Op amps in small-signal audio design - Part 3: Selecting the right op amp

August 02, 2011 //By Douglas Self
Op amps in small-signal audio design - Part 3: Selecting the right op amp
Douglas Self examines the selection criteria for op-amp technology and surveys the most popular options.

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-cancellation circuitry.

The 5532 is not in the forefront for DC accuracy, though it's not actually that bad. The offset voltage spec is 0.5 mV typical, 4 mV max, compared with 3 mV typical, 6 mV max for the popular TL072. I have actually used 5532s to replace TL072s when offset voltage was a problem, but the increased bias current was acceptable.

With horrible inevitability, the very popularity and excellent technical performance of the 5532 has led to it being criticized by subjectivists who have contrived to convince themselves that they can tell op-amps apart by listening to music played through them. This always makes me laugh, because there is probably no music on the planet that has not passed through a hundred or more 5532s on its way to the consumer.

In some applications, such as low-cost mixing consoles, bipolar-style bias currents are a real nuisance because keeping them out of EQ pots to prevent scratching noises requires an inappropriate number of blocking capacitors. There are plenty of JFET-input op-amps around with negligible bias currents, but there is no obviously superior device that is the equivalent of the 5532. The TL072 has been used

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