Op amps in small-signal audio design - Part 3: Selecting the right 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.

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.