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"

Active Tone Control

This topic continues from the more general Passive Tone Control circuit, which begins using only passive filters. This circuit follows the previous design, although the component values are different and in an alternate configuration.

An audio tone control combines both a Base control and a treble control in the same circuit. This 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 [not shown] and the actual tone control.

Tone Control Circuit

LM149 Active Audio Tone Control
Active Tone Control

Input Buffer

The first part of the circuit, comprising the inverting amplifier, is really nothing more than a buffer. The Operational amplifier is configured as an inverting amplifier, with the input applied to the minus input. However the circuit serves no function in regards to the tone control, unless a buffer is a requirement between the filter and presiding circuit. A 0.47uF input capacitor [value not shown] is used as a DC block with the value selected to pass audio frequencies. The 5uF capacitor at the output of the Op Amp is also a DC block and is not part of the filter circuit.

Passive Filter

The passive components with comprise the filter are set up just as the previous circuits are, with a few variations. As noted in the diagram, the top part of the filter functions as the bass control and the bottom part of the filter functions as the treble control.

The values used in this design differ from the previous circuit. However the values will always be different from circuit to circuit as the designer selects different upper and lower 3dB points for the filter.

As the diagram shows the base portion [low pass filter] of the circuit may be adjusted with a 3dB point between 32Hz and 320Hz. While the 3dB of the treble section [high pass filter] may be adjusted from 1.1Hz to 11Hz. The Base Adjustment and Treble Adjustment pages cover those portions of the circuit in more detail.

Operational Amplifier

The Op Amp used for this circuit example uses an LM149, while the previous circuit used an LM301. Although there are differences between different operation amplifiers, by design there meant to be universal parts. So in general many different parts may be substitute for the amplifier used in the actual schematic. In fact the LM148/LM149 series of parts is basically an upgrade or replacement for the universal 741 operational amplifier design.

The LM149 is a quad package Op Amp, so there are four individual amplifiers per package. Although only two amplifiers are used in the circuit above, or one if the buffer circuit is not required. Because this is a quad package, the device is available in either a 14-pin DIP Package [through-hole] or a 14-pin SOIC Package.

The LM149 is specified in MIL-M38510/110, as device type 2, with package style options of; Dual-in-line or Flat pack.

3-Band Tone Control Circuit Schematic

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.

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