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Tuesday, October 13, 2009

Simple Audio Graphic Equalizer Circuit

This is a design circuit for audio graphic equalizers, that are very common as commercial products but circuits for them are very rarely published. This circuit is a simple design circuit. The circuit is needs an op-amp for amplifying the input signal. This is the figure of the circuit.

Only one gyrator stage is shown: all 7 gyrators are the same circuit, only the capacitors change, as shown in the chart. I have shown three of the seven faders to show where they go. A gyrator is a circuit using active devices and transistors to simulate an inductor. In this case the gyrator is the transistor acting with R1, R3 and C2. It could just as easily be a unity gain op-amp. The circuit includes three formulae: one which gives f, the the centre frequency of the band. The second shows how the Q is related to the capacitor ratio. The third shows the impedance presented by the circuit. Note that this includes 3 terms, the first purely resistive, the second is the capacitive contribution from C1 and the third is an inductive term from the gyrator.

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