AEL

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====== Signal Integrator/Phase Shifter ======

In some audio systems, particularly subwoofer and active crossover applications, it is desirable to adjust the phase of a signal without significantly affecting its amplitude. This allows better integration between speakers, compensation for acoustic delays, and improved overall system response. The circuit shown here is a simple active phase shifter based on a TL071 operational amplifier, capable of providing a continuously adjustable phase shift from approximately 0° to 180°. By varying a single control, the phase relationship between the input and output signals can be altered while maintaining an essentially constant signal level.

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**Fig. 1: phase integrator circuit**

===== Circuit Description =====

This circuit is an **active all-pass phase shifter**, sometimes called a phase integrator. Its purpose is to change the phase of an audio signal without significantly changing its amplitude. As the control is adjusted, the output signal can be shifted anywhere from approximately 0° to 180° relative to the input.

The circuit is built around a TL071 op-amp powered from +/-15V supplies. Unlike a conventional amplifier where the feedback network mainly determines gain, this circuit uses a combination of direct input signal and frequency-dependent feedback through capacitor C1 to produce a controlled phase shift.

The audio signal enters at "IN", which is the input to the circuit. One path goes directly through resistor R1 to the inverting input network, while another path passes through capacitor C1 to the non-inverting input of the op-amp. Because the capacitor's reactance changes with frequency, the phase relationship between these two signal paths changes as frequency changes.

Resistor R2 provides feedback from the output to the inverting input. The values of R1 and R2 are equal, which is typical for active all-pass filter designs because it helps maintain approximately unity gain while allowing the phase to vary.

The key component is potentiometer RV1. It is connected between the inverting input node and ground, acting as a variable resistance that changes the feedback ratio seen by the op-amp. As the resistance is adjusted, the balance between the direct signal and the feedback signal changes, altering the amount of phase shift produced by the all-pass network.

When RV1 is adjusted toward its minimum resistance, the inverting input node is pulled closer to ground, producing one extreme of phase shift. As RV1 is increased, the op-amp operates with a different feedback condition and the phase shift gradually changes toward the opposite extreme. The result is a continuously adjustable phase shift from approximately 0° to 180°.

Importantly, RV1 is not a volume control. Ideally, the output amplitude remains nearly constant while only the phase changes. In practice there may be a small level variation due to component tolerances, but the primary function of RV1 is phase adjustment.

Circuits like this are commonly used in subwoofer systems where the acoustic phase between the subwoofer and main speakers needs adjustment for proper blending. They can also be found in active crossovers, studio equipment, and experimental audio processing circuits where phase alignment is important. The reason the control is labeled "PHASE" is that rotating RV1 changes the timing relationship between the input and output waveforms rather than their amplitude.

RV1 should be a linear taper, not logarithmic.

===== Test Results =====

I built the circuit on breadboard and injected a 1kHz sine in to the input pin "IN". Here are some screenshots of the oscilloscope's output. The yellow trace is the input signal, and the blue is the output.

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**Fig. 2: output with control set to minimum; effectively a 0° phase shift. Technically, it'll never be quite 0.**

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**Fig. 3: output shows roughly a 90° phase shift (blue trace) relative to the input signal (yellow trace).**

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**Fig. 4: with the control rotated fully clock-wise, the output (blue trace) shows a 180° phase shift relative to the input signal (yellow trace).**