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Signal chain basics: How 20W amplifier can ruin a speaker system

Posted: 24 Apr 2012 ?? ?Print Version ?Bookmark and Share

Keywords:speakers? amplifiers? bi-amping? miniDSP?

Many engineers who tinkered 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

As 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.

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.

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.

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