SIGNAL CHAIN BASICS #64: How a poorly designed 20W amplifier can destroy a speaker system
Many engineers who played with speakers and amplifiers will tell you a similar story from their youth. When they pushed an amplifier too hard, somehow they blew a driver in their loudspeakers. This usually included some tale of turning the bass knob higher and higher, or increasing the volume knob significantly. So what happened?
They probably blew the tweeter driver in their loudspeaker. But, why did it happen? Most tweeters are designed to drive between 10W and 15W. Only a small amount of energy is needed at high frequencies to drive them. Mid-range and woofers typically are rated for the average power of the overall loud speaker (50W, 100W, and so on.).
Consider what happens when adding gain to a sine wave in an amplitude-restricted system, or music with fixed supply rails. At some point, the signal starts to clip. If you drive a signal beyond clipping, the wave begins to look more like a square wave. With a frequency domain view, we start getting input signal harmonics. With large amounts of clipping come much higher amplitudes on the harmonics. Now many higher order harmonics can easily make their way from bass and midrange drivers to the tweeters, if you have a passive crossover
Because tweeters are rated for far-lower power, the chances of causing damage are much higher. This is a real problem in many systems, especially those running with simple analog processing like operational amplifiers (op amps), or digitally-controlled analog EQ systems. Here are two good solutions:
1. Bi-amping the system
If in an enclosed system such as an active speaker, consider bi-amping your system. Bi-amping lets you drive the tweeter from a separate amplifier. Providing the split between tweeter and woofer is done before the gain on low frequencies, you can isolate the tweeter from damaging high-frequency content of the clipping bass channel.
A bi-amped system allows you to continue running a mostly analog system with the added flexibility of digital tuning. The downside is added cost for the extra amplifiers. However, tradeoffs must be made between a good passive crossover and the cost of the extra amplifier. Using a digital crossover in the digital-to-analog converter (DAC) or codec can ease some of this pain.
Tuning your crossover digitally is much simpler than swapping out different passive components. This also allows the same PCB design to be reused for different size cabinets and speaker drivers. Note that this kind of system only works where you have direct access to both drivers separately.
2. Smart post-process clipped bass signal
Some developers rely on “soft limiting.” It’s a very simple concept, but is rarely seen in home audio systems. Typically, we give the most post-processing to boost low-frequency bass frequencies. Some developers throw 24 dB of bass boost in an attempt to compensate for the poor frequency response of a small two-inch driver.
If the frequencies in the boost are mainly low ones, try adding a low-pass filter after the gain stage to roll off clipping-induced higher frequencies. In an analog system, creating this kind of low-pass filter with a high enough cutoff rate usually requires a multi-order filter, making the system large and expensive. However, in a digital processing system this can be done with ease, providing there are enough MIPS available in the audio processor.
Figure 1 shows an example of a process flow where soft limiting takes place, with digital range control (DRC).
Figure 1: Process flow with high-gain DRC and low-pass filter.
Programmable miniDSP products, from portable audio class such as TLV320AIC325x series of devices to newer PCM514x home audio-grade audio miniDSP DACs, have the ability to implement soft limiting. Smarter implementations are based around the ingenuity of the system developer. Each device integrates a fully programmable miniDSP core, freeing developers from the confines of a fixed processing flow that ties them to someone else’s concept of a good audio system. A power-limiting circuit (PLimit) sets a limit on the output peak-to-peak voltage, Figure 2.
Figure 2: Using PLimit to limit the output
and thus ensure no high-frequency harmonics.
(Click here to see enlarged image)
For some, this may be old information. But for others, it may be the “so that’s why my speakers blew!” moment.
About the Author
Dafydd Roche is the home audio strategic marketing and systems engineer for the Audio Converter group at Texas Instruments. An avid musician in his spare time, Dafydd pours his passion and knowledge of audio and music-making into his work.