See how a bad 20W amplifier design can ruin an entire speaker system

Many engineers who work with speakers and amplifiers will tell you the same thing. If the amplifier is overworked, it will more or less damage the driver of the speaker. This process usually involves gradually turning up the bass knob, or sharply raising the volume knob. What would be the result of doing so?

It can damage the speaker’s tweeter driver. But why does this result occur? Most tweeter drivers are designed for the 10W to 15W power range. Very little energy is required to drive them at high frequencies. Midrange and woofers are generally rated for the average power of the entire speaker (50W, 100W, etc.).

Consider what happens when you add gain to a sine wave in a clipping system (playing music with a fixed power rail). At this point, the signal begins to clip. If you drive a signal beyond the limit, the waveform starts to look more like a square wave. From a frequency domain perspective, we start to get the input signal harmonics. The harmonics have higher amplitudes due to the presence of a lot of clipping. Now, if you use a passive crossover, many of the higher order harmonics can easily be crossed over from the mid-bass driver to the treble.

Since high-frequency-oriented drivers are extremely low-power, they have a much higher chance of damage. This is a real problem in many systems, especially those using simple analog processing (eg: op amps, op amps) or digitally controlled analog EQ systems. Two better solutions are:

1. bi-amp system

If in a closed system, eg: a powered speaker, etc., consider bi-amping your system. Bi-amping allows you to use a single amplifier to drive the treble. The separation between the treble and bass is done before the low frequency gain, which prevents the treble from damaging the high frequency portion of the limited bass channel.

A bi-amp system lets you run most analog systems with highly flexible digital adjustments. The downside is the high cost of adding an amplifier. However, we have to make a compromise between a good passive crossover and the cost of the extra amplifier. Using a digital divider in a digital-to-analog converter (DAC) or multimedia digital signal codec (Codec) can alleviate this problem to some extent.

Fine-tuning your crossover digitally is much simpler than swapping out various passive components. Doing so also allows the same PCB design to be reused for different sizes of cabinets and speaker drivers. Note that this system only works if you have direct access to both drives individually.

2、Intelligent post-processing to limit the bass signal

Some developers use “soft clipping”. It’s a very simple method, but it’s rare in home audio systems. Generally speaking, we will increase the low frequency bass frequencies to the highest. Some developers remove the 24 dB of bass boost to compensate for the low-frequency response of the small 2-inch driver.

If the boost frequency is generally low, try adding a low-pass filter after the gain stage to reduce the high frequencies caused by clipping. In analog systems, building such a low-pass filter with a sufficiently high cutoff rate typically requires a multi-order filter, making the system bulky and expensive. However, it can be easily implemented in digital processing systems, provided there are enough effective MIPS in the audio processor.picture2 An example of the processing flow at soft clipping is shown.

picture1high gainDRC and low-pass filter processing flow

From portable audio amplifiers like the TLV320AIC325x family of devices to the newer PCM514x home audio miniDSP DACs, these programmable miniDSP products are capable of soft limiting. Some smarter implementation methods depend on the vast number of system developers to innovate. Each device integrates a fully programmable miniDSP core, freeing developers from the constraints of a fixed process flow and the need to copy someone else’s audio system design methodology.

picture2usePLimitLimits output and ensures no high frequency harmonics

For some, this content may not be new. But for others, after reading this article, it may be clear “So that’s why my speakers broke!”

Next time, we’ll discuss how to find noise and artifacts in high-speed DACs, so stay tuned.

The Links:   LM14X79 DMF-50260NF-FW-15