Build Your Perfect Paraphase Tone Control

Description

Unlike the widely used Baxandall circuit (established in 1952!), a ‘paraphrase’ tone control provides a straight frequency response as long as the bass and treble controls are positioned identically. This unique characteristic makes the ‘paraphase’ configuration interesting if only the treble or bass needs to be adjusted – simultaneous adjustment of both is not possible. Essentially, the difference in the settings of the tone controls determines the frequency response slope, and the degree of bass/treble correction. The circuit is remarkably simple, based on two networks – C1-C2-C3/R9-R10-R11 and C5-C6-C7/R12-R13-R14.

Picture of the project:

The first network is designed for high frequencies (treble) adjustment, and the second is for low frequencies (bass). The roll-off points, along with C4 and C8, are selected to ensure that the combined output signals re-emerge with a ‘straight’ frequency response at the output. Roughly equal output levels from the networks are maintained by R6, set at 7.15 k and R8, set at 6.80 k. However, the operational principle requires the input signals to the two networks to be out of phase. For optimal operation, the networks are driven by two buffers that provide some additional gain.

Circuit diagram:

The gain of IC1.D is slightly higher than that of IC1.C to ensure the overall response curve remains as flat as possible when the tone controls are set equally. Because each network introduces a loss of approximately 1.72 (times), IC1.D and IC1.C first amplify the signal. The gain is set at around 8 (times), allowing input signals up to 1 V to pass through the circuit at maximum gain and without distortion. The gain also compensates for attenuation if you prefer to keep the tone controls at their mid-positions for a straight response.

Parts and PCB layout:

For audio enthusiasts, the circuit offers rewarding experimentation, particularly concerning the crossover point of the two networks. R3 and R4 determine the control range, which can be increased (within limits) by using lower resistor values here. The specified values ensure a tone control range of approximately 20 dB. IC1.B buffers the summed signal across R15. C9 removes any DC-offset voltage, and R16 protects the output buffer from the effects of excessive capacitive loads. R17, finally, maintains the output at 0 V.

The choice of the quad opamp is relatively uncritical. Here, the unassuming TL074 is used, but even rail-to-rail opamps can be applied as long as they are stable at unity gain. Also, monitor the supply voltage range. A simple circuit board was designed for the project. Linear-law potentiometers can be directly mounted onto the board. Two boards are required for a stereo application. The relevant connections on the boards are then wired to a stereo control potentiometer.

Specification:

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