5 Power amplifiers
When a power amplifier is forced past its maximum output power, it will clip. This means the output voltage of the amp is equal to the supply voltage from its power supply and can't go any higher. The peaks (positive as well as negative) of the signal are clipped off, hence the term "clipping". Clipping in itself isn't bad, neither for the amplifier nor the connected speaker(s). It's the compression of the signal that causes problems. When a signal is compressed (by clipping it) its average power increases, and may be over the amplifier's maximum thermal capabilities, so the amp may overheat. Furthermore, a clipped signal loses low frequency content, due to the fact that low frequencies have a larger amplitude than high frequencies. Low frequencies get clipped first, so to speak. Loss of low end will make you want to turn up higher, although you're already past clipping level at that point. This means the high frequency content will still be able to increase, and this can cause damage especially to tweeters, but also to woofers.
While power amp clipping in itself isn't bad, it should be avoided at all times, if only because power amplifiers are not designed to work outside of their specifications. You never know what ugliness your nice amp will put out if it's pushed too hard.
A socalled clip limiter or soft clip circuit may sound like a good idea, but even those don't protect your speakers from gross user error. What's more, most of them don't even prevent clipping. What they usually do is make the distortion that results from clipping sound less ugly. What they should be doing is what a real limiter does: turn down the volume at the onset of clipping.
A side effect of clipping that is often overlooked is the DC decoupling instability. Most amplifiers can't amplify DC signals (why would they need to?), so DC signals are filtered out. This is called "decoupling". This has a huge advantage for designers: the output offset adjustment, that tells the amplifier where its zero level is, can be done automatically by the amplifier itself, instead of adjusting it by hand at the time of manufacture. This self-adjustment relies on the fact that the amplified signal always has an average value of zero (equal power in both the positive and negative half of the signal). If the signal is non-symmetrical, as is the case with many musical instrument signals, clipping will be more severe on one half (positive or negative) of the signal. As a result, the output offset adjustment starts to shift, as the amplifier will try to keep its output at an average of zero Volts. In turn, this causes a very low frequency (that of the adjustment cycle) at the output. This is often visible as a strong waving of the speaker cones. Needless to say, a speaker cone that moves this much, is likely to fail .
Most amplifiers have several protection circuits on board. The most basic one, the mains fuse, is mandatory, but one or more of the following may be present:
- Output fuses: apart from the mains fuse, additional fuses are located at the outputs to the loudspeakers. They protect the amplifier from short circuit situations and/or the loudspeakers from extreme amplifier failure.
- Overheat: an overheat switch, that just shuts off the amplifier once it runs too hot.
- Short circuit: usually consists of a fuse in the speaker output, that simply blows if there's too much output current.
- DC protection: when an amplifier is clipped severely and becomes unstable or when there's a malfunction, this circuit will prevent the presence of DC at the output to protect the connected loudspeakers.
© Joris van den Heuvel 2001-2009