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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