Audio Tone Control Using The TLC074 Operational Amplifier

Audio Tone Control Using The TLC074 Operational Amplifier

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On: 01 Des, 2018

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This application report describes the design and function of a stereo high-fidelity tone control using a single TLC074 quad operational amplifier. A rail-to-rail operational amplifier is used to provide a midpoint supply voltage and signal ground,...

This application report describes the design and function of a stereo high-fidelity tone control using a single TLC074 quad operational amplifier. A rail-to-rail operational amplifier is used to provide a midpoint supply voltage and signal ground, allowing the use of a single power supply.


Tone controls allow the frequency response of the audio system to be adjusted to compensate for the response of speakers and their enclosures or the listening room, or to simply provide a more pleasing sound. In this design, a variation of the classic and very popular Baxandall 1 , negative feedback tone control circuit provides the familiar Hi-Fi tone control with cut...flat...boost response for both bass and treble frequencies. A single bass control adjusts both channels simultaneously and a single treble control adjusts both channels. A pair of slide pots adjusts the volume of each channel independently. The circuit provides a gain of 6 dB at the maximum volume setting when both tone controls are at their midpoints (flat).

Circuit Description

Each of the two separate channels of the tone control circuit is basically an active filter built around an IC operational amplifier. An active filter design was chosen over a passive filter circuit because active filters have the frequency-response adjusting components located in the feedback loop of the filter amplifiers, providing much lower THD, little or no insertion loss, and a symmetrical response about the axis in both boost and cut, compared with most passive designs. Each channel also includes an input buffer amplifier to provide some gain and isolation from source impedance variations.

A block diagram of the right channel of the tone control circuit is shown in Figure 1. The left channel is identical.

audio tone control schematic diagram.jpg

One dual-element slide pot adjusts the bass response from approximately –20 dB of cut, to flat, to approximately 20 dB of boost for both channels simultaneously. Another dual-element slide pot adjusts the treble response of both channels through the same range. Mid-range frequencies are not affected by the tone controls. An overall flat response (no boost or cut at frequency extremes) is obtained when the tone controls are at their mid-point position. The composite frequency response range curves shown in Figure 2 are provided by the component values indicated in the schematic (Figure 3).

tone control block diagram.jpg

A single TLC074 quad operational amplifier IC contains all the amplifiers required for buffering and filtering both left and right channels. A TLV2461 operational amplifier IC is connected to provide a virtual ground for proper operation of the TLC074 from a single supply voltage.

Figure 3 shows the tone control schematic diagram.

tone control composite frequency response range.jpg


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