Technical & Editorial

AVR - Audio Video Receiver - Build Quality: Part II - Design of High-Performance AVRs and Pre/Pros


Operational Amplifier Selection

As mentioned in Part I, some SSI op-amps (1 – 4 in a package) must be used around the single chip LSI AVR chip for anti-alias filtering (ADC) and reconstruction filtering (DAC). Unless otherwise noted in the chart, these are minimal cost op-amps such as the LM833 and RC4558. They were among the first practical audio op-amps, dating to the 1970s. Many other op-amp part numbers are found in service manuals. These are slight improvements on the original op-amps, where the improvements arose from advances in semiconductor processing technology. The incremental part cost is several pennies.

For 50% more cost per op-amp, the NE5532 can be used. This is a much more complex chip, allowing it to have dramatically lower noise and distortion. Confusingly, the single op-amp version of this chip is given the part number NE5534

Some DAC vendors supply a Printed Circuit (PC) evaluation board for designers to verify the performance of the DACs. Achieving optimal performance is very dependent on associated passive and active parts as well as PC board layout. If the DAC producer has developed an evaluation board for a given part, a PDF on its design will be on the website of the IC Vender. These application documents often provide measurements of the complete board that are more comprehensive than the information provided in the DAC's datasheet for the DAC alone. The IC vendor's evaluation board documentation provides details on how to get the best performance from a specific DAC and is an excellent place to go for more technical detail than I can provide here. Not all DACs from an IC vendor have evaluation boards.

An evaluation board for the vendor's top of the line DACs will include the op-amps that were determined to yield the best performance. Often, lower end op-amps are used in evaluation boards for less expensive DACs in the vendor's line. This is more consistent with the op-amps that will be used in the final product using the less expensive DAC. Typically, the NE5532 is used for all the vendor's evaluation boards except for the top end products. AKM uses the minimum price New Japan Radio NJM 4580 on the evaluation board it sells for it entry level DACs, and the expensive TI LME49710 for AKMs newest top of the line part.

The high-performance data converter manufacturer ESS provides measurements not only for the best available operational amplifiers, but also lower priced parts.

ESS reports performance of the DAC with the following op-amp in descending performance order (page 2).

Analog Devices AD797

National Semiconductor (now a TI subsidiary) LME49710, LM4562

Texas Instruments NE5534

Rohm 4560

Similar guidelines can be found in Douglas Self's Small Signal Audio Design text, published in 2010 by Focal Press.

ESS only recommends use of the minimal cost Rohm 4560 op-amp for use with its lowest priced DAC the ES9006.

The ESS op-amp ladder has a steep cost curve. Better chips than the NE5532 may be had at a premium at about five times the cost. The Analog Devices AD797, for example, is ten times the price of a NE5332. I have never seen it in a multi-channel product.

AVRs with the LSI AVR normally deploy the lower-cost op-amps for the anti-alias and reconstruction filtering functions, though a couple use the NE5532. Pre/Pros built with SSI chips use the NE5332 or better performing op-amps exclusively. The note section in the chart in Part I identifies products with this op-amp.

Op-amps on the next rung of the ladder may be found in some SSI-based Pre/Pros, but only in segments of the signal path where the designer anticipates maximum benefit to performance. They are too expensive to be used exclusively in a multi-channel product. Other op-amps that are more expensive than the NE5532 may be utilized by some engineers, but these are not the units on the next rung of the ladder. Doug Self finds the op-amps between the rungs do not provide a significant advantage.

In some sections of an SSI-enabled Pre/Pro, a very high input impedance of the operational amplifier may be desirable. This requires an op-amp with FETs in the differential pair. No DC bias current can be present in an op-amp with a FET input. Selection of the op-amps for this application is best left to another article. All the operational amplifiers discussed above have bipolar inputs.

Power supply regulation is very important for optimal DAC performance. The ESS application note shows how different op-amp buffers for the analog power supply can make a difference to the THD and SNR of the parts. The application note points out the ESS analog output stage is very sensitive to the op-amp buffer. This is a result of how the output stage works in comparison to other DAC topologies. This is not an issue as long as the AVR or Pre/Pro designer uses the required parts and not low-cost substitutions. Yamaha reduces the output impedance in this stage by adding a transistor to the NE5332, and preceding it with an independent Ricoh R1172 voltage regulator.

The AKM evaluation board documentation sited above shows a complex local regulator which uses more than 25 parts. Such a complex local regulator would not be found in a Pre/Pro but perhaps an expensive two-channel product.

Evaluation boards for Analog Devices, Cirrus, and Wolfson use high performance single chip IC regulators. I don't give the part numbers or attempt to rank the single chip regulators in this report. Consistent with the use of minimal cost op-amps in AVRs, the voltage regulators will also often be of lower quality then what the DAC designers would like used for optimal performance.

If a cheap op-amp is used for current to voltage converter function when a current-mode DAC is specified, the performance inherent in the DAC will likely be lost. In my survey of AVR service manuals, I found companies that use a current mode DAC from TI or ADI have an NE5532 or higher-grade op-amp for the current-to-voltage converter. Some units then revisit op-amps of lesser quality for other sections of the circuitry. The current-to-voltage converter is discussed in more detail in the next section.

Unfortunately, ESS has no control of what the vendors actually use. Board photos of the new Pioneer AVR (SC-LX57), using the ESS ES 9016, show a bottom grade NJM 4570 op-amp used for the I/V and the balanced-to-single-ended converter.