Last year, an audio notch filter with adjustable frequency was published in Elektor. To complete the audio measuring station, Elektor 3-4/2025 presents an audio generator with a high quality whose frequency can be continuously adjusted. The centrepiece of both circuits is a so-called ‘state variable filter’. In principle, it consists of integrators which, with the help of suitable feedback, lead to a high quality bandpass filter despite a simple basic circuit, which cannot be achieved with other circuit concepts. Together with an active amplitude control in the feedback, a high-quality audio generator can be built.

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