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

Technology News | August 1, 2011

By eeNews Europe

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

The NE5532/5534 Op-Amp

The 5532 is a low-noise, low-distortion bipolar dual op-amp, with internal compensation for unity-gain stability. The 5534 is a single version internally compensated for gains down to 3, and an external compensation capacitor can be added for unity-gain stability; 22 pF is the usual value. The common-mode range of the inputs is a healthy ±13 V, with no phase inversion problems if this is exceeded.

The 5532 has a distinctly higher power consumption than the TL072, drawing approximately 4 mA per op-amp section when quiescent. The DIL version runs perceptibly warm when quiescent on ±17 V rails.

Figure 4.20 shows that the 5532 deals well with loads up to its maximum 500 Ω. Its distortion performance is studied in detail in the section above on common-mode distortion.

Figure 4.20: Distortion is very low from the 5532, though loading makes a detectable difference. Here it is working in series feedback mode at the high level of 10 Vrms with 500 U, 1 kΩ loads and no load. The ‘Gen-mon’ trace is the output of the distortion analyzer measured directly. Gain of 3.23, supply ±18 V

The 5534/5532 has bipolar transistor input devices. This means it gives low noise with low source resistances, but draws a relatively high bias current through the input pins. The input devices are NPN, so the bias currents flow into the chip from the positive rail. If an input is fed through a significant resistance then the input pin will be more negative than ground due to the voltage drop caused by the bias current.

The inputs are connected together with back-to-back diodes for reverse-voltage protection, and should not be forcibly pulled to different voltages. The 5532 is intended for linear operation, and using it as a comparator is not recommended.

As can be seen from Figure 4.20, the 5532 is almost distortion free, even when driving the maximum 500 Ω load. The internal circuitry of the 5532 has never been publicly explained, but appears to consist of nested Miller loops that permit high levels of internal negative feedback. The 5532 is the dual of the 5534, and is more commonly used than the single as it is cheaper per op-amp and does not require an external compensation capacitor when used at unity gain.

The 5532 and 5534 type op-amps require adequate supply decoupling if they are to remain stable, otherwise they appear to be subject to some sort of internal oscillation that degrades linearity without being visible on a normal oscilloscope. The essential requirement is that the positive and negative rails should be decoupled with a 100 nF capacitor between them, at a distance of not more than a few millimeters from the op-amp; normally one such capacitor is fitted per package as close to it as possible.

It is not necessary, and often not desirable, to have two capacitors going to ground; every capacitor between a supply rail and ground carries the risk of injecting rail noise into the ground.

Deconstructing the 5532
To the best of my knowledge, virtually nothing has been published about the internal operation of the 5532. This is surprising given its unique usefulness as a high-quality audio op-amp. I believe the secret of the 5532’s superb linearity is the use of nested negative feedback inside the circuit, in the form of traditional Miller compensation.

Figure 4.21 shows the only diagram of the internal circuitry that has been released; the component and node numbers are mine. This has been in the public domain for at least 20 years, so I hope no one is going to object to my impertinent comments on it.

Figure 4.21: The internal circuitry of the 5532

The circuit initially looks like a confusing sea of transistors, and there is even a solitary JFET lurking in there, but it breaks down fairly easily. There are three voltage-gain stages, plus a unity-gain output stage to increase drive capability. This has current-sensing overload protection. There is also a fairly complex bias generator that establishes the operating currents in the various stages.

In all conventional op-amps there are two differential input signals that have to be subtracted to create a single output signal, and the node at which this occurs is called the ‘phase-summing point’.

Q1, Q2 make up the input differential amplifier. They are protected against reverse biasing by the diode-connected transistors across the input pins. Note there are no emitter degeneration resistors, which would linearize the input pair at the expense of degrading noise. Presumably high open-loop gain (note there are three gain stages, whereas a power amplifier normally only has two) means that the input pair is handling very small signal levels, so its distortion is not a problem.

Q3, Q4 make up the second differential amplifier; emitter degeneration is now present. Phase summing occurs at the output of this stage at node 2. C1 is the Miller capacitor around this stage, from node 2 to node 1. Q5, Q6, Q7 are aWilson current mirror, which provides a driven current source as the collector load of Q4. The function of C4 is obscure but it appears to balance C1 in some way.

The third voltage-amplifier stage is basically Q9 with split-collector transistor Q15 as its current-source load. Q8 increases the basic transconductance of the stage, and C3 is the Miller capacitor around it, feeding from node 3 to node 2 – note that this Miller loop does not include the output stage. Things are a bit more complicated here as it appears that Q9 is also the sink half of the Class-B output stage.

Q14 looks very mysterious as it seems to be sending the output of the third stage back to the input; possibly it’s some sort of clamp to ensure clean clipping, but to be honest I haven’t a clue. Q10 plus associated diode generates the bias for the Class-B output stage, just as in a power amplifier.

