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New Japan Radio Co. Ltd. (JRC) developed an operational amplifier IC called MUSES for use in high end audio equipment.

The company will start shipping samples of the IC from March 2008. MUSES employs a lead frame made of Cu with a purity of 99.99% "for the first time anywhere in the world," the company said. In pursuit of higher sound quality, the company decided to ignore in the extra cost of using a higher purity Cu.

The IC is equipped with operational amplifier circuits for two channels, in view of the compatibility with the stereo system. The enhancement in heat and electric conductivities of the lead frame resulted in the superior sound quality, the company claimed.

Yoshitsugu Sugimoto, Manager at the Research Section in the IC Frontier Development Dept. of the JRC Standard IC Division, said, specifically, "Roughness in the mid- to high-frequency ranges decreased."

Although Cu is used in the existing operational amplifier ICs for audio equipment, the purity is about 99.7%. Comparing the 99.99% Cu material with the existing one, the heat and electric conductivities are increased from 360 W/mK to 391 W/mK and from 94 (%1 ACS) to 102 (%1 ACS), respectively.

The company demonstrated playing back music on an audio system equipped with a preamplifier incorporating the latest operational amplifier IC, which is placed between a CD player and a power amplifier. Compared with a system using an existing operational amplifier IC, the sound was clearer and less muffled, giving the impression of the more precise sound localization.

JRC will hold demonstration events using an audio system like this one from October 2007. The company plans to begin the production with a monthly output on the scale of 1,000 units within 2008. MUSES will be on display in JRC's booth at CEATEC JAPAN 2007 in Makuhari Messe from Oct. 2 to 6, 2007.

Concurrently with the CEATEC exhibition, the demonstration events of the audio system will be held from October 2 to 4 at Hotel Green Tower Makuhari located close to Makuhari Messe.

Two operational amplifier IC chips reduced crosstalk

JRC exhibited the prototype of MUSES at A&V Festa 2006 in September 2006. While a lead frame made of the 99.99% pure Cu was used also in this prototype, two major improvements are added to commercialize the product, the company said.

First, in order to reduce crosstalk, one operational amplifier IC chip is used for each channel and all the chips are enclosed in a single package. The prototype also had operational amplifier circuits for two channels, but these circuits were integrated on one chip. Therefore, the chip that appeared to be integrated actually had circuits independent from each other for the reduction of crosstalk between the channels. The company, however, provided each circuit on a separate chip to further reduce crosstalk. As a result, sounds coming from the left and right channels can be clearly separated from each other, the company said.

Another improvement was made to solve the problems resulting from the so-called "common impedance," such as the variation in the ground level depending on the current flowing in the circuit. Specifically, JRC used thicker wires on the chip and increased the number of power supply lines connecting between the lead frame and the chip to two.

These improvements resulted in an increase in the total size of the chip enclosed in a package from the existing one-point-several square millimeters to two-point-several square millimeters (total of the two chips). The size of the chip is increased to "three to four times larger," Sugimoto said.

Consequently, the larger the chip size, the higher the production cost. As in the case of the expensive high purity Cu material that was used, the company prioritized better sound quality over cost and decided to employ the two-chip design.

Improved processing technique for soft Cu enabled the commercialization

According to the company, an improvement added to the production method, in addition to those for enhancing the performance, enabled the commercialization of the latest operational amplifier IC. While 99.99% purity Cu has excellent heat and electric conductivity, its elongation reaches 45% (Cu with a purity of about 99.7% has a minimum elongation of 6%).

When the 99.99% pure Cu is machined to make the lead frame, "Cu clings to the processing machine," Sugimoto said. Furthermore, the upper temperature limit of the 99.99% pure Cu is 200°C (500°C for Cu with a purity of about 99.7%).

When a chip is wire bonded to a lead frame, the temperature at a contact between the wire and the lead frame generally exceeds 200°C. Consequently, the 99.99% pure Cu cannot be processed by a standard bonding machine.

Although the details were not revealed, the company accumulated expertise in processing the high purity Cu that entails such difficulty, thereby establishing the production system with the monthly output of 1,000 units. The company also manufactured the proprietary bonding machine that enables wire bonding at a temperature lower than 200°C.

Differences immeasurable by electrical characteristics

The latest operational amplifier IC operates at a supply voltage ranging from ±9 V to ±16 V. The noise level is 8 nV per root hertz. The standard and maximum input offset voltages are 0.8 mV and 5 mV, respectively. The standard voltage gain is 110 dB, and the standard slew rate is 11 V/μs. The amplifier IC enclosed in a DIP8 package employs the JFET input.

These electrical characteristics are almost unchanged from those of the prototype unveiled in 2006. The THD+N (total harmonic distortion plus noise) is about 0.0002%, which is not exactly numerically superior to that of other companies' operational amplifier ICs for audio equipment.

According to JRC, the difference in electrical characteristics does not directly result in the difference in sound quality so that better electrical characteristics do not always produce better sound. The company, however, did not specify the reason why the latest operational amplifier IC can enhance the sound quality.

As explained by JRC, it is empirically proved that higher purity Cu used in the electronic components of audio equipment leads to a higher sound quality. For example, higher purity Cu used in a speaker cable can produce a higher quality sound. JRC reportedly came up with the idea of employing high purity Cu for the lead frame based on this empirical rule.