This very simple passive circuit provides a simple audio shaping function. It was described by Derek Bowers in Electronic Design in February, 2012.
Early radio receivers and record players nearly always had a knob marked “tone,” which was usually a crude low-pass filter with some form of severity adjustment. At best, these controls could partially compensate for bass loss caused by poor speaker baffling. More sophisticated controls were developed for more modern equipment, including the bass/treble (Baxandall) controls, parametric equalizers, and graphic equalizers.
Nevertheless, a single-adjustment tone control often can be useful for fine balance adjustment or where multiple controls are impractical or unnecessary. A simple version of such a control provides a symmetric response that is flat at the center of the adjustment range.
Moving the control in one direction simultaneously boosts the treble and cuts the bass until about 5.5 dB of boost and 23 dB of cut are obtained. Moving the control in the other direction boosts the bass and cuts the treble in an identical fashion. Figure 2 shows the typical curves obtained from 20 Hz to 20 kHz with a 1-kHz center frequency, for the lower half and upper half of the control range, respectively.
Naturally, for stereo, the circuit would be duplicated and VR1 substituted with a dual, ganged component. Since the circuit is purely passive, it is easy to insert in the signal chain. However, it must be preceded by a low impedance (below 100 Ω) and followed by a high impedance (more than 250 kΩ) for best results. Under these circumstances, insertion loss (at center) approaches 6 dB.
Since this is a purely passive circuit, all component values may be scaled without affecting the ac transfer function. With lower resistor values (and higher capacitor values), the signal-to-noise ratio improves, but the circuit requires a lower impedance to drive it. The values shown represent a good compromise. Overall signal-to-noise ratio in a 20-kHz bandwidth is about –113 dB referenced to 1 V rms with the control in the center position.
Much more complex audio processing circuits are frequently used, but this has a low parts count supported by typical junk boxes. You could scale this design by multiplying all resistor values by a common factor while dividing all capacitor values by that same factor (with some slop to use standard values). For example, 100Ω and 1800Ω resistors and 0.1 uF capacitors.
With lower resistor values you will need even an input stage with an even lower impedance. One simple rule of thumb would be to keep the impedance of the driving stage below half that of the smaller resistor value, and the impedance of the following stage above half that of the variable resistor.
The variable resistor should be a linear one, and ideally it would have a center detent.
Early radio receivers and record players nearly always had a knob marked “tone,” which was usually a crude low-pass filter with some form of severity adjustment. At best, these controls could partially compensate for bass loss caused by poor speaker baffling. More sophisticated controls were developed for more modern equipment, including the bass/treble (Baxandall) controls, parametric equalizers, and graphic equalizers.
Nevertheless, a single-adjustment tone control often can be useful for fine balance adjustment or where multiple controls are impractical or unnecessary. A simple version of such a control provides a symmetric response that is flat at the center of the adjustment range.
Moving the control in one direction simultaneously boosts the treble and cuts the bass until about 5.5 dB of boost and 23 dB of cut are obtained. Moving the control in the other direction boosts the bass and cuts the treble in an identical fashion. Figure 2 shows the typical curves obtained from 20 Hz to 20 kHz with a 1-kHz center frequency, for the lower half and upper half of the control range, respectively.
Naturally, for stereo, the circuit would be duplicated and VR1 substituted with a dual, ganged component. Since the circuit is purely passive, it is easy to insert in the signal chain. However, it must be preceded by a low impedance (below 100 Ω) and followed by a high impedance (more than 250 kΩ) for best results. Under these circumstances, insertion loss (at center) approaches 6 dB.
Since this is a purely passive circuit, all component values may be scaled without affecting the ac transfer function. With lower resistor values (and higher capacitor values), the signal-to-noise ratio improves, but the circuit requires a lower impedance to drive it. The values shown represent a good compromise. Overall signal-to-noise ratio in a 20-kHz bandwidth is about –113 dB referenced to 1 V rms with the control in the center position.
Much more complex audio processing circuits are frequently used, but this has a low parts count supported by typical junk boxes. You could scale this design by multiplying all resistor values by a common factor while dividing all capacitor values by that same factor (with some slop to use standard values). For example, 100Ω and 1800Ω resistors and 0.1 uF capacitors.
With lower resistor values you will need even an input stage with an even lower impedance. One simple rule of thumb would be to keep the impedance of the driving stage below half that of the smaller resistor value, and the impedance of the following stage above half that of the variable resistor.
The variable resistor should be a linear one, and ideally it would have a center detent.
