Three passive tone control circuits are shown in Figure 40. That at A incorporates a preamplifier stage shown in full. With passive tone control circuits of this type, there is an overall loss of audio so that the signal level is reduced. If the amplifier has easily adequate gain, sufficient volume can still be obtained. But if gain is already at maximum, the addition of a tone control network can result in output volume then being insufficient. This depends on the amplifier and other circumstances, arid when it arises the addition of a preamplifier will restore volume.
In A VR1 is the tone control, higher frequencies being reduced as the wiper moves towards C1. VR2 is a gain or volume control. R3 and C3 provide source bias and by-passing, and R2 is the drain audio load, with output from C4. R1 with C2 decouple the positive supply line.
Typical values for A are:
VR1 500k linear VR2 500k log.
R1 1k C1 2.2nF
R2 4.7k C2 470uF
R3 2.2k C3 47uF
FET 2N3819 C4 0.47uF
Operation is from a 12v or similar supply, and R1 can be changed if necessary for higher voltages. In this and similar circuits there is considerable latitude in the choice of values for positions such as C1.
At B VR1 is a top cut control, and VR2 the volume control. C2 is connected to the gate at G, and a 2.2 megohm resistor provides the DC path from gate to negative line, other components being R1, R2, P3, C2, C3 and C4 as at A.
Typical values for B are:
C1 10nF VR1 500k linear
C2 0.47uF VR2 500k log
Yet another top cut control is shown at C. Here, R1 and R2 are the some as R1 and R2 in A, C2 of A being included as at A. In some cases such a tone control can be added to an existing stage without any disturbance to the circuit board. C1 at C can be 47nF, and VR1 25k. Higher values may be fitted for VR1, but tend to make most of the audible effect of this control occupy only a small part of its rotation. C1 can be increased, to give increased top cut. The results obtained with various component values are influenced by the impedance of the circuit.
With such circuits, low frequencies can be made more prominent by reducing treble with VR1, then increasing gain or volume.
Other circuits are able to offer control of both treble and bass frequencies. A passive circuit of this type is shown in Figure 41.
C1 is the input isolating capacitor, and may be unnecessary with some types of input. VR1 is for treble control. Treble is lifted with the wiper towards C2, whose reactance falls as frequency rises. With VR1 wiper towards C3, treble is reduced. VR2 similarly provides lift or cut in bass. Both are linear controls, supplying the volume control VR3 by means of R1. The source and drain circuits for the FET can be as in Figure 40.
Again, values are to some extent a matter of choice, but there is little point in using components which will provide extreme degrees of cut or boost, which wilt never be required. It is also of advantage to have approximately flat reproduction with VR1 and VR2 central.
Typical values for Figure 41 are:
C1 0.47uF R1 270k
C2 470pF R2 390k
C3 1.2nF P2 18k
C4 2nF VR1 500k lin
C5 5nF VR2 1 megohm lin
FET 2N3819 VR3 1 megohm log
