Speaker cable Question

[Danno]

Hey all- Can I use 16 gauge speakon speaker cable from my 1000w@ 4ohm bridged poweramp to my GK 400w 8ohm 4x10 cab?

Thanks, DAN

[MattC]

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P=(I^2)Z

1000W=(I^2)(8w)

125A^2=I^2

11.18A=I

 

So at full power (constant input), the cab would pull ~11Amps. 16AWG cable can carry about 13A safely. Of course, we are talking constant output here- your cab won't be putting our constant power. This cable, provided we are talking reasonable lengths (<15'), should be appropriate.

 

COrrect me if I've misapplied Ohm's law...

[twentyamptwist]

I would use the fattest cable I could. Your rig is only as strong as the weakest link and bass frequencies love amp power, so the bigger the conduit between amp and cab, the better.

 

To find current with ohms law it is:

_P_ .

I E

 

To find a current (I) value you need a voltage (E) value, which I do not see and wouldn't know what voltage the amp is sending to your cab. The power (P) value is 1000 watts. Not sure where the 125A value came from, unless it is supposed to be 125V, which would be mains voltage and not applicable here, so I think the above calculations are flawed. The amp is not sending 125 volts to the cabinet and you cannot use the impedance value in the above formula. You always need two values to calculate the third.

 

The other ohms law is:

_V_

I R

 

where (V) is voltage, (I) is current in amps, and ® is resistance of the circuit in ohms(not to be confused with impedance, also in ohms.) Impedence is the product of inductance combined with resistance in a crossover/driver circuit in a speaker cab.

 

http://ohmslaw.com/ will help you understand more about all of this.

 

20at

[MattC]

Power = (current squared)*(Impedance)

1000 Watts = (Current squared)*(8 Ohms)

125 Amps squared = (Current squared)

~11 Amps = Current

 

The amplifier is modeled here as a current source; the voltage across its output terminals is not needed for this analysis.

 

The impedance of a complex element is indeed a function of both DC resistance and inductance (as well as capitance). Resistance is unique across all frequencies whereas inductive impedance and capitative impedance are not. The speaker is a complex element and as such has a distinct DC resistance, inductance and capitance. Use a meter which can measure all thee and you will discover that a speaker has resistance (about 5 ohms), capitance and inductance.

 

The impedance of a speaker changes in accordance to the frequency of the current going through it, hence the ratings should always include the prefix "nominal" in that the lowest impedance that the amplifier will see is the rated impedance.

 

Impedance is very easily viewed in terms of a crossover where the inductor and capacitor can be viewed direcly. Impedance is of importance here as a result of Thevenin's theorem, stating that the maximum power is transferred between two elements of the same impedance. This is also the concept behind "impedance matching" of sources and inputs.

[LLroomtempJ]

mattc is right. his avatar also rocks.

 

jason

[MattC]

Thanks for the avatar remark. I think its funky but the folks on the political forum see it as a sign of homosexual tendancies. I guess it takes bass players to see the funky banana, not the gay banana.

[Prague]

Keep in mind you can't exceed the cabinet's rating. I roughly figure 7 amps. 16ga will work, just keep the length in check.

[MattC]

Perhaps 7A continuous, but an audio amplifier (escpecially one for bass) is all about peaks, not continuity. RMS power is more like 4 or 5 amps continuous when played hard.

[Prague]

Originally posted by Prague:

I roughly figure 7 amps...[/QB]

Straight ohm's law assumes RMS and 8 ohms resistance (not impedance), so it does overestimate. RMS also assumes a pure sine wave.

 

It's the 1000W figure that is misused.

[MattC]

Ohm's law is used for both complex and non-complex (i.e., DC) sources. When the frequency=0, the imaginary portion of the impedance (that due to the inductance of the speaker) and Z(impedance) = R(resistance).

 

The 1000 Watt figure is used as a starting point- a maximum output under most situations. Because we are dealing with the resistance in a wire (back to the original question), any discrepancies here are not all that important.

[Prague]

Do as you please. 1000W doesn't apply when using a 400W max load. You started at 11A and now you are down to 4 or 5. That's quite a discrepancy.

[Danno]

Thanks for the replies so far. I should have added that I'm running the amp at 700w@8ohms to match the cab and was basically wondering if 16 or 12 gauge would be better?

[Prague]

Bigger is always better.

[Subsonicbass]

12 gauge minimum.

[Danno]

Thanks, you guys rawk!

[alexclaber]

Power loss isn't an issue in bass cable runs, as they're rarely more than a few feet long. However, thicker shorter cables will lower the effective output impedance of the amp and increase the damping factor that the speaker sees. i.e. better cone control and less overshoot on peaks = tighter sound.

 

Not a BIG issue but you might as well get fat cables, they're not exactly pricey when you only need 2'-6' lengths. And they'll last a bit longer than skinny cables - more strands have to be broken before they descend into dubious reliability...

 

Alex