Implement an audiofrequency tiltequalizer filter
Figure 1 In a tilt audiotone control, the tilt control tilts the frequency content of the audio signal by simultaneously boosting the treble and cutting the bass frequencies, or vice versa.
Quad Ltd never published a transfer function for the filter. You need a Spice simulation and many trialanderror cycles to tune it to your desired response. By deriving the transfer function, you can easily select the component values. Surprisingly, the transfer function also shows how you can make the tilt response asymmetric, with different amounts of boost and cut. You begin deriving the transfer function by expressing the input versus the output as a function of dcfeedback resistor, R_{F}, and Z, the complex impedance of the RC branches:
Figure 2 This frequency response is for the extreme wiper positions, where X=0 or P_{1}. All of the other responses, with 0 less than X and X less than P_{1}, lie between these curves.
The frequency response in Figure 2 is for the extreme wiper positions, where X=0 or P_{1}. All of the other responses, with 0 less than X and X less than P_{1}, lie between those curves. To get the frequency responses in decibels, multiply the log of the absolute value of the transfer function by 20: 20log(T_{F}). To get a log/log scale on the graph, substitute 10^{F} for F on the X axis. Pivot frequency F_{P} depends on component value, including the setting of potentiometer P_{1}, as it sweeps between an X value of 0 and P_{1}, where R_{F} must be greater than R:
Figure 3 R_{F} is 16.66 kΩ, R is 7.14 kΩ, and C is 12.24 nF.
Figure 4 Voltages V_{I}, V_{O}, and V are all referred to ground.
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