Wire size in front doors of VT sufficient for splits?

[dephilile]

Hey I’m new to this forum and would really appreciate some help.

I am looking at replacing the speakers in the front doors of my VT. I was wondering if I would need to upgrade the wiring? I’m planning to put in some splits (JBL gto606ce, 210W peak, 70W rms) and running them off a JBL GTO 2000 amp (2 x 75W rms @ 4 ohms). Will the wiring that is in place be sufficient to handle the current?

It’s hard to work this stuff out for audio because it is always fluctuating so I would appreciate it if someone could post some general rules about wire sizes in audio applications because the old rules of P=IV and V=IR are harder to apply.

Thanks for you time

PS. Keep up the good work. It seems to be one of the better forums around.

 

[Tasmaniak]

dephilile..the Ohms law rule are fine to apply to car audio. The speaker wire in your car are fine in reality as well.

 

[dephilile]

Please excuse my ignorance but how do you apply ohms law to audio applications? Voltages in your car a constantly changing (especially if the engine isn’t running) and it’s common knowledge that most car audio manufactures fudge their power ratings on amps and speakers. Do you just reply on the RMS of the speaker/amp?

So, for example. Assuming my engine is running (I usually get voltage drops of about half a volt or so if I fully crank the volume up) so voltage drop is negligible. My splits will be powered by a JBL GTO 2000 amp (2 x 75Wrms @ 4 ohms). So the current flowing through the wire will be.

Power = Current * voltage
So
Current = power/voltage
Current = 75/14
Current = 5.4 amps

Is that right?

 

[Tasmaniak]

LOL...not quite. The power going through your speaker cables is constantly changing. It's AC. All music signals are AC. The voltage and the current changes depending upon the frequency. It is generally high voltage and very low current as such your speaker wire will be fine. (from memory, it's good for about 300 Volts @ approx .06A)

I'm sorry I didn't clarify my above post...it was a little early in the morning. Ohms law is very applicable to speaker wires when trying to determine your "power" at certtain frequencies.

If you wanted to know how much current you will be pulling down your POWER cable then the equation would look something like this....

Current = Power x Voltage
Current = (75 x 2) / 14 (it's actually closer to 14.4 with engine running and should be about 13.6 at rest)
Current = 150 / 14
Current = 10.7 Amps

BUT!!!!!!!!!!!

No amplifier is 100% efficient. In fact it's closer to only 50% Don't forget that to create voltage above 14 volts as required for the speakers, you need to have a power supply. If you pull open your amplifier and traced all the circuitry you will find that 75% of the amplifier is totally devoted to creating MORE voltage. The actual amplifier stage is a very small component.

So, with this in mind we must then take into consideration that the power supply will need it's own current draw on top of the amplifiers. With saying that amps are only around 50% efficent we can safely say that if you are developing the full 150Wrms of power then your current draw will be closer to 16 amps then the original 10Amps.

On a sidenote, it's nice to see someone else who knows Ohms law.

 

[dephilile]

Serious, does the amp up the voltage going to the speaker to like 300volts. WTF! That’s crazy. So why don’t I get electrocuted if I touch the outputs from the amp? I always thought it was like 12volts. I knew it was an AC signal which is why I said use the RMS.

Hmm, I always forget to include heat dissipation in calculations.

BTW, I’m a 2nd year robotics engineering student which is why I know ohms law. We haven’t done much electrical design yet which sux. It’s been mostly linear algebra and calculus :-( and programming

 

[Tasmaniak]

No, the speaker don't have 300 volts poking into their backsides...but the cable could handle 300 volts if you so desired. What reason you have for that is another story.....

 

[dephilile]

Why did you add that bit in that I can handle 300volts @ approx 0.06amps? if you do the power calculation on that (0.06 * 300) that’s only 18 watts (which makes sense since the stock speaker is only 15 watts. 18 watts is a long way from the 75rms output from my amp.

 

[Tasmaniak]

I added that in purely to show the capability of the wire. You will learn in your course that voltage a lot of the times means nothing. It must be able to carry current.

300 volts is HUGE! but it's only at .06A

Without doing the maths, you will find that your voltage is much lower but your current is higher. Peak bass notes, you can expect several amps.

 

[dephilile]

Thanks

Yeah, I understand how voltage doesn’t really matter. The only reason we usually up the voltage (ie. In power lines) is to reduce the amount of power lost due to resistance hey?

What’s this thing u got under your posts. MECP Master Class Installer (US Qualification)? Do you install sound systems for a living?

 

[Tasmaniak]

Thats it exactly, I forget how much voltage is in the power lines but it is quite a bit and it varies of course depending upon the load placed upon them....but it's in the tens of thousands.

Yes, I install for a living.

The MECP Master Class Qualification is an American Qualification that I aquired and I'm damn proud of it. There isn't many of us (less then a thousand with that actual Master Qual) and I'm the first non-US Citizen to gain it. In theory, the qualfication means nothing here. But, it's my little bragging point if you know what I mean.

Regarding the bit about my post, if you like what I have to say and found it useful or if you didn't like what I said and think I don't know what I'm talking aobut and I'm causing trouble, then in the top right section of our posts you will see a set of scales, click on it and you can vote for or against my reputation.