There has always been a lot of interest and effort spent on the endless search for the perfect tweeter. Since the equation of loudspeaker design dictates that high power handling goes hand-in-hand with high mass (heavy wire) and that great high end (frequency) response goes with very low mass, tweeters are always a big area of unsuccessful compromise between performance and rugged construction. There being no quick solutions on the horizon; most quality manufacturers turn to some form of external circuitry to help the fragile tweeters survive in the real world of extreme abuse, misuse, and accidents.

There are a few basic ways to protect tweeters in tough program environments:

The first and most simple type is the fuse. This has the advantage of being simple, easy to understand and build, and is very cheap to manufacture. Its main drawback is that it typically doesn't work well with tweeters. Due to small average RMS power handling requirements of most tweeters verses the much larger instantaneous peak power they can handle it is most often not possible to select a fuse that is of a correct size to allow the peaks and at the same time small enough blow when the system is seriously overdriven. Usually the compromise selection results in either the tweeter becoming a fuse protector or the fuse becoming a nuisance problem needing frequent replacement at inopportune moments.

The second type is a fast acting circuit breaker. These work fine and have the advantage of low cost and good reliability. They unfortunately tend to trip at all the wrong times just like the fuse turning the tweeter completely off and requiring the operator to stop and reset them. A better version (currently hard to find) is an automatic thermal breaker of the type used on small motors. These work well and, when coupled with a fixed pad resistor, give reasonable sound and a good general reliability. They are however very audible when they operate, due to the fixed pad level.

The third type is the fixed threshold shifter or relay attenuation type. An example of this would be the Electrovoice STR, a neat little circuit from the 80s that uses a full wave detector that drives a relay or transistor and is set up to switch when the signal passes a preset voltage threshold. The rang of 6 to 12 volts used to be very common settings for these types of units. The main advantage of these style protectors is that, in high power abusive applications like P.A., they are virtually bullet proof. The main disadvantage is that they are basically a very hard limiter. They are generally quite audible and cause severe dynamic signal level modifications of the program by reducing the signal from a min. 6 dB up to as much as 20 dB depending on the preset level reduction. In addition, they are complex and costly to manufacture and have more parts to break and or get out of calibration.

There is also a variant of this type that is much more pleasant to listen to. This version has the same relay circuit but, instead of a resister, it switches into a dynamic load or thermistor that has a softer knee-type limiting. This gives a fixed threshold and a more genteel natural compression. Them main disadvantage of this type is high manufacturing cost and a high component count.

The fourth type is the thermistor type. This type actually changes impedance with dynamic load and signal. I find this to be the most generally useful and pleasant sounding of the whole bunch. There are two main classes of these: the solid state type and PTC devices. These are in wide use in piezo tweeters, etc. They are very cheap and work fairly well with the exception that they suffer from operational hysterisis (they change value and shift engagement threshold with use). They also have a fair amount of inherent non-linearly and general distortion.

The fifth type is the mechanical thermistor, or calibrated light bulb. This is the one I think, in general, has the best overall performance and sound quality . This was introduced way back in the early 70s as a concept for tweeter protection. We have used variants of this circuit type since the mid 70s. The operation of the light bulb (if properly calibrated) happens in three phases:

Easy-over Compression
In the first phase of operation, the bulb appears to the circuit as a straight piece of wire. At low power it does not effect the circuit at all except for adding a couple of Ohms of resistance (impedance). due to its filament resistance. (Most other devices also add some small resistance or impedance.) At medium power, the bulb begins to heat up rapidly and its internal resistance also begins to go up causing a general rise in the total circuit impedance to the tweeter. This causes a soft, easy-over compression to begin to take place at the threshold predetermined by the bulbs internal current handling characteristics.

Hard Limiting
If the system power continues to rise until the total current capacity of the bulb is reached, a constant current source is achieved. This effectively acts as a hard limiter. This happens by the light bulb reaching the maximum current that it can allow to pass through itself. This in turn causes the load to develop mostly across the bulb. Most of the dynamic signal is dropped across the bulb, thereby protecting the load (tweeter). It is very important to all of the above that the bulb calibration match the tweeter characteristics exactly.

Fuse link
The final phase of operation by the bulb takes place only after some considerable time and in our case after 100 Watts (above kHz) is placed on the tweeter continuously. (This is real abuse - a Very Bad Thing.) The bulb finally becomes a fuse opening up and removing the tweeter from the circuit. This lets the player know that they have gone way over the top power-wise. The bulb is a snap in cartridge type and is located on the back of the rear crossover / jack plate. It is easy to service by removing the crossover plate.

Additionally, the light will give early visual warnings that you are approaching the system limits. It is visible though the surrounds of the loud speakers. Having the light on occasionally is fine. It should not be on a majority of the time. The light only becomes visible when the voltage across it exceeds 24v to 28v or 72 to l00 Watts across the tweeter circuit.

Protecting tweeters is always a compromise. We are always looking for better ways to protect the tweeters and, of course, ways to improve the power handling of the device itself. This represents the best of current technologies that are affordable in the real world today.

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