Opamps for Active Crossovers Rs = 4k7 and Rs = 10 k traces it is abundantly clear that, as usual, source impedances need to be kept low when there is a large CM signal. The results are significantly better than the 5532 in voltage-follower mode­compare above. 409 13.19 The OP27 Opamp The OP27 from Analog Devices is a bipolar-input single opamp primarily designed for low noise and DC precision. It was not intended for audio use, but in spite of this it is frequently recommended for such applications as RIAA and tape head preamps. This is unfortunate, because while at first sight it appears that the OP27 is quieter than the 5534/5532, as the e n is 3.2 nV/rtHz compared with 4 nV/rtHz for the 5532, in practice it is usually slightly noisier. This is because the OP27 is in fact optimised for DC characteristics, and so has input bias-current cancellation circuitry that generates common-mode noise. When the impedances on the two inputs are very different the CM noise does not cancel, and this can degrade the overall noise performance significantly, and certainly to the point where the OP27 is noisier than a 5532. For a bipolar input opamp, there appears to be a high level of common-mode input distortion, enough to bury the output distortion caused by loading; see Figures 13.28 and 13.29. It is