Audio Tone Controls
CIRCUIT
OP_TONE1.CIR Download the SPICE file
Many people at one time or other, regardless of their interest in electronics, have adjusted these controls to suit their preference. Whether its to boost the bass of their favorite CD, cut the noise / static on a talk radio station or compensate for poor loud speaker response, they've reached for the treble and bass tone controls. The nice thing is that these circuits are straightforward to understand and implement. And the only difference between the treble and bass circuits is the location of a single capacitor.
TREBLE CONTROL
What should the frequency response of the treble circuit look like? Ideally, the gain should be flat at low frequencies. At higher frequencies, the gain can be cut or boosted by turning the control knob Clockwise (CW) or Counter-Clockwise (CCW). This amazing feat is accomplished via the simple inverting operational amplifier and several cleverly placed components.
The key to understanding its operation is to look at the low and high frequency extremes. First, at low frequencies, the capacitors open up leaving this circuit.
This is classic inverting amplifier with gain G = - R2 / R1. The R3-R5 resistor string just hangs between the input and output, not disturbing the summing point (negative input node) or the overall gain. Alternatively, at high frequencies the capacitor looks like a short.
Again, its the inverting amplifier, but with its gain modified by the parallel resistors R3-R5.
MAX BOOST With the pot turned fully CW, how is the gain (G = -R2/R1) increased? Because R3 parallels R1, its reduced resistance in the equation's denominator causes the gain to increase. The maximum gain is calculated by
R2 is also decreased to some degree. But, because R4 is very large, R2 gets reduced by a much lesser amount.
MAX CUT How much are the high frequencies cut with the control knob fully CCW? Just slide the wiper of the pot forward, and see the effect on gain. The gain is reduced by R5 directly paralleling R2.
±3dB FREQUENCY When set to the maximum boost or cut, what is the frequency where the response increases or decreases by +3 or -3dB (1.414 or 0.707)? It's estimated by
f-3dB = 1 / (2 ∙ π ∙ R1 ∙C1)
assuming a few things like R1 = R2, R3 = R4, R1 >> R3 and R1 << R4. Given the initial circuit values, the treble cuttoff frequency is around f-3dB = 1 / (2∙π ∙100kΩ∙820pF) ≈ 2 kHz.
CIRCUIT INSIGHT Run a SPICE simulation of OP_TONE1.CIR. Plot the AC Response of the treble output at V(3). Because the pot R4 is initially set at midpoint, the response should be flat! Ho-hum - nothing exciting at this point.
POTENTIOMETER MODEL To model the 500 kΩ pot R4, just break it up into two resistors, R4A and R4B.
This uninspiring, but useful model, lets you set easily the pot according to
| Position | R4A | R4B | |
| CW | 0.5 kΩ | 500 kΩ | |
| Mid | 250 kΩ | 250 kΩ | |
| CCW | 500 kΩ | 0.5 kΩ |
or any position in between. Just remember that R4A + R4B ≈ 500 kΩ.
Now, set the pot fully CW: R4A = 0.5 kΩ and R4B = 500 kΩ. Rerun the simulation. Has the gain increased at higher frequencies? Set the pot the fully CCW position by swapping the R4A and R4B values. Did the higher frequencies get cut?
HANDS-ON DESIGN Need more treble boost and cut? Reduce R3 and R5 to values like 5 or 2 kΩ. Modify your circuit and look for your added boost or cut.
BASS CONTROL
The bass control circuit induces some Deja Vu at first glance except for the position of one component - the capacitor. This time C1's mission is letting R3-R5 affect the gain at low frequencies, while leaving R1 and R2 alone at high frequencies. Based on your newly acquired treble control experience, understanding the bass control should be a snap.
Looking at the frequency extremes, you have this circuit at high frequencies
R1 and R2 are equally effected by R3-R5 and the circuit maintains a unity gain. At the more interesting low frequencies you have
Notice, this circuit bears a striking resemblance to the treble control at high frequencies.
MAX BOOST AND CUT The maximum boost or cut are
±3dB FREQUENCY The ±3dB frequency is estimated by
f-3dB = 1 / (2 ∙ π ∙ R3 ∙C1)
assuming R1 = R2, R3 = R4, R1 >> R3 and R1
<< R4. the bass cuttoff
frequency is around
f-3dB = 1 / (2∙π ∙10kΩ∙82nF) ≈ 200 Hz.
CIRCUIT INSIGHT Simulate the circuit OP_TONE1.CIR. Plot the AC Response of the Bass output at V(13). With the pot at midpoint, the response should be flat! Now adjust the pot either fully CW or CCW to boost or cut the low frequencies. Run a new simulation and view the new shape of your audio spectrum.
HANDS-ON DESIGN Do you need more bass boost and cut? Reduce R3 and R5 by a factor of 2 to 10. ( Just remember to increase C1 by the same factor! ) Modify your circuit for an added bass control range.
MID-RANGE BOOST AND CUT
You may say - not only treble and bass controls, but we've also spun the mid-range controls to sculpt the sound as we like it! The mid-range filter is just a combination of a treble and bass filter. This sounds like the makings of a future design topic. In the meantime, try including a capacitor in both locations, one across R4 and one to the wiper of R4. Initially try capacitors with the same value. And thinking ahead even further, maybe we could stack a few of these mid-range controls together, each tuned to a different frequency, to create a nice graphic equalizer!
SPICE FILE
Download the file or copy this netlist into a text file with the *.cir extention.
OP_TONE1.CIR - TREBLE AND BASS TONE CONTROLS * * INPUT VOLTAGE VS 1 0 AC 1 * * TREBLE CONTROL (HIGH-PASS FILTER) R1 1 2 100K R2 2 3 100K XOP1 0 2 3 OPAMP1 * C1 2 5 820PF R3 1 4 10K * ADJUST POT: CW = R4A < R4B, CCW R4A > R4B R4A 4 5 250K R4B 5 6 250K * R5 6 3 10K * * * BASS CONTROL (LOW-PASS FILTER) R_1 1 12 100K R_2 12 13 100K XOP2 0 12 13 OPAMP1 * C_1 14 16 82NF R_3 1 14 10K * ADJUST POT: CW = R4A < R4B, CCW R4A > R4B R_4A 14 12 250K R_4B 12 16 250K * R_5 16 13 10K * * * OPAMP MACRO MODEL, SINGLE-POLE * 1-pos in, neg in, 6-output * .SUBCKT OPAMP1 1 2 6 * INPUT IMPEDANCE RIN 1 2 10MEG * GAIN BW PRODUCT = 10MHZ = DCGAIN x FPOLE1 * DCGAIN=100K AND FPOLE1=1/(2*PI*CP1*RP1)=100HZ EGAIN 3 0 1 2 100K RP1 3 4 1K CP1 4 0 1.5915UF * OUTPUT BUFFER AND RESISTANCE EOUT 5 0 4 0 1 ROUT 5 6 10 .ENDS * * ANALYSIS .AC DEC 10 1HZ 100KHZ * * VIEW RESULTS .PRINT AC VM(3) .PROBE .END
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