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audio/videoZONE Products for the week of January 21, 2008
National Semiconductor Says…
Competition For BUF634 Audio Buffer: At Last
Low Distortion Has Current and Voltage Needs Satisfied
A new buffer from National Semiconductor Corporation delivers the industry’s lowest noise and harmonic distortion for professional audio applications. Part of National’s high-fidelity LME audio amplifier family, the LME49600 produces the necessary output current to drive multiple low-impedance headphones and the voltage swing to drive several high-impedance headphones. Additionally, the buffer is well-suited for a wide range of other applications, including line drivers, analog-to-digital converter (ADC) input drivers, low-noise and wide-frequency voltage regulators, and it can also drive the headphone amplifier output stage in mixer consoles as well as capacitive loads in low-power audio amplifiers.
The LME49600’s high-fidelity specifications ensure that it can respond to highly dynamic inputs, accurately reproducing these signals without degrading them by adding distortion. The device delivers 180 MHz bandwidth, a high slew rate of 2000 V/us and has 2.6 nV/rtHz input referred voltage noise density. In a closed-loop configuration with National’s LME49710 single operational amplifier (op amp), the total harmonic distortion plus noise (THD+N) is a vanishingly low 0.00003 percent. Combining the LME49600’s +/- 250 mA output current capacity with the LME49710 and an LM4040 low-noise bandgap reference produces a very high-performance, low-noise, wide-bandwidth, audiophile quality, voltage regulator.
EN-Genius Says…
Nobody who has worked in professional audio in the last ten years needs to be told what the LME49600 does. This is direct competition to the TI (Burr-Brown) BUF634, with the same pinout in the TO-263 version of the part (they also offer DIP-8 and, bizarrely, TO-220). National makes a big deal about “Industry’s lowest noise” in both the original news release headline and the first paragraph. It is not untrue, but it is a relatively small reduction and rather out of context for the operational conditions.
With rails up to ±18 V (±15 V is common in this market) plus a 250 mA output current capability the LME49600 can drive either multiple low-impedance headphones with the current needed, or multiple high-impedance headphones with the voltage swing required. The part also has the BW pin introduced in the TI part which, when connected to the negative rail, improves bandwidth at the cost of quiescent. There is far less difference in bandwidths in the LME49600 (100 MHz to 160 MHz with a 10 Ω load) than there was with the BUF634 (20 MHz to 160 MHz, also with a 10 Ω load) but there is a cost in quiescent with the National part in the lower bandwidth position taking 8 mA (cf 1.5 mA with the TI part). Most of the use of the parts is not in portable equipment, however.
In other specifications both parts offer the same slew rate at 2000 V/µs, same closeness to unity gain with different loads and the same offset against temperature characteristic of ±100 µV/ºC. The offset itself is lower, with the newer VIP process, at ±17 mV and the input voltage noise density at 10 kHz has improved from 4.0 nV/rtHz to 3.0 nV/rtHz in the narrower bandwidth setting and down to 2.6 nV/rtHz in the wider bandwidth condition.
Settling time for the National parts has worsened compared to the TI (where it was 50 ns in both bandwidth settings) to 200 ns in the narrow position, and 60 ns in the wide. That should not be important in audio applications. The supply rail range minimum is ±2.25 V.
While both products say they can push 250 mA, the short-circuit current of the LME49600 is considerably higher, at ±490 mA than the earlier ±350 mA – suggesting that the National part can, in fact, be pushed a little harder. One cause for potential concern, in some existing circuit designs that might replace the TI part, is that the input resistance (data sheet incorrectly says impedance) of the LME49600 is 7.5 MΩ in low bandwidth and 5.5 MΩ in the high bandwidth state. The BUF634 shows an input impedance (correctly used) of 80 MΩ in parallel with 18 pF.
The gem specifications are THD+N, which are typically 0.0000x%, and who really cares what number x is…
TI has not neglected the professional audio market. Their sample rate converters, digital audio controllers, and the like, are top notch. But the focus has tended to be on the high-volume consumer markets rather than the much more profitable professional and broadcast users. Until we develop digital microphones and delivery transducers like headphones we need analog audio products. The BUF634 is a neglected part: it should have been moved to a newer process maybe five years ago, and TI should have been listening to the grumblings out on the street about lack of support. Launching a new product fabricated in an up-to-date process, probably with a much higher yield, puts National in the catbird seat for quite a while. The blogging activity since the part preannouncement at AES 2007 indicates that a huge number of designs will move over to the LME49600 as soon as they can, National has cheekily priced the part at ten cents less than the published TI part (you have to love the new pricing attitude on Kifer Road). And with the declared distortion numbers from National (I don’t believe TI ever published any numbers), together with a way to measure them, the audience is going to be much more confident about what they are buying. The LME49600 will do extraordinarily well, but not because of the “Industry’s lowest noise.”
Design wins will be the usual suspects: headphone amplifiers; monitoring outputs; line drivers; other audio and in-loop op amp applications. With the higher basic bandwidth other applications in video and similar technologies might well appear.
The LME49600 is in production in TO-263-5 priced at $4.50 in 1000-piece lots.
Data Sheet
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