length of ground

I brought an underseat active sub and wired the ground direct to the battery.

Worked fine for 2 or 3 months then stopped due to a blown fuse which when replaced immediately blew again, and again, and again, then I run out of fuses.

Rang retailer and he said it was "user error" as the ground was too long (around 4 foot).

I agreed to send it back (at my cost) for them to look at. More than a month later they said the fuse keeps blowing as a FET blew on the amplifier and was therefore not covered by the manufacturers warranty.

They therefore want me to pay £25 for the repair. It only cost me £100 a few months ago.

Does their diagnosis seem correct to you experts? Or are they trying to pull a fast one?

I'm not an expert but I have connected the ground for my amps 2 different ways. Either direct to the battery alongside the power wire or a chassis ground.

As far as I know, the ideal ground is always straight to the battery. Being too long? As long as you are using the correct gauge wire, 4 feet should have no effect on it working. If there is a condition that is blowing the fuse, it would do it whether you were direct to the battery or on a chassis ground.

Have you tried testing the ground wire (after it blows the fuse) to see if 12v is running thru it? Maybe something shorted out somewhere?

I dont' know enough about the internals of an amp to say that an FET can causes that, but anything is possible.......

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a short ground through the chassis is always better than a long ground through wire direct to the battery. trying to pass current through long lengths of wire means the amp has to work harder to force that return current through that wire. a bad ground is one of the main reasons why an amp just seems to stop working for no reason.

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soundnsecurity wrote:

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a short ground through the chassis is always better than a long ground through wire direct to the battery. trying to pass current through long lengths of wire means the amp has to work harder to force that return current through that wire. a bad ground is one of the main reasons why an amp just seems to stop working for no reason.

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I can see that a bad ground can cause the amp not to work and draw extra current but would that blow a FET in the Amp?

A bad ground means LESS current and the amp etc works less (not that it "works"; voltage (and resistance) determines what amperage can be "pushed").
But bad grounds can cause equipment to blow for various reasons, though usually not outputs (except thu over-voltage or otherwise over-driven by bad-ground effected circuity).


IMO warranty-wise, the FET blew because it was out of spec, ie, it was faulty, and hence is covered under warranty UNLESS their warranty specifically exempts output FETs etc (which here would probably be overruled in court).   

IMO they need to provide more justification to their claim - ie, too low an output impedance (low-ohm speakers or shorts) or show that f.ex because of the long ground (and hence voltage rise of power GND) the lower signal ground caused FET overload (despite the lower current max could it can supply).
IE - what makes them so sure that FET has not died as part of its normal J-curve (meaning initial high failure rate of devices etc)?

Mind you, it can get complex.
But try that, it may win if they are bluffing...


PS - what has "length" got to do with it? Isn't it resistance, or more correctly impedance? (That's a dig at Regulators that specify resistances of eg one Ohm or less for house and AC wiring - it should be an IMPEDANCE, not resistance. The same applies to vehicles though the can't achieve the same distance-induced issues.)

And since "too long" implies LESS current, why did the FET then blow?

Of course any such warranty requirement will be clearly stated somewhere - eg, the Owners & Installation Manual. If not, it is THEIR shortfall.
Beware that your local consumer protection etc laws & regs may not provide such protection.