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The function of tone controls is to provide frequency-selective amplification. The tone control permits manual adjustment of the frequency response of an amplifier. Any fixed low-frequency, or high-frequency, compensating network can be changed to a tone control circuit by substituting a manually variable circuit element for one of the fixed circuit elements. The two types of control that are most frequently used are the bass-boost and treble-cut controls.
A control giving low-frequency boost is shown in the figure below. This control is similar to the low-frequency compensation network of the previous section, except for the addition of a variable resistance across the capacitor. This resistance serves to control the amount of bass boost.
Design a bass-boost control for the second stage of the two-stage transistor amplifier. Design requirements are as follows.
Low-frequency turnover: 800 Hz
Bass boost: 20 dB
The design procedure is as follows. Input impedance of second stage and bias network is approximately 875 ohms.
For a 20-dB boost at low frequencies,
For a 800-Hz turnover (cut-off frequency),
R' must be chosen to keep the loss at low frequencies down to a negligible amount.
If R' > 10 × 875 ohms, say 10,000 ohms, this loss is about 1 dB.
The coupling capacitor is broken into two sections to prevent interference with the transistor bias, and to keep direct current out of the bass control. The minimum value of the series combination of C' and C'' should be about 10 μF, and C'' must be large compared to C. For a practical design, choose C' = 15 μF and C'' = 30 μF. The figure below shows the response of such a tone control. The control element (R') should have a logarithmic taper.
The function of bass boost is in reality one of attenuating the midband by a constant amount. In general, an additional stage must be used to make up for this loss.