Scratch Filter Circuit and Calculator

A scratch filter is an audio circuit that removes the scratch noise inherent in vinyl records. It is usually a low-pass filter that rolls-off at around 10 kHz thereby severely attenuating the high-frequency end of the audio spectrum. There are three main types of filter circuits, passive networks, transistors based, and op amp based, and they all perform to varying standards. An active op amp filter usually performs the best and requires the least amount of external components. In this article we look at an op amp based circuit design that is accompanied by a calculator so that you can tailor-adjust the corner frequency for your specific needs.

The default parameters set in the calculator are for a two-pole (-12 dB/Octave) active low-pass Butterworth filter (Q=0.707), with pass-band gain Ao=-1, corner frequency Fc=10 kHz, and supply voltage +12 V. In this calculator we start by choosing a value for capacitor C1, which is an arbitrary choice, and the calculator then computes the values of the rest of the filter components.

Initial Parameters

Nearest Component Values Chosen

Trim Function Calculator

This part of the calculator is for the trim function. In this part, you enter the chosen component values and it calculates the corner frequency you are likely to get. This is useful if you wish to try other values of R2 and R3 to see how close they bring the overall characteristics to the desired corner frequency.

Full Component List