How to Protect your Amp (audio amplifier)
There are numerous schematics on the internet covering amplifier protection and also many people looking for the ideal protection circuit. Unfortunately with any electronic device, although possibly meticulously designed the way in which it is going to be used may determine the eventual outcome – thrashing an entry level home theater amplifier will lead to its early demise – pricing is around getting as many functions on a budget more than supplying reliability at a price.
Although this article may seem a misnomer because the picture actually shows a loudspeaker protection device in case your amplifier goes faulty it does in a manner also allow for stability of the components before connecting to the low impedance load.
Amplifiers all use one very costly device, the mains transformer. These transformers can be picked to provide high current outputs for 24/7 usage (costly) or a cheaper device which allows only one or two channels to be driven at full power for a short period of time. There are many ways in which to determine the VA rating of a transformer but core size and copper usage is normally the main factors taken into consideration when designing a transformer to fulfill a need. Most home theater amplifiers do not use a transformer which is designed for the party animal. If you are fortunate only a fuse will blow, less fortunate the temperature cut off protector goes open circuit and really unfortunate, the amplifier burns out the final and driver stage transistors. Bridge rectifiers go short circuit and may cause the transformer to overheat if the fuse doesn’t blow causing transformer failure. Mains transformers are very hardy but if they are underrated by a factor of four they will fail under harsh conditions. Maybe I am being a bit anal about the transformer loading but good amplifiers have very beefy transformers. Yes, many amplifiers for home theater designed for an 800VA transformer only use a 200VA transformer. For normal usage this is entirely adequate. (read: for movies).
Thermal Loading (thermal runaway)
Many home theater amplifiers are only designed to drive 8 Ohm loads but the owners often fail to heed the manufacturer warnings and operate at full tilt not knowing that the loudspeaker impedance IS going to drop lower than the specified with specific program material. This means more current and therefore more heat. Inadequate ventilation is another problem – home theater amplifiers do not use adequate heat sinking for all channels to be driven simultaneously over lengthy periods of time. Coupled to this, driving an amplifier into smaller than designed loads (more heat) and stacking with the amplifier below the optical pickup (CD/DVD) decreases ventilation. Transistors run hot, in most cases too hot and will cause the output stage to run outside it’s limits.
Professional Use and SMPS (and of course class D)
High powered amplifiers designed for commercial use (bands and PA) all have three things in common – more than adequate power supplies, plenty of heat-sinking and paralleled output devices designed for intended thrashing into low impedance loads. They are not cheap. Many amplifiers designed for big club and band use are shipped with switched mode power supplies. This makes them cheaper to manufacture but NOT to repair. The lifespan of an amplifier using switched mode power supplies will very unlikely make it into two decades of use only because the capacitors inside the power supply work very, very hard. Conventional transformers and huge electrolytics in my opinion are more robust. This is changing though, capacitors are also manufactured to be more reliable and can withstand higher temperatures than many years back. Class-D amplifiers are becoming more popular pushing demand up on lightweight amplifiers – one of the major advantages being shipping.
So how to protect your amplifier?
The worst case scenario is for a transistor to go short circuit applying DC to the loudspeaker. Most amplifiers of modern design are running at a minimum of +/- 35 rails and at this potential the voice coil will burn within a few seconds. Fortunately MOST amplifiers do have built in DC to speaker protection. The circuit is essentially a DC detector (capacitor, resistor and bridge rectifier) feeding a three stage amplifier driving a relay. It is always good practice to turn the volume down before switching on the amplifier – high currents are generated through the voice coil when playing music loudly and as the contacts close there will be arcing – this will cause problems later on due to the resistance across these contacts. However, if a transistor fails and there is DC offset the current travelling through the contacts is much larger which may cause the relay contacts to weld together. A good many circuits do not use the protection relays adequately – first of all these relays are wholly inadequate for their purpose and secondly the normally closed contact should go to earth or through a high wattage low ohmic value resistor (4 to 8 Ohms). Most safety circuits leave the normally closed contact floating.
If your amplifier does not have DC protection you can either build your own or purchase the kit from Velleman (featured) which has it’s own power supply. I also use a separate enclosure with the protection kit built in for testing amplifiers – you never know what may happen.
