Amped blogger Neville Ward is a repairs engineer, specialising in restoring poorly studio and guitar equipment to full glory. In this blog, he talks about protection circuits on amplifiers, and the simple design flaw found on many amps...
Amplifiers need protection, mainly from amplifier users, but also from loudspeakers. This is because transistors can be destroyed very quickly, much faster than a fuse can blow, and loudspeakers place a constantly varying stress on the amplifier. The result is that your bass player may end up telling you that either you do the song in a different key, or s/he will need a new bass rig.
Valves can withstand overloads for longer than a fuse takes to blow, so valve amplifiers don't need any special protection circuitry. Of course, it is possible to build a transistor amplifier that has so many transistors to share the load that they would never fail, but this would make the amplifier too expensive. Even then, it would still need protection from accident and abuse, like short circuits in the loudspeaker or cable.
All modern transistor amplifiers have protection with very few exceptions, one being some amplifiers with regulated power supplies. If the power supply can limit the power delivered to a safe level, the amplifier may not need protection circuitry of its own, but only if it has generous safety margins.
Most amplifiers have quite narrow safety margins. This is fine if the protection circuit is accurate and works fast enough, but many aren't and don't. In fact, I would say that most of the amplifiers that have crossed my bench have protection circuits that are close to useless. People often ask me what is a good amplifier to buy, but I never know, as I never see the ones that don't break.
No amplifier is 100% efficient, but most can comfortably manage well over 50%, which means that more power ends up in the load (the loudspeaker) than the amplifier. The protection the amplifier needs is not against the power that it delivers to the load, but against the power that is wasted in the amplifier.
So, for reasons of cost, size and weight, the amplifier can only actually cope with a certain proportion of the total power available from the power supply, and if all of it ends up in the amplifier, it will usually fail within a few milliseconds. A short circuit across the output terminals will do this. Protection is therefore essential, and protection against short circuits is fairly easy to implement.
Real loudspeakers present strange, complex loads to an amplifier, loads that vary constantly with frequency and sudden changes in amplitude, i.e. with music. These strange loads result in less power in the loudspeaker and more power lost in the amplifier.
The difference is not small, often amounting to five times as much power, which would need an amplifier five times the size to cope with it. Such an amplifier would be uneconomical to produce.
The protection circuit therefore needs to limit the amplifier's output when a strange load appears. This means that the amplifier will clip at much reduced power levels when driven at certain frequencies into certain loads.
It means that the power stated on the back of the amplifier is not the whole story; it is variable dependant on several conditions. Can you hear it happening? Does it make an audible difference? Yes, it does.
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