AN-1651| Application Note

Keeping Up with the Expanding Demands of High-Performance Audio

Keeping Up with the Expanding Demands of High-Performance Audio
Engineers designing audio equipment for the high-end audiophile or professional recording markets have a limited selection of high-resolution signal amplifiers that will meet end-user demands for both outstanding objective and critical subjective performance. Suitable devices must provide a combination of state-of-the-art specifications for low noise and ultra-low distortion along with high GBP and preferably unity gain stability. While nearly all suppliers develop and provide devices optimized for video, instrumentation, or low power, these applications demand a specification mix that does not necessarily align with the needs of the high-performance audio engineer. Accordingly, National Semiconductor has developed a series of high-performance, high-fidelity audio amplifiers specifically optimized for these applications. The LME49710 / LM4562 are representative of this line of devices and have been optimized for the high performance and professional community. They provide the designer with unprecedented lev
els of THD (0.00003%) and IM (0.00005%) plus very low noise (2.7 nV / Hz) and GBWP of 55 MHz with output current of 26 mA into a 600 load. Supply current is extremely low, serving the needs of both portable products as well as multimedia components utilizing multiple amplifiers. This document will provide several design examples of highperformance applications using the LME49710/ LM4562 that demonstrate their capability in meeting the demands of the highest audible performance. Note that the design philosophy and choices discussed herein will frequently refer to the practices embraced by the professional and high-performance community as having both objective (measurable) as well as subjective (audible) merit. The quantification of the contribution of these techniques has been published in many other documents and will not be included in this document. Included in this approach are capacitor dielectric choices, power supply impedance considerations, and circuit topology. Objectively, it is understood that b
est practices are required with device parameter consideration relative to circuit topology as well as grounding/shielding practices, power supply purity, and other well documented practices generally applied to sensor conditioning and instrumentation applications.

National Semiconductor Application Note 1651 Joe Curcio July 2007

stances. Although this would seem to indicate that moving magnet is preferred (at least from the standpoint of the preamp designer), it is commonly accepted that the moving coil configuration provides superior audible performance most likely due to the lower mass of the cantilever assembly. The key parameters of concern with RIAA equalized phono pre-amplifiers are low noise, low total harmonic distortion, low intermodulation distortion, and bandwidth. When interfacing with low output moving coil cartridges with source impedances of less than 100, the first amplifier should target an input noise voltage density of < 5 nV / Hz. Of course, THD and IMD should be a low as possible. Although not commonly emphasized, a high PSRR across the full signal bandwidth is critical to the audible performance of the circuit. While the more commonly discussed specifications of distortion, noise, bandwidth and slew rate are important to characterize the performance of the amplifier, if one envisions an audio amplifier as a dev
ice that modulates the power supply current into the load in direct response to the input signal, it is clear that the power supply rails must present a low and more importantly flat impedance across the audio bandwidth to preserve the audible spectral balance and overall integrity of the input signal. The PSRR of the LME49710/ LM4562 is outstanding across the full audio bandwidth and therefore is highly immune to power supply impedance discontinuities. However, given the low level signals provided from moving coil phono cartridges, the effect is more pronounced and therefore the impedance of the supply rails is addressed in this application. In order to avoid the distortion contribution of coupling capacitors, the two circuit topologies presented will use direct coupling from input to output. Overall DC gain will be 74 dB (44 + 30) for the fully passive EQ design and 70 dB (44 + 26) for the active-passive EQ design. Typical VOS of the LME49710 is 0.05 mV while Ibias is 7 nA. With the aforementioned DC gain,
voltage offset at the output will typically be less than 1 volt. To compensate and provide for full DC coupling, servo offset amplifier U3 has been included. Although we will not be discussing a fully active RIAA design, when this topology is used, the demands on slew rate and unity gain stability will intensify.

RIAA Phono Pre-Amplifier
Phono amplifiers are utilized to amplify and equalize to the RIAA standard signal from magnetic phono cartridges. Although phono cartridge technology can be based on several forms of electrical generation, but in rare cases the highest quality are electro-magnetic using either a fixed coil (moving magnet) or fixed magnet (moving coil) designs. The moving magnet designs provide higher output voltages generally in the region of 1.0 mV for each cm/s tip of recorded velocity while moving coils deliver 0.1 mV under the same circum-

RIAA Equalization
Table 1 shows the RIAA equalization response while Figure 1 displays the same graphically. The response is normally defined relative to 1 kHz with approximately 20 dB rise and attenuation relative to this frequency. The total dynamic range is actually greater than 40 dB as the high frequency rolloff continues beyond the second high-frequency inflection point (2.122 kHz) beyond audibility.

AN-1651

2007 National Semiconductor Corporation

300243

www.national.com