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

[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-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 in this application for many years but its HF linearity is not first-class and distortion across the band deteriorates badly as output loading increases.

However, the op-amps in many EQ sections work in the shunt-feedback configuration with no CM voltage on the inputs, and this reduces the distortion considerably. When low bias currents are needed with superior performance then the OPA2134 is often a good choice, though it is at least four times as expensive as the TL072.

The question of op-amp selection is examined in much more detail in the rest of this chapter, where the most popular types are surveyed.

Op-Amps Surveyed: BJT Input Types
The rest of this chapter looks at some op-amp types and examines their performance, with the 5532 the usual basis for comparison. The parts shown here are not necessarily intended as audio op-amps, though some, such as the OP275 and the OPA2134, were specifically designed as such. They have, however, all seen use, in varying numbers, in audio applications. Bipolar input op-amps are dealt with first.

The LM741 Op-Amp
The LM741 is only included here for its historical interest; in its day it was a most significant development and, to my mind, the first really practical op-amp. It was introduced by Fairchild in 1968 and is considered a second-generation op-amp, the 709 being first generation.

The LM741 had (and indeed has) effective short-circuit protection and internal compensation for stability at unity gain, and was much easier to make work in a real circuit than its predecessors. It was clear that it was noisy compared with discrete circuitry, and you sometimes had to keep the output level down if slew limiting was to be avoided, but with care it was usable in audio.

Probably the last place the LM741 lingered was in the integrators of state-variable EQ filters, where neither indifferent noise performance nor poor slewing capability is a serious problem (see Chapter 10 for more details on this application). The LM741 is a single op-amp. The dual version is the LM747.

Figure 4.19 shows a region between 100 Hz and 4 kHz where distortion rises at 6dB/octave. This is the result of the usual dominant-pole Miller compensation scheme. When slew limiting begins, the slope increases and THD rises rapidly with frequency.

Figure 4.19: The THD performance of an LM741 working at a gain of 33, on ±15 V rails, giving 3 and 6 Vrms outputs, with no load. At 6 Vrms, slew distortion exceeds 1% before 20 kHz is reached; there is visible slew limiting in the waveform. THD is, however, very low at 100 Hz, due to the high NFB factor at low frequencies