tlc074 tone control

Application ReportSLOA042 - JANUARY 2000

1

Audio Tone Control Using The TLC074Operational Amplifier

Dee Harris Mixed-Signal Products

ABSTRACT

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

Contents

1 Introduction 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 Circuit Description 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 Tone Control Frequency Response 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 Summary 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 References 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

List of Figures

1 Tone Control BLock Diagram 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Tone Control Composite Frequency Response Range 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Audio Tone Control Schematic Diagram 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Bass and Treble Tone Control Response 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Maximum Bass and Treble Boost 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Bass and Treble Flat 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Maximum Bass and Treble Cut 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 Audio Tone Control Using The TLC074 Operational Amplifier

1 IntroductionTone 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 DescriptionEach 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.

In

Buffer Amplifier

Feedback Network

Filter Amplifier

Out

U2:A U2:D

mid

C1

Tone Control

R1(Log)

2×

Figure 1. Tone Control Block Diagram

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).

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5

–5

–15

100

f – Frequency – Hz

10

0

–10

–20

10k

15

15

Out

put L

evel

– d

B

20

20k1k

Flat

Full Boost

Full Cut

Figure 2. Tone Control Composite Frequency Response Range

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.

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VDD

C6 0.1 µF

R Out

R In

C80.033 µF

_

+

R610 kΩ

R820 kΩ

_

+

C143300 pF

R1310 kΩ

midmid

C16 ‡

100 Ω

R2047 kΩ

R10B100 kΩ R17

3.3 kΩ

R1510 kΩ

R1210 kΩ

C100.033 µF

+

R18B100 kΩ

++++++++++++

L Out

U1 = TLV2461 Single Op-AmpU2 = TLC074 Quad Op-Amp

+C3 10 µF

C133300 pF

C70.033 µF

C12.2 µF

_

+

R510 kΩ

R720 kΩ

R1†

50 kΩ

_

+

C123300 pF

R1110 kΩ

midmid

C15 ‡

100 Ω

R1947 kΩ

R10A100 kΩ

R163.3 kΩ

R1410 kΩ

R1910 kΩ

C90.033 µF

R18A100 kΩ

C113300 pF

U2:D

U2:CU2:B

U2:A

VDD

VDD

C42.2 µF

_

+

R420 kΩ

R320 kΩ

U1

C5 0.1 µF

mid

GND

Bass Treblemid

Vol

L InC2

2.2 µF

R2†

50 kΩ

mid

Vol

† Volume controls are audio taper (log) potentiometers. ‡ Component locations marked C15 and C16 on the SLOP109 EVM board should be populated with 100- Ω resistors.

Figure 3. Audio 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.

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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 = Cx and 1 octave down = 2 Cx.)

5

–5

–15

100


10

0

–10

–20

10k

15

15

Out

put L

evel

– d

B

20

20k1k

Case B Case A

Case A: C7, C9 = 0.033 µF C11, C12 = 3300 pF

Case B: C7, C9 = 0.068 µF C11, C12 = 6800 pF

Figure 4. Bass and Treble Tone Control Response

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.

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Measured Performance

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.

5

–5

–15

100


10

0

–10

–20

10k

15

20

20

20k1k

1

0.005

TH

D +

Noi

se —

%

100 10k20 1k 20k

0.01

0.02

0.05

0.1

0.2

0.5

2


Vo = 170 mV Vo = 170 mV

Out

put G

ain

— d

B

THD + Noise vs Frequency Output Gain vs Frequency

Figure 5. Maximum Bass and Treble Boost

5

–5

–15

100


10

0

–10

–20

10k

15

20

20

20k1k

1

0.005

TH

D +

Noi

se —

%

100 10k20 1k 20k

0.01

0.02

0.05

0.1

0.2

0.5


Vo = 100 mV Vo = 100 mV

Out

put G

ain

— d

B


Figure 6. Bass and Treble Flat

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5

–5

–15

100


10

0

–10

–20

10k

15

20

Out

put G

ain

— d

B

20

20k1k

1

0.005100 10k20 1k 20k

0.01

0.02

0.05

0.1

0.2

0.5

2


TH

D +

Noi

se —

%

Vo = 60 mV Vo = 60 mV


Figure 7. Maximum Bass and Treble Cut

4 SummaryThe 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.

5 References1. Negative-Feedback Tone Control, by P. J. Baxandall, Wireless World, October 1952.

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tlc074 tone control

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