Question for the audiophiles

[betterman]

Hey guys got a strange problem thats hard to describe so bare with me. Oh and I don't know most of the technical terms so just correct me if I'm wrong.

Ok, my friend just purchased a 1200 watt amp and 2 10" subs from my brother to add to his current system. The system he had before the subs was 4 type R speakers run by 2 sony 300 watt amps. One amp running one the left side adn one amp running the right side. Power cable from battery has inline fuse and is run back to a power splitter box and then to the 2 amps. Oh and he has a sony 4 X 50 deck and a sony 6 disc cd changer. All of which works fine and still works fine. The problem he has is with the new subs and the amp. I don't know the brand of the amp but I know the subs were (they are blown now) thumps. We ran the power for the amp from the power splitter that both other sony amps are powered from. The power wire that runs too the new amp also has a inline fuse (30 amp). The ground wire for the new amp is grounded in the same spot that the other two amps are grounded to. The sound inputs for the new amp are run from the through outputs on one of the sony amps.

What the problem he is having is that he keeps blowing the fuse on the power wire that runs from the power splitter to the 1200 watt amp. The fuse seems to blow anytime the volume is turned past one quarter. We replaced the fuse several times but always with the same result. We tried adjusting the settings on the amp several times which also never helped. We tried running just one sub and that also didn't help. The second last time we replaced the fuse one sub blew (makes a really weird and quiet grinding noise). Then we replaced the fuse one more time just for the heck of it and the other sub blew. Sometimes the fuses would last for several minutes at fairly high volume levels and the subs sounded good during this time but then the fuse would just blow without any warning. Note that the inline fuse closest to the battery never blew. I may have to add more details to this problem later but for now this is all I can think of. Do you think its just that the amp is no good or is something with the wiring that is the problem.

 

[yamahaSHO]

If you have a fuse to the near the battery, I'd get rid of the other fuses (except for the ones supplied in the amp). What size fuses are you running? How is the power cable run? What size fuses do the amps run (actually mounted to the amp). Is your ground wire as adequate as the power lead?

 

[betterman]

It's a cheap amp, no fuses actually on the amp. I always thought that was weird that there were no actual fuses on the amp. Not to sure what size the main fuse near the battery is, I guess I can ask my friend to check. Should the size of the ground cable be equal to the size of the power cable? Because the ground cable is not the same guage as the power cable its definately a thinner cable. I still don't see why both subs blew though, can power surging to the amp blow the subs?

 

[K-Dawg]

You definitely have a wiring problem. Yes, the ground wire should be at least the same size as the power wire. What size power wire does he have. 1200 watts RMS should have at least 4 gauge wire.
If the amp is truly a 1200 watts RMS amp, I believe a 30watt fuse is too small. FYI, I think the JBL BP1200.1 (true 1200 watts RMS) has 3 30 watt fuses.
Its a possibility that the amp could be shot and may have internal problems. It doesn't sound like the amp is of high quality, or puts out 1200 watts RMS. Check the wiring first.

 

[yamahaSHO]

Just a note... Current actually flows from negative to positive. The ground cable MUST be as big as the positive! I've never heard of an amp that does not have fuses. I'd dump it really quick. If you're not blowing the fuse by the battery, then do not worry about that fuse.


FYI, fuses are not rated in watts. They are rated in amperes. I think you are correct about the JBL... All I remember is when I blew those fuses, I had a **** of a time finding the correct rated fuse.

EDIT: The JBL uses 40A x 3 fuses.

 

[sho808]

Just a note... Current actually flows from negative to positive. The ground cable MUST be as big as the positive! I've never heard of an amp that does not have fuses. I'd dump it really quick. If you're not blowing the fuse by the battery, then do not worry about that fuse.


FYI, fuses are not rated in watts. They are rated in amperes. I think you are correct about the JBL... All I remember is when I blew those fuses, I had a **** of a time finding the correct rated fuse.

Many good older amps don't have accessable fuses on the outside thus leaving one to believe there is no fuse but many amps had fuses "the old short round glass ones" on the circuit board it's self. Rockford Fosgate Punch 45's 75's & 150's were this way but they were also supplied with inline fuses that went near the amp if I remember correctly. There were many other GOOD older late mid 80's to early 90's amps that were like this.

 

[yamahaSHO]

I did not start messing with car audio until I could drive.... 1997, I think

 

[K-Dawg]

FYI, fuses are not rated in watts. They are rated in amperes.

Doh! You're right. And to think, I'm an engineering major! It's way past my bedtime.

 

[yamahaSHO]

Meh... not harm, no foul. I've taken many Electronics Principles courses and work with this crap everyday. I'm way too tired of this stuff. Not to mention, I don't like what my degree is in now... I think I should change up.

