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The Tweeter protector is designed to monitor the signal (voltage) being fed to a speaker, and disconnect the speaker when that signal exceeds a pre-set level. This unit is completely signal powered, and presents an additional load over 109Ω, so it is not expected to cause any issues when used with a speaker system of 16Ω or less.
This module is designed as an an 'after market add on' to an existing speaker system (noting that fitting this module will constitute a modification, so may invalidate any manufacturers guarantee on the speaker). Once the circuit has tripped, the speaker will remain disconnected, until it is reset by a significant drop in signal.
A typical second, or higher order crossover, presents a resonant circuit to an amplifier if the driver is disconnected, this can be problematic with some amplifiers,so this circuit is designed to be installed before the high pass filter in the crossover.
The input filter is used to approximate the expected frequency response of a passive crossover to avoid false triggering. Often a Speaker data sheet will provide the crossover frequency the manufacturer chose.
The input filter consists of CA1, CB1, and L1, while R1 provides a constant/predictable load for the filter, and CA3 compensates for the drop in impedance when the circuit is triggered.
As it is sometimes difficult to get a capacitor value close to the one you need, I decided to allow the input filter to be made up of two parallel capacitors – CA1 and CB1, so in the following calculations C1 = CA1 + CB1. The following equations are used to calculate the component values for the input Filter:
C1=0.1125/(150 × F)
L1=0.2251 × 150 × F
where: 150 is the resistance of R1 in ohms, F is the desired crossover frequency in Hertz, C1=CA1+CB1 in farads and 0.1125 and 0.2251
It is safer to set the input filter slightly lower than your speaker crossover, so for example if you have a speaker crossed at 3000Hz you may want to set the input filter at 2750Hz.
Once tripped, this circuit will present a lower resistance to the input filter, which in turn will shift the crossover point of this filter to a higher frequency. As this may cause the circuit to reset while the problem still exists CA3 is included to compensate for the lower resistance load while the protection circuit is in the tripped state.
To calculate the value of CA3 we use the same formula as above, except we presume a load of 109 Ohms, and then subtract the value of C1:
CA3=(0.1125/(109 × F)) – C1
I do need to mention the power rating of R1. At 5 Watts, R1 will only handle the full output from a 200 Watt (into 4 ohms) amplifier. This may not seem like much if the speaker system is rated at 1000 Watts or more, but the thinking here, is that a typical Audio system will be delivering less than 10% of power to the Frequencies usually looked after by the Tweeter. The main exception to this rule is when feedback occurs. Of course feedback will cause the protection circuit will kick in, which will stop the feedback, and so R1 will not end up dealing with significant power. However, If you think there may be situations where the amplifier is going to deliver considerable power levels at high frequencies, then you might want to make R1 a higher powered resistor, which would need to be mounted off the PCB.
Noting here that a mildly inductive resistor for R1 will make the circuit slightly more prone to triggering at higher frequencies, but this would not be considered a problem.
The Tweeter protector goes between the Amplifier input and the high-pass filter on the crossover board. (Note this will almost certainly invalidate any warranty)
The high-pass filter will usually consist of a capacitor in series with the Tweeter, and will usually have an inductor across the tweeter, there may also be other circuitry across the tweeter to stop tweeter resonance, and also have a resistor in series with the tweeter to attenuate it.
The circuit to the left shows a typical crossover, Resistors R3A and R3B make up the 'pad resistor' referred to in the Calculations above. In this case the pad resistor would be 15 Ω, if there is no pad resistor then use 0 for the pad resistor, when calculating the trigger voltage.
Depending on the type of capacitor used in the crossover, it may be easier to de-solder, and lift one end of the capacitor, than cutting a PCB track.