Definition of Engineering Phrases
"A"
"B",
"C",
"D",
"E",
"F",
"G",
"H",
"I",
"J",
"K",
"L",
"M",
"N",
"O",
"P",
"Q",
"R",
"S",
"T",
"U",
"V",
"W",
"X",
"Y",
"Z"
3-Band Active Tone Control
This topic continues the coverage of audio tone controls. The first entry started with a Passive Tone Control circuit using different RC filter configurations and introduced an active filter. The second entry showed a fully designed 2-Band Active Tone Control circuit, and now this third section covers a 3-Band Active Tone Control circuit.
Component values are sometimes provided as examples, but the final filter characteristics are left to the designer, although design equations are provided. Alternate configurations are possible for any of the circuit schematics, as these are all just low-pass or high-pass filters [active filters in the case of the Op Amps].
This audio tone control combines adjustments [RC filters] for a Base control a Midrange control and a treble control all in the same circuit. The design combines an operational amplifier, as the active element, with a passive filter section. The design also uses a buffer amplifier between the external input to the circuit and the actual tone control filters.
Tone Control Circuit
As indicated this circuit contains three individual filters, or one filter with three individual controls, that adjust the low band [Base], mid band [Mid-range] and high bands [Treble] of the audio range.
3-Band Active Tone Control
Input Buffer
As with the previous active tone control design the first part of the circuit is nothing more than a buffer and its presence does not effect the operation of the filter [tone control]. If the implementation does not require a buffer, than the first TL082 amplifier does not need to be used. That includes the 10k input bias resistor, and the 5uF blocking capacitor. The 1uF capacitor at the output of the Op Amp is also a DC block and is also not part of the filter circuit, but should be used as isolation between the filter and what ever input circuit is connected to it.
Passive Filter
The configuration of the passive filter section differs from the previous examples. This circuit uses an individual adjustable resistor for each frequency band and a different resistor-capacitor pair per section. Notice how the [high-pass] treble circuit uses a series capacitor to the potentiometer, the [low-pass] bass circuit uses a capacitor in parallel with the potentiometer, and the [band-pass] mid-range circuit uses both a series and parallel capacitor. Of course the values of the components are changing based on the 3dB point of the filters.
Operational Amplifier
The Op Amp used for this circuit example uses an TL082 Dual JET operational amplifier. Although that is just another example and many other amplifiers could be substituted for the Op Amp. The TL082 has an internally trimmed input offset voltage so does not require an Off-Set Voltage Adjustment. The TLO82 also does not include a frequency compensation pin, so no compensation capacitor is required. However the amplifier still requires a 0.01uF capacitor between each supply voltage pin and ground [not shown in the schematic for simplicity].
The TL082 is a dual package Op Amp, so there are two individual amplifiers per package [which share power lines]. Both amplifiers are used in the circuit above, or one if the buffer circuit is not required. The TL082 is available in either a 8-pin DIP Package [through-hole] or a 8-pin SOIC Package [surface mount].
For government usage the TL082MJGB is defined in the government vendor drawing; 5962-9851501; OP AMP, DUAL, J-FET INPUT.
Related Audio Circuits;
Base Adjustment, lower frequency compensation.
Treble Adjustment, higher frequency compensation.
Midrange Adjustment Circuit, mid-frequency compensation.
Loudness Adjustment Circuit, Tone and volume control switch.
Crossover Network, multi-speaker separation
Audio Amplifier Circuit, Operational Amplifier.
Audio Dual Channel Amplifier Circuit, Operational Amplifier.
Audio Mixer Circuit, Operational Amplifier.
