The LM4562 dual high performance audio op amp is featured in the preamp module for the Class-A amplifier, elsewhere in this issue. It has ultra-low distortion, low noise, high slew rate and high gain-bandwidth product.

In fact, its total harmonic distortion + noise (THD+N) is so low you cannot measure it directly with current equipment such as the Audio Precision Test Set!

Even without its super-low distortion figures, the LM4562 is an impressive op amp. Its typical input noise density is 2.7nV/ Hz while slew rate is quoted at ±20V/μs.

Gain-bandwidth product is 55MHz and open-loop gain is 140dB for a 600Ω load. In addition, it has excellent figures of 120dB for PSRR (power supply rejection ratio) and CMRR (common mode rejection ratio).

But it is the harmonic distortion performance that made us pick it as the only suitable op amp to be teamed with the new 20W Class-A amplifier current being featured in the magazine. We wanted an op amp that would not degrade the amplifier’s performance in any way.

The LM4562 turned out to be the right choice but we have to admit that the preamplifier module featured in this month’s issue was our third attempt to actually extract that fine performance. As in any low distortion design, PC board layout is critical.

Having obtained a great performance figure, typically less than .0005% from the module, we had two problems:

One, that distortion is about the same as the residual distortion of the Audio Precision test set and

Two, the THD+N of the op amp is more than an order of magnitude (ie, ten times) better again, at 0.00003%.

So how does National Semiconductor manage to quote a value that cannot be measured?

Well, there is always a way! Although the THD+N cannot be measured directly, a simple set-up enables the ultra-low distortion to be measured indirectly. The circuit is as in Fig.1 and is taken directly from the National Semiconductor data sheet.

The op amp is connected as a unity-gain buffer but with a low resistance R1 (around 10Ω) placed between the inverting and non-inverting inputs and a higher resistance R2 placed in the feedback loop.

The result is a closed loop gain of 1 but a noise gain of G=1+(R2/R1). This is because R1 and R2 effectively form a voltage divider.

This means that the error (ie, harmonic distortion) and noise signal is amplified by this factor and this allows the measurement to be made on currently available equipment!

Another way of looking at it is to regard the op amp as having its open loop gain reduced to a figure of 1 + (R2/R1) and this means that much less feedback is available to reduce the circuit non-linearity.

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