Loudness Control
Circuit : Andy Collinson
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When listening to music at low volume levels, bass and treble frequencies are attenuated more than mid frequencies. This loudness control alters the frequency response curve to correspond roughly with the equal loudness characteristic of the ear. The circuit shown, is for a single channel, so for a stereo system, two such units should be built. This circuit has a boost of 13dB at 20Hz and approximately 9dB at 20kHz.

loudness control circuit Specification
Power 12 Volts nominal 5mA current drain
Gain 13dB @ 20Hz, 9dB @ 20kHz
Input Impedance 27Kohm @ 1Khz

This circuit uses a three stage, passive, RC filter to tailor the frequency response, followed by an op-amp in non-inverting mode to provide the gain. The frequency response is shown below:
Frequency Response
frequency response plot
Frequency Shaping Network
C1, is the input capacitor, and blocks any DC from preceding stages. R1,C2, R4,C5 are the first stage of a frequency shaping network. Each stage behaves as a frequency dependant attenuator and is easier to understand with the following diagram: frequency shaping network

R2 and C1 form one branch of an attenuator redrawn as XL1, and C4 and R5 form the second branch, equivalent to XL2. Across the audio spectrum, the impedance of XL1 and XL2 will vary, offering different amounts of attenuation at different frequencies.

The impedance of C2 is chosen at the treble frequency, 20KHz to be small compared to R2, and at the bass frequency, 20Hz, C5 is chosen to have a high impedance. The result is a constantly changing impedance network, as the effective values of XL1 and XL2 vary. You can think of the network as a frequency dependant attenuator, and the frequency response of this single network is shown below: frequency response

Cascading two more of the same stages, results in a more pronounced peak at the bass and treble frequencies. As all three stages are passive, the op-amp is needed to provide the necessary gain. Gain in the op-amp is set by R9 and R10, C9 in parallel with R9 rolls off the high frequency response above 20kHz.

Input Impedance
The input impedance will change, which is expected and the impedance across the audio spectrum is shown below:
input impedance

The op-amp is made by Analog Devices, type LT101. If not available, other low noise op-amps can be used instead such as LF071, NE5534, etc. All plots made with LTspice.

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