Overload

Two things you should never do: tease an angry dog, or feed an amplifier too high a frequency. Both will result in bad stuff. Your rear end may be riddled with teeth marks from the dog, and bright and aggressive sound is likely from your amplifier. The dog's obvious but perhaps the amplifier is less so. Any amplifier with high open loop gain reduced to reasonable levels by feedback can react poorly when fed a signal beyond its capabilities. Walt Jung discussed some of this problem in a paper I mentioned earlier Slewing Induced Distortion or SID. Jung, you'll recall, was the author of the first book I read on audio design, and the lessons learned from my journey - whose course was set by his words - helped me understand the subject we are discussing today. One of the problems faced by DAC designers is the very issue at the heart of this post, high frequency overload. Designers without a great deal of experience will tend to take the easy route when it comes to filtering high frequency noise from the output of a DAC. The DAC's output is fed directly into an amplification device and the noise reducing low pass filter is wrapped around the amplifier in a feedback loop. The results of this classic technique are twofold: great measurements, and bad sound. When viewed on test equipment, noise is lowered as expected and the DAC passes even the closest scrutiny. But if the designer takes the unusual step of listening to the end result (yes, believe it or not this part of the design process is rare), he may be mystified at the aggressive nature of what comes through his speakers. There are several ways he can address this problem. The first would be to change the speed of the amplifier circuit so it no longer goes into overload, but this path has other problems that can lead to quicksand. Tomorrow I'll tell you a better way.