The most interesting signal path is the semi-local Miller loop through C2, from node 3 to node 1, which encloses both the second and third voltage amplifiers; each of these has its own local Miller feedback, so there are two nested layers of internal feedback. This is probably the secret of the 5532’s low distortion.

Q11 is the source side of the output stage and, as mentioned above, Q9 appears to be the sink. Q12, Q13 implement overcurrent protection. When the voltage drop across the 15 Ω resistor becomes too great, Q12 turns on and shunts base drive away from Q11. In the negative half-cycle, Q13 is turned on, which in turn activates Q17 to shunt drive away from Q8.

The biasing circuit shows an interesting point. Bipolar bias circuits tend not to be self-starting; no current flows anywhere until some flows somewhere, so to speak. Relying on leakage currents for starting is unwise, so here the depletion-mode JFET provides a circuit element that is fully on until you bias it off, and can be relied upon to conduct as the power rails come up from zero.

The LM4562 Op-Amp

The LM4562 is a new op-amp, which first became freely available at the beginning of 2007. It is a National Semiconductor product. It is a dual op-amp – there is no single or quad version. It costs about 10 times as much as a 5532.

The input noise voltage is typically 2.7 nV/√Hz, which is substantially lower than the 4 nV/√Hz of the 5532. For suitable applications with low source impedances this translates into a useful noise advantage of 3.4 dB.

The bias current is 10 nA typical, which is very low and would normally imply that bias cancellation, with its attendant noise problems, was being used. However, in my testing I have seen no sign of excess noise, and the data sheet is silent on the subject. No details of the internal circuitry have been released so far, and quite probably never will be.

It is not fussy about decoupling and, as with the 5532, 100 nF across the supply rails close to the package should ensure HF stability. The slew rate is typically ±20 V/µs, more than twice as quick as the 5532.

The first THD plot in Figure 4.22 shows the LM4562 working at a closed-loop gain of 2.2× in shunt-feedback mode, at a high level of 10 Vrms. The top of the THD scale is 0.001%, something you will see with no other op-amp in this survey. The no-load trace is barely distinguishable from the AP SYS-2702 output, and even with a heavy 500 Ω load driven at 10 Vrms there is only a very small amount of extra THD, reaching 0.0007% at 20 kHz.

Figure 4.22: The LM4562 in shunt-feedback mode, with 1 kΩ, 2k2 feedback resistors giving a gain of 2.23. Shown for no load (NL) and 1 kΩ, 500 Ω loads. Note the vertical scale ends at 0.001% this time. Output level is 10 Vrms, ±18 V supply rails

Figure 4.23 shows the LM4562 working at a gain of 3.2× in series-feedback mode, both modes having a noise gain of 3.2×. The extra distortion from 500 Ω loading is barely detectable.

Figure 4.23: The LM4562 in series-feedback mode, with 1 kΩ, 2k2 feedback resistors giving a gain of 3.23×. No load (NL) and 500 Ω load. Output 10 Vrms, ±18 V supply rails

For Figures 4.23 and 4.24 the feedback resistances were 2k2 and 1 kΩ, so the minimum source resistance presented to the inverting input is 687 Ω. In Figure 4.24 extra source resistances were then put in series with the input path (as was done with the 5532 in the section above on common-mode distortion) and this revealed a remarkable property of the LM4562 – it is much more resistant to common-mode distortion than the 5532.

Figure 4.24: The LM4562 in series-feedback mode, gain 3.23×, with varying extra source resistance in the input path. The extra distortion is much lower than for the 5532. Output 10 Vrms, ±18 V supply rails

At 10 Vrms and 10 kHz, with a 10 kΩ source resistance the 5532 generates 0.0014% THD (see Figure 4.6) but the LM4562 gives only 0.00046% under the same conditions. I strongly suspect that the LM4562 has a more sophisticated input stage than the 5532, probably incorporating cascoding to minimize the effects of common-mode voltages.

Note that only the rising curves to the right represent actual distortion. The raised levels of the horizontal traces at the LF end are due to Johnson noise from the extra series resistance. It has taken an unbelievably long time – nearly 30 years – for a better audio op-amp than the 5532 to come along, but at last it has happened. The LM4562 is superior in just about every parameter, but it has much higher current noise. At present it also has a much higher price, but hopefully that will change.

AD797, OP27, OP270 and OP275 Op-Amps

The AD797 Op-Amp
The AD797 (Analog Devices) is a single op-amp with very low voltage noise and distortion. It appears to have been developed primarily for the cost-no-object application of submarine sonar, but it works very effectively with normal audio – if you can afford to use it. The cost is something like 20 times that of a 5532. No dual version is available, so the cost ratio per op-amp section is 40 times.

This is a remarkably quiet device in terms of voltage noise, but current noise is correspondingly high due to the high currents in the input devices. Early versions appeared to be rather difficult to stabilize at HF, but the current product is no harder to apply than the 5532. Possibly there has been a design tweak, or on the other hand my impression may be wholly mistaken.

