Paraphase Tone Controller

The
first is for the high frequencies (treble) response, the second, for
the low frequencies (bass). The roll-off points have been selected, in
combination with C4 and C8, for the sum of the two output signals to
re-appear with a ‘straight’ frequency response again at the output.
Roughly equal output levels from the networks are ensured by R6 = 7.15 k
and R8 = 6.80 k. However, the operating principle requires the input
signals to the two networks to be in anti-phase. For best operation the
networks are driven by two buffers providing some extra gain.

The
gain of IC1.D is slightly higher than that of IC1.C to ensure the
overall response curve remains as flat as possible at equal settings of
the tone controls. Because each network introduces a loss of about 1.72
(times), IC1.D and IC1.C first amplify the signal. The gain is set at
about 8 (times) allowing input signal levels up to 1 V to pass the
circuit at maximum gain and distortion-free. The gain also compensates
the attenuation if you prefer to keep the tone controls at the mid
positions for a straight response.

To
audio fans, the circuit is rewarding to experiment with, especially in
respect of the crossover point of the two networks. R3 and R4
determine the control range, which may be increased (within limits) by
using lower resistor values here. The values shown ensure a tone
control range of about 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 too high capacitive loads. R17, finally,
keeps the output at 0 V.

The choice of the quad opamp is
relatively uncritical. Here the unassuming TL074 is used but you may
even apply rail to rail opamps as long as they are stable at unity
gain. Also, watch the supply voltage range. A simple circuit board was
designed for the project. Linear-law potentiometers may be fitted
directly 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.

Spec:

• Current consumption (no signal) 8 mA
• Max. input signal 1 Veff (at max. gain)
• Gain at 20 Hz 13.1 dB max. –6.9 dB min.
• at 20 kHz 12.2 dB max. –7.6 dB min
• Gain (controls at mid position) 2.38 x
• Distortion (1 Veff, 1 kHz) 0.002% (B = 22kHz) 0.005% (B = 80 kHz)

COMPONENTS LIST
Resistors
R1-R4 = 10k
R5,R7 = 1k
R6 = 7k15
R8 = 6k80
R9,R10,R11 = 8k2
R12,R13,R14 = 2k2
R15 = 1M
R16 = 100R
R17 = 100k
P1,P2 = 100k preset or chassis-
mount control potentiometer, linear law
Capacitors
C1,C2,C3 = 47nF MKT, lead pitch 5mm
C4 = 68nF MKT, lead pitch 5mm
C5,C6,C7 = 10nF MKT, lead pitch 5mm
C8,C10,C11 = 100nF MKT, lead pitch 5mm
C9 = 2µF2 MKT, lead pitch 5mm or 7.5mm
Semiconductors
IC1 = TL074
Miscellaneous
K1,K2 = line socket, PCB mount, e.g.
T-709G (Monacor/Monarch)