Audio Tone Control Using The TLC074 Operational Amplifier
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On: 25 Jul, 2014
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Are you want to make audio tone control? In this time i want to write about audio tone control using TLC074 like on title above. The design and function of a stereo high-fidelity tone controlusing a single TLC074 quad operational amplifier. A rail...
Are you want to make audio tone control? In this time i want to write about audio tone control using TLC074 like on title above. The design and function of a stereo high-fidelity tone controlusing 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 powersupply.
Tone controls allow the frequency response of the audio system to be adjustedto compensate for the response of speakers and their enclosures or the listeningroom, or to simply provide a more pleasing sound.In this design, a variation of the classic and very popular Baxandall1, negativefeedback tone control circuit provides the familiar Hi-Fi tone control withcut...flat...boost response for both bass and treble frequencies. A single basscontrol adjusts both channels simultaneously and a single treble control adjustsboth channels.A pair of slide pots adjusts the volume of each channel independently. The circuitprovides a gain of 6 dB at the maximum volume setting when both tone controlsare at their midpoints (flat).
2. Circuit Description
Each of the two separate channels of the tone control circuit is basically an activefilter built around an IC operational amplifier. An active filter design was chosenover a passive filter circuit because active filters have the frequency-responseadjusting components located in the feedback loop of the filter amplifiers,providing much lower THD, little or no insertion loss, and a symmetrical responseabout the axis in both boost and cut, compared with most passive designs. Eachchannel also includes an input buffer amplifier to provide some gain and isolationfrom source impedance variations.A block diagram of the right channel of the tone control circuit is shown inFigure 1. The left channel is identical.
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 channelssimultaneously. Another dual-element slide pot adjusts the treble response ofboth channels through the same range.Mid-range frequencies are not affected by the tone controls. An overall flatresponse (no boost or cut at frequency extremes) is obtained when the tonecontrols are at their mid-point position.The composite frequency response range curves shown in Figure 2 are providedby the component values indicated in the schematic (Figure 3).
A single TLC074 quad operational amplifier IC contains all the amplifiers requiredfor buffering and filtering both left and right channels. A TLV2461 operationalamplifier IC is connected to provide a virtual ground for proper operation of theTLC074 from a single supply voltage.
Figure 3 shows the tone control schematic diagram.
The input buffer amplifier provides a gain of approximately 2 (RF/RIN) with thespecified resistor values. Input capacitor C1 blocks dc and sets the minimumlow-frequency response of the tone control circuit at approximately 16 Hz (–3 dB)with the value of 2.2 μF. Volume control R1 has an audio taper to provide aperceived linear response in volume, proportional to the physical position of theslider. The adjustment range of the buffer amplifier is from 0 V to approximately2 times (6 dB) the audio signal input voltage.
The tone adjusting action in each channel of the tone control circuit is providedby an equalizing amplifier (or active filter) created by placing afrequency-dependent negative feedback network around an operationalamplifier. Almost any overall gain-versus-frequency characteristic can be definedby the design of the feedback network.
3. Tone Control Frequency Response
The overall tone control circuit frequency response can be shifted up or down bychanging the values of capacitors C7, C9, C11, and C12 in the tone adjustingnetworks.To shift the frequency response downward, for example, increase the values ofthe capacitors in the tone adjusting networks. Doubling the values of C7, C9, C11,and C12 shifts the break frequency downward a full octave (Case B, Figure 4).Conversely, halving the values of C7, C9, C11, and C12 shifts the breakfrequency upward a full octave. (1 octave up = ½Cx and 1 octave down = 2 Cx.)
Note that to keep the boost and cut break frequencies the same, the value of C7must equal that of C9, and the value of C11 must equal that of C12. In addition,although the bass and treble break frequencies can be adjusted separately ifdesired, to maintain the overall shape and symmetry of the response, all fourcapacitors must be increased or decreased by the same factor.
Figures 5, 6, and 7 are measurements from the TI Tone Control EVM (SLOP109),using the TLC074 IC amplifier. All measurements taken with Vdd = 5 V, Vin =100 mVrms, RL = 47 kΩ, and f = 3 kHz.
The familiar Baxandall Hi-Fi tone control is updated from vacuum tubes withmodern high-performance operational amplifiers. Two channels (for stereo) canbe implemented using only two very small IC packages and a few small passivecomponents. This design places the frequency determining components in thefeedback loop of an operational amplifier, reducing distortion and insertion loss,and providing symmetrical boost and cut responses.
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