Again, a plethora of information on the internet. An amplifier should have adequate ventilation and heat sinking. If possible the heat sink cooling fins should be outside and not inside. It may look neater inside but when running hot the heat sink often serves no purpose without having some sort of air scavenging taking place. The safe operating area (SOA) of all transistors are given out by their respective manufacturers, their derating values can be a real eye opener. Bottom line is that the cooler a device runs the longer it will last. Electrolytic capacitors are no exception to this. A quick remedy in most cases for overheating is to blow air directly onto the heat sink such as in the CPU cooler of your PC. My own belief however is to use adequate passive cooling (i.e. not to rely on cooling fans). In many cases though this is not practical – high powered amplifiers will always use some sort of cooling fan, either directly on the cooling fins or in a push-pull configuration. Remember that heat rises so ensure adequate ventilation above the amplifier.
Thermal Dissipation (Blow, don’t Suck)
High powered amplifiers usually use some sort of thermal detection circuit to determine the temperature of the heat sink. This can either power up a cooling fan, speed it up or shut down the amplifier. Cooling fans should never be positioned where it would blow directly onto the thermal and bias compensation circuit. Often this is a transistor mounted directly on the heat sink next to the output devices. I mention this because this circuit is designed to compensate for temperature increase, reducing bias when needed. In some cases this transistor is mounted in thermal insulation to prevent this kind of occurrence from happening. I never blow cool air across the components – always directly over the fins which are usually situated backward of the components. A very good training ground to explore avenues of cooling semiconductor and thermionic valve equipment is to see how manufacturers cool down high powered transmitters. A case in point, to show how effective the cooling is, a transmitter I worked on once would trip within 30 seconds of high voltage being applied (valve gear). The blower start up capacitor had failed. Without this simple blower the temperature rose to unacceptable levels within 30 seconds whilst keyed (on and modulated). Class A audio is a case in point and my point about passive cooling is relevant. Always go for more heatsinking and less reliance on forced air. In very high powered equipment this is impossible – water or oil cooling is an absolute must.
Some amplifiers (actually, many) use a reduced drive circuit to the power stages when running at very high temperatures. Although many audiophiles are strictly against this I also take into account that if there is adequate cooling these circuits remain more or less inactive. On the other hand, if one had adequate cooling this would not be necessary. So in my opinion a manufacturer designing his equipment to be stable at for instance up to 80 degrees Celsius should take the necessary precautions by applying forced air cooling rather than a reduced drive topology. The less components the better. (to explain this from another angle – the manufacturer should apply feedback to the cooling system, just as a motor manufacturer doesn’t slow the engine down if the engine starts running hot. Temperature sensor, relay, cooling fan). Read up about Class D amplifiers and their advantages. Reducing power is one thing but what about short circuit loads.
Short Circuited Load
This is by far one of the most common problems after thermal issues. Many home theater amplifiers have a protection mode circuit which will show when there is a short circuit. Audiophiles should know better but often they make the same mistakes as you and I – wires come off a speaker and short. Sometimes in the enclosure itself. As mentioned above, drive reduction is one thing when temperatures become too high but a simple circuit which detects current through the emitter resistors to remove drive completely from the output stage is another. I am over simplifying things but if one has built a regulated power supply with current limiting will get the gist of the story. The amplifier is no different, the emitter resistors are a very low value and make excellent over current detectors.
Switch on thump
Often the circuit used for DC protection incorporates a slow switch on circuit as well using only an RC timer. A capacitor charges up through a resistor which in turn switches a transistor either on or off – this allows for a second or two of amplifier stability to be attained before connecting your loudspeakers to the amplifier. Again, unfortunately most manufacturers do not take into account that the volume may be set to a critical level which will cause the user to soil his pants when switching on. I feel that amplifiers should always reset to a minimum level when powering on. Just a thought mind you.
