reference fig7 illustrates an active delay circuit comprised of the single op amp 340 capacitors 342 344 346 and 348 and resistors 350 352 354 and 356 which provides a relatively constant group delay up to the limit of human hearing with negligible variation in frequency response the circuit as shown provides up to 360 u00b0 of phase shift and additional similar circuits may be cascaded to provide the desired delay period as described above since the circuit efficiently gives two real or complex poles and symmetric zeroes it provides a flat response limited only by component shortcomings and assures unity gain at low frequencies independent of the rc tolerances without requiring trimming thus the circuit as shown delays for a predetermined time dependent upon component selection the input signal 12 before it is output via intermediate terminal l as the delayed input signal 68 as shown in fig8 and explained above the detection signal 64 produced by the transient noise detection unit 24 is applied via signal path 74 and intermediate terminals k to the control input of electronic switch 72 in the normal operating state the pair of complementary field effect transistors fet 358 and 360 are gated so that the delayed input signal 68 applied to the fet 358 via intermediate terminal l is allowed to pass through fet 358 and resistor 362 to the basic switch output point 364 in this state fet 360 is gated so that it does not pass the estimate signal applied via intermediate terminal n and capacitor 366 upon application of the detection signal.