Design Article
Op amps in small-signal audio design - Part 3: Selecting the right op amp
Douglas Self
7/27/2011 12:04 PM EDT
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.
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.
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.
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 4.31: 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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Related links:
Op amps in small-signal audio design - Part 1: Op amp history, properties | Part 2: Distortion in bipolar and JFET input op-amps
PRODUCT HOW-TO: Differential line driver with excellent load drive
Using Op Amps with Data Converters - Part 1 | Part 3 | Part 4 | Part 5
Yet More On Decoupling, Part 4: Op amp macromodels: A cautionary tale
Discrete audio amplifier basics - Part 1: Bipolar junction transistor circuits | Part 2: JFETs, MOSFETs and other circuit configurations
Op amps: to dual or not to dual? Part 1 | Part 2
Are you violating your op amp’s input common-mode range?
Distortion in power amplifiers, Part I: the sources of distortion | Part II: The input stage | Part III: The voltage amplifier stage | Part VII: frequency compensation and real designs
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Dr. HD
7/28/2011 7:18 PM EDT
It isn't true that the LM4562 has no "single" version. The LM4562 is actually the dual of the LME49710 and is therefore exactly the same as the LME49720. The LM4562 was released first, and the single version was then developed. By the time the single was released, National had changed their numbering scheme; the original plan was to phase out the LM4562 number and use just LME49720, but the former had gained too much traction in the market place. Interestingly enough, the LM4562 is less expensive, in small quantities at least.
National have some other very impressive op amps in the LME series, most notably the LME49713, which is a current-feedback op amp with similarly ultra-low distortion, ultra-wide bandwidth, ultra-high slew rate, lower noise, and higher output current capability (at least ±93 mA)
Other op-amps worth mentioning are the new AD8597 (single) & AD8599 (dual) from TI. These have been released since Doug's book was published and are recommended by TI over the AD797; which is very nice of TI given that the AD797 is considerably more expensive!
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Dr. HD
7/29/2011 5:22 AM EDT
doh! The AD8597, 8599 and 797 parts are of course from Analog Devices. Note to self: the clue's in the part number!
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Rene Prevo
8/3/2011 7:29 AM EDT
The article talks about low noise, but what are levels of the noise in the audio range (1Hz to 20000 Hz)?
RP
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Hughston
8/3/2011 5:35 PM EDT
To calculate the noise in the audio bandwidth you have to add all the noise components in an RMS fashion then multiply them by a bandwidth factor like 1.57 x sqrt (BW). The factor depends upon the slope of the filtering beyond the 3 dB points. 1.57 in this case is for single pole filtering. There are app notes for this on the ADI and Intersil web sites.
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kendallcp
8/3/2011 5:50 PM EDT
That's not quite accurate. You need to multiply the noise spectrum by the frequency response that it's exposed to, and then rms it up (square it and integrate over the bandwidth). If the response is a single-pole low-pass, that 1.57x factor (over sqrt(BW))pops up automatically from the integration. The methods are equivalent if the noise density is flat with frequency, but it often isn't.
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sinsinsin49
8/5/2011 3:07 AM EDT
What I don't understand is that the modern, real audio opamps: LME49990, OPA1611, and OPA211 were omitted! They all are superior in performance compared to those that have been presented.
With properly designed LME49990 based circuitry it is actually possible to do 24-bit quality analog work.
Please explain.
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Jay Sinnett
8/10/2011 10:31 AM EDT
In the circuit description of the 5532, Doug says he doesn't understand the function of Q14. I am not the designer, but I think he was on the right track when he referred to clamping. It's plain to see that Node 3 rides at about 2Vbe above the neg supply (Q8 + Q9). If node 3 (the collector of Q9) starts to go below 1Vbe, then Q14 turns on and sucks current out of Node 2, limiting the drive to Q8 + Q9. In effect, this prevents the collector voltage of Q9 from ever going below about 1Vbe. In other words, it prevents Q9 from going into "hard saturation." Hard saturation causes slow recovery time - so the purpose of Q14 is to keep the circuit recovery time fast whenever the output stage has approached the negative rail. At least that's my guess.
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