With bass and treble controls set to maximum boost (both
wipers at the top of resistors VR1 and VR2), and the inactive components greyed
out, the circuit will look like Fig. 0.0.4. Both bass and treble potentiometers
that may have either linear or logarithmic tracks depending on the circuit
design, are much higher values than other components in the circuit, and so
with the VR1 and VR2 wipers set to maximum resistance both potentiometers can
be considered to be open circuit. Nor does C4 contribute to the operation of
the circuit because of the high resistance of VR2, and C1 is effectively
shorted out by the wiper of VR1 being at the top end of its resistance track.
The full bandwidth of signal frequencies is applied to the
input from an amplifier having low output impedance, and the higher frequency
components of the signal are fed directly to the output of the tone control
circuit via the 2.2nF capacitor C3, which has a reactance of about 3.6KΩ at
20kHz but over 3.6MΩ at 20Hz, so blocks the lower frequencies.
The full band of frequencies also appear at the junction of
R1 and C2, which together form a low pass filter with a corner frequency of
around 70 to 75 Hz and so frequencies appreciably higher than this (the mid and
high frequencies) are conducted to ground via R2.
Having R2 in series with C2 prevents the attenuation of the
mid band frequencies exceeding about -20dB. The lower frequencies are fed to
the output via R3. Because R3 has quite a large value (to effectively isolate
the effects of the two variable controls from each other, the input impedance
(Zin) of the circuit following the tone control must be very high to avoid
excessive signal loss due to the potential divider effect of R3 and the Zin of
the following stage.
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