The negative feedback loop is closed before the filter to prevent instabilities. Feedback cannot be taken from the speaker because of the phase shift of the output filter, which varies from 0 at dc to nearly 360 at 120 kHz. Since the filter is linear, feedback may be taken from the filter input, which has no phase shift. Unfortunately, this point is a high frequency square wave which must be integrated to determine its average voltage. The input is mixed with the square wave output by resistors R4 and R5 shown in Figure 2. The resultant signal is integrated, which accurately simulates the effect of the output filter. The output of the integrator will be zero only if the filter input is an accurate inverted reproduction of the amplifier input. If the output is higher or lower than desired, the integrator will generate a negative or positive error voltage. This error voltage is applied to the input of the switch controller, which makes the required correction. The integrator introduces a 90 phase shift at high frequencies which leaves a phase margin of nearly 90.