Audio Electronics

Posted on Feb 4, 2014

How to use a desktop computer to solve the complex arithmetic needed to design state-variable filters for audio use. Construction of a tunable notch filter for measuring harmonic distortion is featured. Audio Amateur Corp. did a beautiful job (as always)of editing and presenting my article. For those of you who are planning to build my notch filter however, several typos

Audio Electronics
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snuck in which should be clarified. Figure 5 on page 13 has both inputs on the left labeled as "Speaker Level Input". J1, the differential input, is correctly labeled, but J2, the single ended input, should have been labeled "Line Level Input". In the schematic diagrams, I didn`t show the power supply connections to the opamps, but probably should have. Connect +15 Vdc (filtered, regulated, and bypassed) to pin 8 of each LF353. Connect -15 Vdc (filtered, regulated, and bypassed) to pin 4 of each LF353. On page 14 under the heading "Constructing the Prototype" I make a statement regarding the input preamp U4B that "distortion was higher when I configured the preamp for unity gain. " The increase in distortion is actually due to a design error on my part which is easily corrected. It turns out that JFET input opamps have a unique distortion mechanism that only comes into play when used in a non-inverting circuit, as is U4B in Figure 5. Input capacitance varies with common mode voltage, making the inputs non-linear at high frequencies when the source impedance is high. This is the case for the inverting input of U4B when unity gain is selected. The solution is to configure SW3 so that R26 is shunted by a much smaller parallel resistance when unity gain is selected. The following circuit diagram shows the correction: I learned about this issue from a Burr-Brown data sheet, available online at: OPA 134 Audio Operational...

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