Speakers Blowing Headunit?

[M@tt 77]

A friend at work has a scooby wrx but has mentioned that his standard head unit appears to have blown for the 2nd time and the dealer seems to think that it's because the previous owner fitted uprated door speakers. The head unit seems to function ok ie all lights etc come on but there is no sound output but there is the ocassional crackle from the speakers if he hits a bump or boots it really hard.

 

now i initially though the wiring might be dodgy ie a cable had worn through and was earthing against the body somewhere but as all speaker wires are seperate i would have though it would just affect one speaker instead of all of them which is what is happening.

 

The first headunit was replaced and it exibited exactly the same symptoms

 

Now i'm not very good on ice but to me that seems very unliikely that the speakers could blow the head unit but i could understand the head unit blowing the speakers if the speakers were too pants but vice versa seems weird to me.

 

any ideas?

[d-9 0]

If the speakers are dead and have melted the voice coil, which effectivly shorts out the headunit, this could kill the headunit but would more likely cause it to turn off. (most headunits are pretty robust and take all sorts of abuse).

 

Sounds like crap if the speakers are still working

[SweetBadger 98]

A friend at work has a scooby wrx but has mentioned that his standard head unit appears to have blown for the 2nd time and the dealer seems to think that it's because the previous owner fitted uprated door speakers. The head unit seems to function ok ie all lights etc come on but there is no sound output but there is the ocassional crackle from the speakers if he hits a bump or boots it really hard.

 

now i initially though the wiring might be dodgy ie a cable had worn through and was earthing against the body somewhere but as all speaker wires are seperate i would have though it would just affect one speaker instead of all of them which is what is happening.

 

The first headunit was replaced and it exibited exactly the same symptoms

 

Now i'm not very good on ice but to me that seems very unliikely that the speakers could blow the head unit but i could understand the head unit blowing the speakers if the speakers were too pants but vice versa seems weird to me.

 

any ideas?

 

It's certainately possible but unlikely I would have thought...

 

All speakers will have a rated impedance, and the amp in the head unit will be desgined to drive speakers within a specific impedance range (usually 4 - 16 ohms). The lower the nominal impedance for a speaker the more current the speaker will draw, and the more current will flow through the output transistors of the amp. This in turn will produce more heat in the head unit's output devices (P(wasted) = I^2 R(transistor)) and could shorten the life of the headunit if it's running too hot. A well sorted amp will double its 8 ohm rated power output when connected to a 4 ohm load however this is rarely the case. How loud does your mate play his music? If he's constantly using it at high volumes with low impedance speakers then he could be pushing the head unit to it's limits... Another way to kill an amp quickly is to connect extra speakers in parallel with the existing speakers, connecting two 4 ohm speakers in parallel effectively produces a 2 ohm load which most head units will not be able to deal with for a sustained period.

 

It sounds more like a short circuit in the speaker wires somewhere if you ask me, I'd advise double checking all the connections to the head unit and making sure the speaker impedance is matched to the headunit. Although the speaker wires are seperate, it might only take one short-circuit to trigger the protection circuitry in the head unit and cut all speaker outputs.

 

With regards to a headunit blowing the speakers, it's a common mis-conception that a highly powered amp will blow speakers with a lower power rating. It's far more likely that a speaker will be blown when a lower powered amp is being driven too hard. The amp clips (distorts) producing nasty high frequency harmonics which can quickly melt the coils in a tweeter. A speaker can usually handle more than its rated power if driven with a clean signal, it might be worth it if your mate got hold of a reasonable power amp and just used the head unit to provide the signal, at least then if ithe amp blew it would cost less to replace than head unit.

 

While I'm on the subject, installing monster amps is not always the way to go to get a loud hifi system. A very effective way of making your system louder is by installing higer efficiency speakers - A speaker with an efficiency of 90db\W\m will be twice as loud as a speaker with 87db\W\m when driven with the same power, however doubling the power of your amp will not produce a system that will be anywhere near twice as loud...

[dee205 3]

Well put together mate.

[M@tt 77]

cheers for the reply's

 

he's not running an amp or anything and i don't think he canes the stereo massively

 

so i'll suggest he checks his wiring and also the ratings on his speakers

[jonD6B 0]

Don't Suburu's have a positive earth or something like that? Could it be something to do with that and a conflict in the wiring somewhere? ( I know what I mean) I think!