

Example: 50 volts applied across a device whose resistance is 2 ohms will cause a 25 ampere current to flow through that device.

(BTW, the formula is often written V = I*R and mistakenly called "Ohm's Law." This is actually a derivation of the real Ohm's Law, with which I won't bore you here.)

If you are asking about what "ohm" means in term of audio equipment, such as speaker cabs and amplifiers, then the best advice is to pay attention to matching the specified cab resistance (really an average across the frequency range) to the load resistance required by the amplifier. This is important so as not to stress the amplifer by requiring of it either too much current or voltage (clipping) -- conditions which can cause failures in amplifier power output tubes, transistors, and/or transformer.

Resistance is sometimes called impedance. This is not really accurate; however, it is true that at low frequencies ...

Resistance approx. = Impedance

Hope this helps -- Greg

-=tension & release=-


   
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(@wes-inman)
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Joined: 20 years ago
Posts: 5582
 

Greg

Thanks for that basic electronics lesson that was very well written and easy to understand.

SteveoBoutToRock

For guitar players, the main thing to know about Ohms is how it relates to amplifiers and speakers.

Amplifiers are designed to work best at a certain resistance. Sometimes you will see specs like this on the back of an amp;

100 watts @ 4 Ohms, 70 watts @ 8 Ohms.

So if you hooked up a speaker cabinet rated at 4 Ohms the amplifier will be able to pump 100 watts of power into it. If you hooked up an 8 Ohm speaker cabinet, the amplifier will only be able to provide 70 watts of power. This is because the 8 Ohm cabinet has more resistance to current flow than the 4 Ohm cabinet. But as you can see in this case, the amplifier would work with a 4 or 8 Ohm cabinet.

What you have to be careful not to do is hook up a speaker or speaker cabinet with a LOWER ohms rating than the amplifier specifies. For instance, you wouldn't want to hook up a cab rated 2 Ohms to this amp.
There is not enough resistance. Too much current will flow through the amp. It was not designed to handle this much current and you will fry the amp.

Using a cab or speaker rated higher than the amp's ohm rating will not usually cause problems, but the amp will not be able to perform to full potential. It is kind of like driving with your foot on the brake pedal.

Some things to know.

Speakers can be wired in Series or Parallel. Most speakers are wired in parallel. There is a BIG difference.

Two 8 Ohm speakers wired in parallel produce a 4 Ohm load.
Two 8 Ohm speakers wired in series produce a 16 Ohm load.
Two 4 Ohm speakers wired in parallel produce a 2 Ohm load.
Two 4 Ohm speakers wired in series produce a 8 Ohm load.

There is a formula to figure out the ohms of various speakers wired in series or parallel.

You will often see 4 X 12 speaker cabinets using all 8 Ohm speakers rated 8 Ohms. How do they do that? They will wire 2 speakers in series for a 16 Ohm load. They will wire the other two speakers in series for a 16 Ohm load. Then they will wire these two sets of two speakers in parallel for an 8 Ohm load.

This might be more than you asked for, but it's good stuff to know, especially when you start looking for speaker cabs for your amp head.

If you know something better than Rock and Roll, I'd like to hear it - Jerry Lee Lewis


   
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(@steveobouttorock)
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Joined: 20 years ago
Posts: 87
Topic starter  

thanx a lot guys

be good at what you can do-


   
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