Chapter 13
Ultrasonic and Scratch Filtering
Ultrasonic Filters
Scratches and groove debris create clicks that have a large high-frequency content, some of it ultrasonic and liable to cause slew rate and intermodulation problems further down the audio chain. The transients from scratches can easily exceed the normal signal level. It is often considered desirable to filter this out as soon as possible (though of course some people are only satisfied with radio-transmitter frequency responses).
If an MM input stage is provided with an HF correction pole, in the form of an RC 1st-order roll-off after the opamp, this in itself provides some protection against ultrasonics because its attenuation continues to increase with frequency and it is inherently ...
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