 

[NoSlo]

A little math: 13v x 30A fuse = maximum of 390 watts. If the amplifier is really a 1200 watt amplifier, it probably can pull more than 1200 watts of electricity through the power cable. You most likely need to get a bigger wiring harness with 2 or 4 gauge wire going from the battery or master fuse block back to the amp so voltage doesn't drop under big loads, with a 100 amp ANL fuse near the battery for this connection, a power distribution block back by the amp etc. and big ground wires with a good ground (try the center back seat belt bolts). You don't need a fuse on the individual amps as long as the main power feed is fused near the battery. For more protection however, if you have multiple amps you might also fuse each one seperately near the amp with an appropriate size fuse for that amp (i.e. 100A fuse for 1000W sub amp, 30A fuse for 300W front amp, 10A fuse for 100W rear channel amp, 3A fuse for active crossover). Remove the 30A fuse by the amp; as you can see it is inadequate for the amp's power rating.

 

[betterman]

Alright, sounds like the fix is to put in a bigger ground wire, power wire and ditch the second inline fuse. Thanks for all the help guys hopefully it will fix the problem.

 

[SHOZ123]

A little math: 13v x 30A fuse = maximum of 390 watts. If the amplifier is really a 1200 watt amplifier, it probably can pull more than 1200 watts of electricity through the power cable. QUOTE]


Input amps=output amps is a no compute. There should be an input amperage rating somewhere. #4 wire is probaly over ****. #6 would be easier to work with. Put a big fuse on the wire at the wire's rating not necessarily the amps rating. #6, 100A, #4, 150A. Of course you can go smaller.

 

[NoSlo]

The fuse rating has nothing directly to do with the cable size used. The fuse should be specified as the minimum required to not blow under maximum current draw anticipated by the amplifier, to offer maximum protection. The fuse protects against amplifier circuitry fault and wiring short circuit. Even if I ran one inch thick power cable to a 50W amp that would draw 5 amps, I would still want to use a fuse appropriate for the amp power rating - 5 or 10 amps.

You can look at the amplifier manual for the Phoenix Gold X1200.1 amplifier. It is a 1 channel 1200W amplifier, rated for 1200W into 4, 2, and 1 ohm loads. The manufacturer recommends a 100A fuse and 4 Gauge power and ground wire.

ftp://208.187.38.55/Phoenix_Gold/Manuals/Amplifiers/XENON/xenonampmanual.pdf


The fuse rating 100A for a 1200W amp is perfect. If the amplifier is 100 percent efficient and I want it to make mega booty bass at max RMS power, it is going to draw 1200W through a perfect power cable; it will quickly exhaust any headroom in the power supply capacitors, even exhaust 'power stiffening' capacitors in the trunk at this power level. The current drawn is equal to the power divided by the voltage; I=P/V. Given a standard 13.8 volt car power supply, 1200W/13.8V = 86.96 amps. No amplifiers are 100 percent efficient, so it will draw more (unless you can tell me how less than 1200 watts of power in can make 1200 watts power out). Normal musical peaks are transient, so you would rarely see the amp hit the max power level, unless you overdrive it to clipping. Those 'DJ BASS TEK' cds break this rule though with their sine wave bass notes.

The reason for specifying large gauge cable is to avoid voltage drop at the amps, robbing them of power and potential and reducing headroom.

Gauge Resistance per 1000m
0 0.322424
2 0.512664
4 0.81508
8 2.060496

AWG 8 cable has 2.06 ohms resistance per 1000m (per the Handbook of Electronic Tables and Formulas). In a 10m circuit of power and ground cable, like you might find in a car, that resistance would be .0206 ohms. Very low resistance, you may think. However, your 100 percent efficient amp drawing 86.96 amps also has a very low resistance. Mr Ohm and his law says that resistance is R=V/I, or 13.8V/87A = 0.159 ohms. The resistance of the wire is 13 percent of the resistance of the amp, this starts to make a very effective voltage divider. With a total resistance of 0.1796 ohms for both the amp and wire, the voltage drop across the wire is 11.5 percent, or 1.58 volts! That means the amp power supply voltage drops from 13.8V to 12.2V at max load of 1200W. Of course, with a good switching power supply, the amp will find out quickly that it can't get 1200W from that 12.2 volt power supply using just 87 amps. To get 1200W at 12.2V = 98.4 amps - of course more current means lower amp resistance which means more voltage drop.

If we make a theoretical optimum amplifier that can pull all the current it needs at whatever voltage drop is given to it to get 1200W, and solve for the voltage drop across the 8 AWG gauge cable you get a 2.116 volt drop! The amp would be running off 11.68 volts. The total current would be 102.7 amps, and while the amp would be dissipating 1200W, the CABLE would be dissipating 217 watts. What happens with a real world amp is it just distorts without making it's rated power.

Upgrade our amp to 4 AWG power wire with 0.0081 ohms over 10m and solve for the voltage drop, current, and power dissipation by the cable:
The optimum amp makes 1200W power with a 0.7452 volt drop. The cable only dissipates 68.6 watts. Current is 92A.

Tell me how 4 gauge wire is overkill?

 

[yamahaSHO]

Agreed, the fuse(one by the battery) is supposed to be big enough that it does not blow when the amp is at full power. It is really there to prevent fires due to shorting before it reaches the amp(s). In today's amps, most come with fuses attached to them. Those are the fuses that will protect the amp. You only need seperate in-line fuses on a multi amp system if your amp(s) does not contain the fuses it needs.