The AD797 incorporates an ingenious feature for internal distortion cancellation. This is described on the manufacturer’s data sheet. Figure 4.25 shows that it works effectively.

Figure 4.25: AD797 THD into loads down to 500 Ω, at 7.75 Vrms. Output is virtually indistinguishable from input. Series feedback, but no CM problems. Gain = 3.23×

The OP27 Op-Amp
The OP27 from Analog Devices is a bipolar-input single op-amp 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 en is 3.2 nV/√Hz compared with 4 nV/√Hz for the 5534, in practice it is usually slightly noisier.

This is because the OP27 is in fact optimized 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 – which is the case in RIAA preamps – the CM noise does not cancel, and this appears to degrade the overall noise performance significantly.

For a bipolar input op-amp, there appears to be a high-level common-mode input distortion, enough to bury the output distortion caused by loading (see Figures 4.26 and 4.27). It is likely that this too is related to the bias-cancellation circuitry, as it does not occur in the 5532.

Figure 4.26: OP27 THD in shunt-feedback mode with varying loads. This op-amp accepts even heavy (1 kΩ) loading gracefully

Figure 4.27: OP27 THD in series-feedback mode. The common-mode input distortion completely obscures the output distortion

The maximum slew rate is low compared with other op-amps, being typically 2.8 V/µs. However, this is not the problem it may appear. This slew rate would allow a maximum amplitude at 20 kHz of 16 Vrms, if the supply rails permitted it. I have never encountered any particular difficulties with decoupling or stability of the OP27.

The OP270 Op-Amp
The OP270 from Analog Devices is a dual op-amp, intended as a ‘very-low-noise precision operational amplifier’, in other words combining low noise with great DC accuracy. The input offset voltage is an impressive 75 mV maximum.

It has bipolar inputs with a bias-current cancellation system; the presence of this is shown by the 15 nA bias current spec, which is 30 times less than the 500 nA taken by the 5534, which lacks this feature. It will degrade the noise performance with unequal source resistances, as it does in the OP27. The input transistors are protected by back-to-back diodes.

The OP270 distortion performance suffers badly when driving even modest loads. See Figures 4.28 and 4.29. The slew rate is a rather limited 2.4 V/µs, which is only just enough for a full output swing at 20 kHz. Note also that this is an expensive op-amp, costing something like 25 times as much as a 5532; precision costs money. Unless you have a real need for DC accuracy, this part is not recommended.

Figure 4.28: OP270 THD in shunt-feedback mode. Linearity is severely degraded even with a 2k2 load

Figure 4.29: OP270 THD in series-feedback mode. This looks the same as in Figure 4.28 so CM input distortion appears to be absent

The OP275 Op-Amp
The Analog Devices OP275 is one of the few op-amps specifically marketed as an audio device. Its most interesting characteristic is the Butler input stage, which combines bipolar and JFET devices. The idea is that the bipolars give accuracy and low noise, while the JFETs give speed and ‘the sound quality of JFETs’. That final phrase is not a happy thing to see on a data sheet from a major manufacturer; the sound of JFETs (if any) would be the sound of high distortion. Just give us the facts, please.

The OP275 is a dual op-amp; no single version is available. It is quite expensive, about six times the price of a 5532, and its performance in most respects is inferior. It is noisier, has higher distortion, and does not like heavy loads. See Figures 4.30 and 4.31.

The CM range is only about two-thirds of the voltage between the supply rails, and Ibias is high due to the BJT part of the input stage. Unless you think there is something magical about the BJT/JFET input stage – and I am quite sure there is not – it is probably best avoided.

The THD at 10 kHz with a 600 Ω load is 0.0025% for shunt and 0.009% for series feedback; there is significant CM distortion in the input stage, which is almost certainly coming from the JFETs. (I appreciate the output levels are not the same but I think this only accounts for a small part of the THD difference.) Far from adding magical properties to the input stage, the JFETs seem to be just making it worse.

Figure 4.30: An OP275 driving 7.75 Vrms into no load and 600 U. THD below 1 kHz is definitely non-zero with the 600 Ω load. Series feedback, gain 3.23×

Figure 4.31: OP275 driving 5 Vrms into 1 k and 600 Hz U. Shunt feedback, gain 2.23×, but note noise gain was set to 3.23× as for the series case. The ‘Gen-Mon’ trace shows the distortion of the AP System 2 generator; the steps at 200 Hz and 20 kHz are artefacts generated by internal range switching

Coming up in Part 4: Selecting the right op amp: JFET-input types surveyed.

Printed with permission from Focal Press, a division of Elsevier. Copyright 2010. "Small Signal Audio Design" by Douglas Self. For more information about this title and other similar books, please visit www.elsevierdirect.com.

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