[Part 1 reviews a brief history of op amps and then looks at
various op amp properties from a perspective of audio design.
Part 2 looks at distortion in BJT and JFET-input op amps, and
using rail bootstrapping to reduce common-mode distortion.]
Selecting the Right Op-Amp In audio work, the 5532 is pre-
eminent. It is found in almost every mixing console, and in a
large number of preamplifiers. Distortion is almost unmeasurably
low, even when driving 600 Ω loads. Noise is very low, and the
balance of voltage and current noise in the input stage is well
matched to moving-magnet phono cartridges; in many applications
discrete devices give no significant advantage. Large-quantity
production has brought the price down to a point where a
powerful reason is required to pick any other device. The 5532
is not, however, perfect. It suffers common-mode distortion. It
has high bias and offset currents at the inputs, as an
inevitable result of using a bipolar input stage (for low noise)
without any sort of bias-cancellatio