The LM4562 Opamp

486  Opamps for Active Crossovers

Figure 16.17: The LM4562 in series-feedback mode, with 1 kΩ, 2k2 feedback resistors giving a gain of 3.2x. No load (NL) and 500 Ω load. 10 Vrms output. ±18 V supply rails.

amount of extra THD, reaching 0.0007% at 20 kHz. Compare this with the 5532 performance in Figure 16.7, where loading brings up the distortion in the flat region below 10 kHz to 0.0008% and gives a THD of 0.0020% at 20 kHz with a 500 Ω load, as opposed to only 0.0007% for the LM4562.

Figure 16.17 shows the LM4562 working at a gain of 3.2x in series-feedback mode, both modes having a noise gain of 3.2 times. The extra distortion from the 500 Ω loading is very low.

Common-Mode Distortion in the LM4562

For Figures 16.16 and 16.17 the feedback resistances were 2k2 and 1 kΩ, so the minimum source resistance presented to the inverting input is 688 Ω. In Figure 16.18 extra source resistances were then put in series with the input path (as was done with the 5532 in the section on common-mode distortion), and this revealed a remarkable property of the LM4562—with moderate levels of CM voltage it is much more resistant to common-mode distortion than the 5532.At 10 Vrms and 10 kHz, with a 10 kΩ source resistance, the 5532 generates 0.0014% THD (see Figure 16.9), 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 minimise the effects of common-mode voltages. Note that only the rising curves to the right represent actual distortion. The

Opamps for Active Crossovers  487

Figure 16.18: The LM4562 in series-feedback mode, gain 3.2x, with varying extra source resistance in the input path. The extra distortion is much lower than for the 5532.

10 Vrms out, ±18 V supply rails.

raised levels of the horizontal traces at the LF end are due to Johnson noise from the added series resistances, plus opamp current noise flowing in them.

As we saw with the 5532, the voltage-follower configuration is the most demanding test for CM distortion, because the CM voltage is at a maximum. Now the LM4562 does not work quite so well. At 10 kHz the distortion with a 10 kΩ source resistance has leapt up from 0.00046% in Figure 16.18 to 0.0037% in Figure 16.19, which allowing for the noise component in the former must be an increase of some ten times. This is a surprising and unwelcome result, because it means that despite its much greater cost the performance of the LM4562 in this configuration is no better than that of the 5532; compare Figure 16.10.

The reason for this rapid increase in distortion with CM voltage appear to be a non-linearity mechanism that is activated quite suddenly when the CM voltage exceeds 4 Vrms. This is illustrated in Figure 16.20, which shows distortion against level for the voltage-follower at 10 kHz, with different source resistances. The left side of the plot shows only noise decreasing relatively as the test level increases; the steps in the lowest trace are measurement artefacts. Since the CM voltage for the seriesfeedback configuration is only about 3 Vrms, the non-linearity mechanism is not activated, and CM distortion in that case is very low.

488  Opamps for Active Crossovers

Figure 16.19: The LM4562 in voltage-follower mode, with varying extra source resistance in the input path. CM distortion is much higher than for the series-feedback amplifier in Figure 16.16.

10 Vrms out, ±18 V supply rails.

The conclusion has to be once more that if you are using voltage-followers and want low distortion, it is well worthwhile to expend some time and trouble on getting the source impedances as low as possible.

It has taken an unbelievably long time—nearly 30 years—for a better audio opamp than the 5532 to come along, but at last it has happened. The LM4562 is superior in just about every parameter, apart from having more than twice as much current noise and no better CM distortion in voltagefollower use. These issues should not be serious problems if the low-impedance design philosophy is followed.At present the LM4562 has a much higher price; hopefully that will change, but it may take a very long time, based on the price history of other opamps. It has already begun to appear in hi-fi equipment, one example being the Benchmark DAC1 HDR, a combined DAC and preamplifier.

There is, however, or at any rate appears to be, a slight problem with using the LM4562 as a voltagefollower. On several occasions I have built such a voltage-follower, supplied it with a low source impedance such as 40 Ω, and attempted to measure the noise performance. This has been rendered impossible by the stage picking up and demodulating radio stations. In the same circuit position a 5532 works fine. I assume some internal oscillation is causing heterodyning; no oscillation is visible on the output.Acure I found helpful is connecting the inverting input to the opamp output with a 100 Ω