Glass Fuse/bulb Protection

The instrument panel lamp types I have seen to protect tweeters are Festoon glass tube types, that fit in a glass fuse holder,  rated 12.8 Volts and 2.1 amps.  These have about a 6 ohm resistance when they are heated to full temperature of 2800 degrees Kelvin.  A cold lamp has between 10 and 15 times lower resistance.  A 100 Watt amplifier provides 28.3 Volts into an 8 ohm speaker.  If a tweeter in a 2 way cabinet receives 30% of the signal, on average, from a full frequency range source, then it gets about 15.5 volts on average at almost 2 amps, producing 30 watts.  At the maximum volume from the amp, the lamp would heat up and limit tweeter voltage, so you may hear a slight volume reduction, but the tweeter would be saved from destruction.  A cold lamp will have 10 to 15 times lower resistance than a fully hot bulb, so at low volumes a human would not be able to notice the loudness reduction. These lamps work well in a full range speaker cabinet, however, if the signal coming from the amp has mostly high frequencies, then the tweeter could see higher voltages, which could push it over its 30 Watt long-term power rating, or over its diaphragm excursion limit, and it might or might not survive, depending on the safety factor the manufacturer used.  Failure modes are 1) Mechanical, due to over-travel of the diaphragm causing fatigue cracks, or immediate tears, and 2) Thermal, overheating which melts the voice coil wires.  The lamps can be put in parallel for higher wattage tweeters, or multiple tweeters.  The best way to prevent tweeter damage is DO NOT DISTORT THEM.  If you hear distortion, turn it down.  Do not let idiots run your PA board in a band, turning the volume up until it distorts.  If it sounds bad, it IS bad. Use a power amp slightly higher rated than your speakers, as too small an amp will clip voltage at its DC power supply limit, stopping the speaker cone or diaphragm motion momentarily, reducing the reactive component of coil impedance, sending large DC overcurrent spikes into the speaker.  This will eventually destroy the speakers, and may blow the output transistors in the amp.  Or, set a smaller amp gain low enough not to clip. If a larger power amp is used, set the gain low enough that the pre-amp or mixer will never drive the speaker over its mechanical or thermal rating.  Overdriving the tweeter can also cause the same output stage failure on the amp. Most of this also applies to woofers, in a similar manner, although they are more robust than tweeters.  I welcome comments.  Thank you.

Edited by Sparky02 - 26 August 2017 at 1:45am