Speaker Fire

[unclej]

a buddy of mine stopped into the store yesterday with an old 70's sansui stereo receiver that he's had since it was new. i don't do solid state so i told him that i couldn't help him but what he described got me curious. he was listening to something on it when suddenly it quit and within a couple of minutes one of the speakers actually burst into flames..not just smoke..actual flames. i wouldn't have thought that there could be enough current going to a speaker to cause that. any ideas of what could have gone wrong?

[JoeAArthur]

Speakers are not very efficient machines. Most of the power they receive is converted to heat and not the movement of the cone which creates sound. This heat has to be eliminated and one thing that helps a great deal is the airflow caused by the movement of the speaker itself. You may have noticed vents in the back of the magnet structure.

Let's say the power amp is of the dual polarity type, the kind that doesn't use an electrolytic capacitor between the output transistors and speaker load. Let's also say one output transistor decides to give up the ghost and shorts across its emitter and collector. This would be like placing the B+ voltage of a tube amp directly across the speaker.

When the DC voltage hits the speaker it will be slammed fully forward or backward and it will stay there. Without air movement the heat created by the unrelenting power supply current will quickly build up and eventually the flash point of some material like the paper cone will be reached and it will burst into flame.

[unclej]

Speakers are not very efficient machines. Most of the power they receive is converted to heat and not the movement of the cone which creates sound. This heat has to be eliminated and one thing that helps a great deal is the airflow caused by the movement of the speaker itself. You may have noticed vents in the back of the magnet structure.

Let's say the power amp is of the dual polarity type, the kind that doesn't use an electrolytic capacitor between the output transistors and speaker load. Let's also say one output transistor decides to give up the ghost and shorts across its emitter and collector. This would be like placing the B+ voltage of a tube amp directly across the speaker.

When the DC voltage hits the speaker it will be slammed fully forward or backward and it will stay there. Without air movement the heat created by the unrelenting power supply current will quickly build up and eventually the flash point of some material like the paper cone will be reached and it will burst into flame.

thank you for that concise, completely understandable explanation. i couldn't imagine how it could get enough voltage to create a fire but that makes perfect sense.