492  Opamps for Active Crossovers

Figure 16.23: The LME49990 in series-feedback mode, gain 3.2x, with varying extra source resistance in the input path; note that a 1 kΩ source resistance actually gives less distortion than none. The CM distortion is lower than for the 5532 but higher than for the LM4562. 10 Vrms out, ±18 V supply rails.

resistance, the LME49990 is not the best choice, and either the 5532, the LM4562, or theAD797 will give considerably lower distortion.

The SOIC-only format of the LME4990 is not helpful for experimentation or home construction. I soldered my samples to an 8-pin DILadaptor and plugged that into a prototype board without any problems. The LME49990 samples were kindly supplied by Don Morrison.

The AD797 Opamp

TheAD797 (Analog Devices) is a single opamp 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 opamp section is 40 times. TheAD797 incorporates an ingenious error-correction feature for internal distortion cancellation, the operation of which is described on the manufacturer’s data sheet. The measurements presented here seem to show that it works effectively.

Opamps for Active Crossovers  493

Figure 16.24: LME49990 distortion in a voltage-follower circuit with a selection of source resistances;­ test level 10 Vrms, supply ±18 V.

The AD797 is a remarkably quiet device in terms of voltage noise, but current noise is correspondingly high due to the large standing currents in the input devices. Early versions appeared to be quite difficult to stabilise at HF; the current product seems to be easier to handle but still a bit harder to stabilise than the 5532 or the LM4562. Possibly there has been a design tweak, or on the other hand my impression may be wholly mistaken. It has taken quite a long time for theAD797 to make its way into audio circuitry, possibly because of discouraging early reports on instability but more likely because of the high cost.At the time of writing (2010)AD797s are showing up in commercial audio equipment, a contemporary example being the Morrison E.L.A.D. line-level preamplifier, and they are also appearing in DIYdesigns.Audio Precision use this opamp in their 2700 series state-of-the-art distortion analysers.

Figure 16.25 shows theAD797 in shunt-feedback mode, to assess how it copes with output loading; as you can see, the effect is very small, with a very modest rise above 10 kHz with the 500 Ω load. Below 10 kHz the loading has no detectable effect at all, which makes it a better opamp than the 5532—see Figure 16.5. The step at 20 kHz is a measurement artefact. When the loads were applied, a 33 pF capacitor had to be put across the 2k2 feedback resistor to obtain stability—this was not required with either the 5532, the LM49990, or the LM4562, and shows that theAD797 does require a little more care to get dependable HF stability.