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Author: N.H. Crowhurst
Another kind of control often required in amplifiers is the tone control. This name is used to describe an arrangement that will continuously adjust frequency response, increasing or reducing the high- and the low-frequency output. A tone control usually acts to boost or reduce the frequencies toward one end of the range amplified. (The control for the high-frequency end is called a treble control, for audio purposes, and that for the low-frequency end is called a bass control.)
A fixed circuit that adjusts frequency response to a curve previously decided upon is called an equalizer. (Sometimes equalizers may also be required to remove undesired resonances, which requires another kind of circuit.) These circuits are called equalizers, because they are usually needed to equalize for the characteristics of something else in the system: to correct for the response deficiencies of a microphone, recorder, pickup, playback head on a tape machine, a loudspeaker, for the studio or listening-room acoustical characteristics, or in some instances, for personal preference in the kind of sound desired.
Tone control and equalizer circuits can work in one of two basic ways. One system adjusts the frequency response of an amplifier on the way through, while the other adjusts it by varying the amount of different frequencies fed back. There are also two kinds of adjustment to the response. One is a reduction of some frequencies in comparison with the rest. The other is an accentuation or boost of some frequencies compared with the rest. Unless we add another stage the amplifier has only a certain amount of total gain and a tone-control circuit containing only resistances, capacitances, and possibly inductances, cannot give us any more gain. For this reason the only way to accentuate some frequencies is to cut down frequencies in the rest of the range.
A simple way to achieve tone control is to use a voltage divider between two stages, rather like the method used for volume control. (Here, however, the voltage divider is fixed rather than variable.) A variable voltage divider connected in parallel with the fixed divider through two small capacitors will affect the high frequencies only. (This occurs because the small capacitors block current to the resistors in the variable voltage divider at lower frequencies.)
Control of the low frequencies can be achieved by inserting capacitors in series with the resistor used for dividing the audio voltage. A capacitance in series with the lower resistor in the drawing will develop a considerable audio voltage, particularly at the low frequencies, resulting in a larger voltage being passed on to the next stage at the extremely low frequencies than over the rest of the audio range. Putting a capacitor in series with the upper resistor develops the greater part of the voltage at the low frequencies and reduces the amount developed across the lower resistor* This produces an attenuation or loss of the extremely low frequencies.
By combining the arrangement and using a potentiometer across the two capacitors, we provide a continuous adjustment that will go from bass boost to bass cut. Most modern tone control circuits combine the two arrangements with two controls, one for the treble boost and cut, the other for bass boost and cut.
An alternative system of tone control uses feedback. In this case, a similar control arrangement is placed in the feedback network. The action of this control is the reverse of that just discussed. If more of the high frequencies are fed back, the amplification at the high frequencies is reduced, resulting in treble cut. If less of the high frequencies are fed back, then the amplification of the high frequencies is increased, resulting in a treble boost.
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