Download Audio Tone Control Using the TLC074

Application Report
SLOA042 - JANUARY 2000
Audio Tone Control Using The TLC074
Operational Amplifier
Dee Harris
Mixed-Signal Products
ABSTRACT
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.
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Circuit Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3 Tone Control Frequency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
List of Figures
1
2
3
4
5
6
7
Tone Control BLock Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tone Control Composite Frequency Response Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Audio Tone Control Schematic Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bass and Treble Tone Control Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Maximum Bass and Treble Boost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bass and Treble Flat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Maximum Bass and Treble Cut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
3
4
5
6
6
7
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1 Introduction
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 Baxandall1, 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).
2 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.
C1
Feedback Network
U2:A
In
R1
(Log)
mid
U2:D
2×
Buffer Amplifier
Out
Tone Control
Filter Amplifier
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 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).
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Audio Tone Control Using The TLC074 Operational Amplifier
SLOA042
20
Full Boost
15
Output Level – dB
10
5
Flat
0
–5
–10
–15
Full Cut
–20
15
100
1k
10k
20k
f – Frequency – Hz
Figure 2. Tone Control Composite Frequency Response Range
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.
Audio Tone Control Using The TLC074 Operational Amplifier
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VDD
VDD
R3
20 kΩ
U1 = TLV2461
Single Op-Amp
U2 = TLC074
Quad Op-Amp
+
C3
10 µF
C5
0.1 µF
_
mid
+
C4
2.2 µF
R4
20 kΩ
U1
GND
R19
10 kΩ
R In
C11
3300 pF
R14
10 kΩ
R10A
100 kΩ
C1
2.2 µF
R16
3.3 kΩ
R7
20 kΩ
R5
10 kΩ
R1†
50 kΩ
C7
0.033 µF
_
U2:A
R18A
100 kΩ
C12
3300 pF
R11
10 kΩ
C9
0.033 µF
C15 ‡
100 Ω
_
+
Vol
U2:D
+
mid
C6
0.1 µF
mid
Bass
Treble
R19
47 kΩ
R Out
mid
VDD
R12
10 kΩ
C8
0.033 µF
C13
3300 pF
R15
10 kΩ
L In
R10B
100 kΩ
C2
2.2 µF
R2†
50 kΩ
R17
3.3 kΩ
R8
20 kΩ
R6
10 kΩ
R13
10 kΩ
_
R18B
100 kΩ
C14
3300 pF
C16 ‡
100 Ω
_
C10
0.033 µF
+
Vol
+
U2:C
U2:B
mid
L Out
R20
47 kΩ
mid
mid
† 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 the
specified resistor values. Input capacitor C1 blocks dc and sets the minimum
low-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 a
perceived linear response in volume, proportional to the physical position of the
slider. The adjustment range of the buffer amplifier is from 0 V to approximately
2 times (6 dB) the audio signal input voltage.
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Audio Tone Control Using The TLC074 Operational Amplifier
SLOA042
The tone adjusting action in each channel of the tone control circuit is provided
by an equalizing amplifier (or active filter) created by placing a
frequency-dependent negative feedback network around an operational
amplifier. Almost any overall gain-versus-frequency characteristic can be defined
by the design of the feedback network.
3 Tone Control Frequency Response
The overall tone control circuit frequency response can be shifted up or down by
changing the values of capacitors C7, C9, C11, and C12 in the tone adjusting
networks.
To shift the frequency response downward, for example, increase the values of
the 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 break
frequency upward a full octave. (1 octave up = Cx and 1 octave down = 2 Cx.)
½
Case A: C7, C9 = 0.033 µF
C11, C12 = 3300 pF
20
15
Output Level – dB
10
5
0
Case B
Case A
–5
–10
–15
Case B: C7, C9 = 0.068 µF
C11, C12 = 6800 pF
–20
15
100
1k
10k
20k
f – Frequency – Hz
Figure 4. Bass and Treble Tone Control Response
Note that to keep the boost and cut break frequencies the same, the value of C7
must 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 if
desired, to maintain the overall shape and symmetry of the response, all four
capacitors must be increased or decreased by the same factor.
Audio Tone Control Using The TLC074 Operational Amplifier
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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.
THD + Noise vs Frequency
Output Gain vs Frequency
2
Vo = 170 mV
Vo = 170 mV
20
1
15
10
Output Gain — dB
THD + Noise — %
0.5
0.2
0.1
0.05
5
0
–5
–10
0.02
–15
0.01
0.005
20
–20
100
1k
20
10k 20k
1k
100
10k 20k
f – Frequency – Hz
f – Frequency – Hz
Figure 5. Maximum Bass and Treble Boost
THD + Noise vs Frequency
Output Gain vs Frequency
Vo = 100 mV
Vo = 100 mV
20
1
15
Output Gain — dB
THD + Noise — %
0.5
0.2
0.1
0.05
10
5
0
–5
–10
0.02
–15
0.01
0.005
20
–20
100
1k
10k 20k
20
100
f – Frequency – Hz
Figure 6. Bass and Treble Flat
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Audio Tone Control Using The TLC074 Operational Amplifier
1k
f – Frequency – Hz
10k 20k
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THD + Noise vs Frequency
Output Gain vs Frequency
2
Vo = 60 mV
Vo = 60 mV
20
1
15
Output Gain — dB
THD + Noise — %
0.5
0.2
0.1
0.05
10
5
0
–5
–10
0.02
–15
0.01
0.005
20
–20
100
1k
f – Frequency – Hz
10k 20k
20
100
1k
10k 20k
f – Frequency – Hz
Figure 7. Maximum Bass and Treble Cut
4 Summary
The familiar Baxandall Hi-Fi tone control is updated from vacuum tubes with
modern high-performance operational amplifiers. Two channels (for stereo) can
be implemented using only two very small IC packages and a few small passive
components. This design places the frequency determining components in the
feedback loop of an operational amplifier, reducing distortion and insertion loss,
and providing symmetrical boost and cut responses.
5 References
1. Negative-Feedback Tone Control, by P. J. Baxandall, Wireless World, October 1952.
Audio Tone Control Using The TLC074 Operational Amplifier
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Audio Tone Control Using The TLC074 Operational Amplifier
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