It may have been noticed that none of my amplifier designs include anything more than a supply line fuse to limit damage from overload and shorted outputs. Speaker protection in the form of an output coupling capacitor combined with overall feedback is included in all my recent designs, and before this I used relay protection for both output offset and thermal cutout. Speakers can be very expensive to repair, and excessive heatsink temperature can be dangerous, particularly for those of us with children or pets, so a thermal cutout at no more than 70deg.C is strongly recommended.
Protection of the amplifier output stage against shorted or excessively low impedance load is less essential. I try to avoid connecting my amplifiers to any load unless I am entirely certain it is safe to do so, and I suggest that for most DIY constructors the time taken to ensure such safety is far less than the time taken to design or build protection circuits. The use of well insulated 'figure-8' speaker cables may be safer than some of the high capacitance types with low separation between conductors. Mistakes do happen of course, but it could be argued that shorting our amplifier output and watching the smoke drifting upwards is a valuable learning experience, and the cost of replacing a few power transistors will encourage us to be more careful next time.
Some modern high power transistors are actually quite difficult to destroy, and a simple 3A supply line fuse in one case came to the rescue during testing when I foolishly applied a 100kHz full power square wave to a 4uF capacitor load. This is not something I would recommend anyone try, they may not be so lucky. The supply line fuses are essential, otherwise there is little to limit the supply or output current under fault conditions, and although occasional stories about speaker cables bursting into flame with shorted loads seem highly unlikely this sort of risk must certainly be avoided. The 2A fuse I now specify should make damage even less likely, as does the power output specification of typically 30watts while using 200watt transistors, which gives a wide margin of safety compared to designs which push the transistors to their limit.
If load-line protection is to be included there are simple and well known ways to do this. The simplest, giving just a straight-line approximation to the output transistor safe operating area (SOA) is shown below on the left for one half of a class-B direct coupled output stage using plus and minus 30V supplies:
Shown here as a single transistor, Q2 is normally a darlington or cfp. There are two small problems with this circuit, one is that supply line noise is injected into the circuit via R1, and also the protection transistor is already biased on slightly even with no amplifier output voltage or current, with as much as 300mV base-emitter voltage. In practice neither 'problem' is likely to be serious, but both can be solved or at least improved just by taking R1 to earth (0V) instead of the positive supply, as in the second circuit. The resistor values need to be recalculated, and some very approximate values are shown for the load line example used later. One disadvantage is that the supply voltage needs to be known to calculate correct values, so variations of the supply voltage are a slightly greater problem. The base-emitter voltage of the protection transistor now varies from -600mV to +600mV instead of the 0mV to +600mV of the first version, and is close to zero for zero amplifier output, so any tendency for the transistor to conduct at high signal levels and thereby add distortion should be reduced. In practice for either circuit a diode in series with the collector may be needed to prevent reverse bias on the protection transistor, and also various resistors and diodes can be added to give a closer match to the safe operating area of the output transistor.
If the maximum possible power output is to be permitted before limiting occurs a more complex circuit is needed. Looking at published versions it is a little surprising that there is rarely any attempt to match anything more than the dc safe operating region of the power transistors at a fixed temperature. Circuits allowing increased dissipation for shorter time periods are hard to find, while reducing maximum dissipation as case temperature rises seems almost unheard of, although temperature derating is invariably included in transistor data sheets. To see why this could be important consider the load lines for an amplifier similar to my own with a 60V supply designed to drive 3ohm speakers. If 3ohms is the dc resistance for a simple single moving coil driver then there will be additional reactive impedance components at different frequencies, and for a range of values of the reactance the load lines are as shown next:
A worse case would be if the lowest impedance and highest phase angle occurred at the same frequency, but even for a more complex speaker with several drive units and crossover network this appears not to be the case. Looking at a few published graphs of speaker impedance shows that the minimum impedances invariably occur at the frequencies where the phase angle is close to zero, i.e. the impedance is resistive. (It is also resistive when the impedance is a maximum.) I am not certain how accurate this is or whether there are exceptions to the rule, though I have a vague memory that it is only strictly correct for 'minimum phase' networks, which speakers are not because of time delay effects involving reflected acoustic energy, though this effect is generally small.
The SOA of a typical transistor, rated at 200watts, which we could choose for this amplifier is shown next:
Comparing the voltages and currents it can be seen that the load lines fit easily within the dc SOA of the transistor, so there should be no problem. Unfortunately the thermal derating mentioned earlier changes this result. This particular device is derated from 200watts down to 135watts with its case temperature at 70deg.C, and the load line passing through 30V and 5Amps gives peak dissipation 150 watts, so the safe operating area is exceeded. (The maximum current should probably also be derated, though this is mentioned less often in data sheets). The maximum power is only exceeded by about 11%, and this is unlikely to happen at very low frequencies, so the 100msec SOA can be assumed to be applicable, and destruction is highly unlikely. (Actually we should use a derating based on heatsink temperature rather than case temperature because our 70deg thermal cutout will be attached to the heatsink, so case temperature can be higher.) Had we constructed some sort of protection circuit operating within the derated dc SOA however this would operate and cause heavy distortion. Including both temperature and time compensation however the protection would not operate, while at lower case temperatures higher short-term power into lower impedance loads would be possible. The added complexity compared to typical protection circuits need be little more than an extra capacitor and resistor, plus thermal contact between the protection circuit transistor and the heatsink, which although not very exact is better than nothing. An approximate calculation of the effect of this thermal contact is shown here.
If the sensor transistor is not in thermal contact with the heatsink then there is some difficulty knowing what temperature it will be operating at. A board mounted component will be at a virtually unpredictable temperature depending on distance from heatsink, ventilation of the case, room temperature, and also with various thermal time constants involved. So how can the value of Vbe at which the sensor transistor conducts be determined? Thermal contact with the heatsink I suggest is not only a good way to increase available output at low temperatures, but also a more predictable way to add SOA protection.
The use of protection circuits to sense voltage, current, case temperature, and time duration are just an attempt to predict the junction temperature and to prevent it exceeding its maximum value, typically 170 deg.C, but achieving a high level of accuracy is difficult. A far simpler method would be to detect the junction temperature directly, and reduce the power dissipation only when the 170 deg limit is approached. This appears to be the method used in some National Semiconductor integrated circuit power amplifiers such as the LM1876 and LM4766. There are power transistors available (the SAP15N darlington for example) with built in temperature sensor diodes, but these appear to be only intended for stabilising quiescent current, and I am not sure whether they could also be used effectively in a protection circuit.
Turning down the volume control is all we need do to avoid the simplest straight line protection circuit operating with low load impedance. The only reason to use more complex protection is to get closer to the maximum possible power output of the transistors chosen, but a just audible 1dB increase needs a 26% increase in power, so any improvement needs to be greater than this to have any real value.
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