On The Level

A detailed look behind the science of amplifier stability and how it can realistically be achieved.

Amplifier stability is a term that gets thrown around a lot in car audio circles. Yet, few installers I’ve talked to seem to know the real meaning of the word or understand what stability really implies. I thought an article on the meaning of the term might be in order. First, it’s important to always keep in mind that there’s no such thing as an unconditionally stable amplifier. While it might not be impossible to build such an amp, you can be sure that it would take a NASA-sized budget to do it.

Just what is the real meaning of the term “stable” as it refers to an amp? In the field of electronics, the term stable is used to describe how well a component, or amp in this case, is able to maintain a constant operating condition over a period of time with or without an external influence. While it’s true that an amp can become unstable without any outside influence, most of the time in car audio the key factor is the external influence. No amp will last forever, and the failure of just one critical component can cause an entire amp to fail to remain stable at any time. Hopefully, the use of high quality components will alleviate this form of instability, and this is why we should be careful when selecting amps.

In its simplest form, an amplifier is a two-port electronic circuit that receives a small input signal at one port and replicates a larger copy of that signal at its output port. Contrary to popular belief, an amplifier doesn’t really pass a signal from input to output but just makes a larger copy of the small input signal. The larger copy of the signal can occur due to an increase of either voltage, current, or both. The requirement of an amp is the same as an office copier in that we want the copy to look, or, in this case, to sound just like the original. But, due to the inherent non-linearities of electronic components, this is difficult to do and requires a little slight of hand on the part of the circuit designer. One of the tricks played by amp designers is to put an automatic correction circuit into every amplifier to insure that the output resembles the input to the highest degree.

This correction is done in the form of something known as negative feedback. Although the term negative sounds bad, it really isn’t. Negative feedback is the term used to describe the process of taking some of the signal at the output of the amp and feeding it back to the input of the amp. The feedback is called negative because the signal polarity is reversed — when this signal is combined with the original signal, it tends to cause a correction of any change from the input signal to the output signal.

Like everything else, it’s easy to get too much of a good thing. On the outside, it would appear that negative feedback is a cure for the lack of perfect components and, up to a point, it is. But there’s always a limit, and this limit can cause problems. A typical amp is designed to work over a wide range of frequencies, and this can cause problems with feedback. At the frequency extremes, the amp will experience a phase shift. When the shift becomes excessive, what was negative feedback can suddenly become positive feedback.

When the amp design fails to maintain an adequate phase margin, and positive feedback occurs, then it can turn the amp into an oscillator - the amp can actually generate a signal at it’s output without an input. Of course, no one would design an amp with this problem, but under certain conditions the load placed on an amp can affect phase margin. If the load is reactive enough in nature, it can cause an amp that’s stable with a simple resistive load to break into oscillation when a high enough signal is produced. Operation under such conditions can cause excessive distortion and, in extreme cases, can be destructive to both the amp and the load. To an engineer, this would be considered one form of external or load-related instability. In extreme cases, some speaker wires can exhibit high capacitance, especially braided types, as well as complex passive crossovers with lots of reactive components. In this case, reactive components would refer to caps and coils.

Another common form of instability is related to power supplies in amps. Every amp has a range of voltage that it works well at: the wider the range, the more stable the amp is under changing conditions. If the amp fails to remain undistorted or cuts out if the voltage fluctuates, it’s demonstrating instability related to the power supply design. Since it’s normal for a car’s electrical system to vary over a range of 10.5 to 14.5 volts, this is not exactly a trivial matter. A variation of this degree in the average house voltage of 115 would cause a fluctuation from about 100 volts to 140 volts. One thing is certain: if your house voltage was constantly changing over a range this large, you would not have many appliances working after only a very short time. Your refrigerator, color TV, and computer would definitely show you the meaning of one form of instability.

Another form of stability is related to load impedance. Many installers are somewhat familiar with this and are constantly performing speaker impedance calculations to determine the proper way of connecting speakers to amplifiers. Every amp has design limits that are based on voltage and current limits of the output stage. Higher impedance loads require higher voltages and lower currents for a given amount of power. Likewise, lower impedance loads require higher current and lower voltage for the same amount of power. For example, to drive an 8-ohm speaker at 200 watts requires that the amp produce 40 volts and 5 amps. To drive a 2-ohm speaker at the same 200-watt power level requires 20 volts and 10 amps. In each case, the power produced is the same, but how well the amp can produce the 200 watts is dependent on how well the load is matched to the output stage of the amp. If a speaker requires 40 volts and it’s matched to an amp that can only produce 20 volts, then it will fall short of the required 200 watts no matter how much current the amp can produce.

Requiring an amp to exceed its design requirements for either voltage or current results in a form of instability where the amp no longer amplifies and the output no longer follows the input. The feedback circuits can no longer correct for errors and the amplifier produces distortion. Rarely does this cause any harm as the amp will return to normal if the output load is corrected. A similar situation occurs if the amp is correctly loaded, but the input signal is too large. This causes a condition commonly known as clipping and also produces distortion. During clipping, the normally closed loop amplifier momentarily becomes open loop as the feedback can no longer correct for errors. This is another form of instability as the entire operation of the amp fails to behave in a linear fashion. This also is rarely a problem as a stable amp design quickly recovers from momentary overload and returns to normal operation when the input signal is brought back within design limits.

Another common form of instability is thermal related. Every electronic device has limits within which it will work properly. Operation outside these limits will almost always result in unstable operation or outright failure. Thermal problems in an amp are a double-edged sword in that they are compounded by the fact that transistors have a negative temperature coefficient. This means that the hotter they get, the more they add to the condition that caused them to heat up in the first place. A hot transistor conducts better than a cold one, so when it overheats it’s only a matter of time before it conducts so much current that it actually melts. Virtually all high power amps use more that one output transistor, and transistors act like selfish hogs. No two transistors are exactly identical, and one of them always conducts more than the others. The hottest transistor in the amp draws more than all the others until it fails and causes the entire amp to shut down. Good amp designs incorporate circuits that serve to limit “current hogging” of individual transistors. They also incorporate circuits that monitor output device temperatures and act to limit the conduction of current in the event of excessive temperatures. In such a case, the amp may continue to output power at a reduced rate. In extreme conditions, the protection circuitry may actually turn the amp off in order to allow it to cool and save itself from certain destruction.

Many factors can cause thermal instability. Shorted speaker wires, excessive output loading, signal overload, and high ambient temperature are the usual causes. The hotter the surrounding air, the less of a thermal stability margin the amp will have. If the air around the amp is hot enough, the amp will have no thermal margin and it will become unstable. Although not usually published, most amps have a thermal stability margin that can be calculated. At a certain temperature, the amp can dissipate a certain amount of heat and produce a certain amount of power. As the temperature rises, the ability of the amp to shed excess heat is reduced, therefore, the available output power is reduced. Eventually, at some eventual temperature, the amp will be unable to produce any output power and, hopefully, its internal protection circuitry will shut it down.

Just how the amp exhibits this instability is usually a matter of design quality, but the better amps will almost always recover. Of course, long before this ever happens, the owner can reduce the drive signal, reduce the load, or increase the thermal margin by adding a fan, etc. So, it should be clear that all amps have a certain tendency towards instability, but as long as an amp is operated within its design limits it should demonstrate stability - that is, its operating conditions will not change.