Switch on thumps can be bad news. I had an early quasi-comp amplifier using 2N3055s many years back which would power the speaker through a 4700u capacitor and in all honesty the travel of the bass drivers was frightening to watch on switch on. Before I got rid of the amplifier I can recall a total rework of the output stages at least three times. They would always blow on switch on. Yet, some like minded individuals never had the same problem. I could never find the cause of this except to ‘think’ that the startup current surge through the loudspeakers caused the damage. Audio enthusiasts often connect an electrolytic capacitor in series with the amplifier output to protect their speakers and say they hear no difference. Again IMO the less compoknents the better, especially ones in which unwanted reactances are brought into the equation. In hindsight, something which may have had a big impact is the fact that counterfeit transistors were making their appearnace at the same time the amplifier used to give problems. A friend of mine that worked at the CSIR (Council for Scientific and Industrial Research, South Africa) mentioned to me that they had received a batch of dodgy transistors. They replaced the entire batch with Toshiba devices. I did the same and the problem never manifested itself again (as far as I know). All amplifiers built now have anti-thump circuitry built in with a resistor to ground through the relay. (I use 8 Ohm non-inductive 20W. Approx. $2.50 each)
Overdriving the Amplifier and Burning Speakers
Compressing the audio signal to stop the user from over driving the amplifier can be dangerous for your loudspeakers. Compressing a signal is used when recording a signal, or more commonly in keeping microphone levels fairly constant. Broadcasters use this technique and it was very common with CB radio ‘Power Mic’s’. This increases the modulation of the carrier wave by reducing dynamic range and increasing average power. It is not be used to drive an audio amplifier as it raises the average power (meaning more heat).
The best form of power reduction is done through a limiter circuit – this reduces the voltage which drives the amplifier leaving the dynamic headroom the same or as close to possible as that intended by the recording engineer. Limiters are usually connected before the amplifier, have a very fast attack time and decay is set to a few seconds. A badly set up limiter, which is essentially a non-linear auto gain amplifier (or pre-amp) is often used to prevent excessive excursions on bass drivers. A well known amplifier designer and manufacturer recommends having an amplifier of about 2 and a half times the rated power of the loudspeakers but setting the amplifier levels conservatively. This is far easier to do than with active limiting which more often than not requires a specialist to set up. Special equipment is also often required for the set up of a limiting circuit for ACCURATE reproduction of the audio signal. See these two pages on limiters and loudspeaker protection:
a) Speaker Failure Analysis – Rod Elliott
What loudspeakers are you driving:
Loudspeakers are not all the same. A primitive boombox is more often than not easier to drive than a good quality loudspeaker system. Voice coils and crossover networks, cabling and connectors all offer resistive and reactive loads to the amplifier. Often resellers match loudspeakers to amplifiers. Not knowing the one from the other, a user may make a purchase which sounds horrible at home but really cooked on the showroom floor. Room accoustics are one thing, speakers that are difficult to drive, totally another. Some of the most expensive loudspeaker systems are not known for their efficiency neither their quality when driven from the wrong source. Good power amplifier design almost always covers all factors. The loudspeaker system is essentially a reactive load. The loudspeaker is an electric motor with all the reactive properties which means that there will be currents leading or lagging the voltages which means a phase angle. In amplifiers what is the maximum phase angle allowable. In real terms loudspeakers make an awkward load compounded by the fact that many utilise crossover networks formed by additional capacitance and inductance. Loudspeaker manufacturers are not concerned about the loading on the source, just that the amplifier must be designed to drive them. Interesting but a fact.
Read up on electrostatic speakers.
To sum up
To sum up I think it is very relevant to have a very good understanding of amplifier output power and how it is calculated. By this you need to know first of all, the most important formula in all things electronic: Ohm’s Law. Know the differences between average power and RMS power. Know the difference between feeding a square wave into a loudspeaker and a pure sine wave. Know the inductive nature (or better still, the reactive nature) of loudspeakers and their crossovers.
- Most amplifiers fail because of abuse.
- Purchase an amplifier for it’s designed use. (Power supplies and transistor SOA).
- Never drive an amplifier into clipping. Loudspeakers love pure sine wave.
- Ensure adequate ventilation.
- Ensure designed for load impedance.
- Never ground the loudspeakers of a a bridged amplifier.
- Valve amplifiers – never disconnect a load. Semiconductors – never short the output.
- Turn down the amplifier volume before switching on.
- Do not compress the audio signal to limit drive.