Even though we publish new blogs daily from your intrepid editor and from a number of well-known writers, sometimes it's good to have a look back at some of our well-received articles from the recent past. Here is one that discusses the consequences of driving a solid state amplifier too hard. With no provisions to limit output power gracefully, turning up the bass too far may blow the tweeter.
This apparent contradiction is explained in detail in an article published a year ago in Planet Analog by current author and TI audio systems engineer Dafydd Roche. You've seen his work recently here.
Let us know if these articles are of interest to you and if so, we'll post a few more.
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In an incorrectly designed amplifier the most common result of excessive input drive is distortion and the inability to deliver any more power, due to power supply limitations. BUT it is also possible to design an amplifier that keeps delivering more power as it is overdriven, with the distortion levels continuing to rise, along with the output power. So the harmonics can continue to rise, as distortion products, as the total output power increases, and the poor treble speakers get far more power than they were ever intended to handle. The result is speaker destruction. Quite pitiful.
Yep – exactly the mechanism to which Mr. Roche referred in the original article. Hence the idea of doing soft limiting.
In a very well designed amplifier, what happens is that the driver circuits don't drive into distortion. But having all of the stages perfectly matched is a lot of extra design effort.
Hi Brad–nice article and was useful to me. Not being a real analog guy I hadn't considered the idea of the square wave being the sum of all those frequencies and how that would interact with a crossover. Back when I was young and spent too much money on home audio, I can remember endless discussions of “the best” crossover designs…
Recently Daniel Chow of Altera blogged about some of these signal concepts in a nice 2-part series over on The Connecting Edge. Readers here might want to check it out:
Basics of Signal Spectra
Thanks again for the article.
I'll head over to The Connecting Edge and have a look. Thanks.
And of course extra design means extra cost. Not preferrable.
By observation it is clear that a square wave contains a lot more power than a sine wave, and then bu a simple Fourier analysis, it becomes clear that the additional energy isin the odd harmonics. Thus we have all of the amplifiers power passing through the crossover and into the highest frequency speakers, which are generally not rated to handle the speaker package's full power rating. The result is almost always the failure of a few components due to overheating.
It is fairly common misconception that the harmonics of a clipped waveform destroy tweeters. If we think about the Fourier series expansion of a square wave, you will see it contains a fundamental and all the odd harmonics. However, the level (relative to the fundamental) of each harmonic isdivided by that harmonic's order. In other words, 1/3 for the 3rd harmonic, 1/5 of the 5th, 1/9 for the 9th, etc.
Now lets think of common dance music. The bass note is going to be beating between 60 and 80 Hz. This is where clipping occurs. Now lets assume we have a 2-way system with a crossover at 1kHz. The first harmonic of a 70 Hz fundamental above the crossover point is the 15th, which is at 1/15th the input level of the fundamental.
Now lets assume our 2-way speaker has a 100W woofer and a 10W tweeter. If we send a clipped 120 watt signal to the speaker, Less than 10 watts of that clipped signal is going to the tweeter. In other words, the tweeter is not going to blow from the clipping bass signal.
Lets think about what else is happening. As you turn up the volume, the bass signal clips but the music between bass beats still increases without clipping. It is very easy for the level of this program material to exceed the power of the tweeter. In other words, the harmonic energy of the bass note is often only a minor contributor to the tweeter's failure.
Sawdust – Something to consider with regard to what signal [frequency] is being squared when you calculate the energy contained in the harmonics. In some cases, it's higher frequency bass or even mid-range signals that are clipping. Combine that with a cross-over network that isn't as good as the one you supposed in your analysis and you could blow out a tweeter. Just suggesting….