Equal loudness circuit using tap potentiometer

Figure 1. 88 shows several equal loudness control circuits composed of tap potentiometers
. In the figure (a),
the A RC compensation network is connected between the tap of the potentiometer and the input terminal and "ground" respectively. When the sliding point of the potentiometer is rotated near the tap
(i.e. low volume), the high and low frequencies can be obtained. compensate. Its principle is as
follows: when the volume is low, the input signal is
divided by RP and the attenuation is large. However, because the capacitive reactance of cl decreases with the increase of frequency , the high audio
component in the signal can pass through cl and the potentiometer tap, directly sent to the output end, the attenuation is greatly reduced, that is, the treble is relatively improved; the island connected between the potentiometer tap and the ground has a large capacity, and can attenuate the medium and high audio frequencies at the same time, that is, the It has the effect of relatively enhancing the bass ; the maximum attenuation of the middle and high notes (that is, the relative improvement of the bass) is determined by the size of R. When the volume is high, the sliding point of the potentiometer moves to the upper end, away from the tap position, and the upward RC component has no effect, and the frequency response of the output signal is flat. The compensation effect of this loudness control circuit is related to the tap position of the potentiometer : the higher the tap position, the earlier the compensation will take effect, but the amount of improvement will decrease.     Figures (b), (c) - (d) are other figures. A commonly used compensation circuit form. Figure (b) is a double-tap type with a more complicated circuit, but it can provide appropriate compensation for large, medium and small volumes and has the best effect. Figure (c) is a single-tap conversion type. When the switch is set to the "1" position, it is suitable for normal signal input like circuit (a) ; when it is set to the "2" position, the compensation network loses its function and is suitable for strong signal input. Figure (d) is a simple single-tap type, which only compensates for bass, not treble, and is mostly used in popular amplifiers.