The NA1150 audio amplifier sets a new standard, driving speakers with PWMCarrying on the legacy of the NJM4558

In October 2024, Nisshinbo Micro Devices Inc. began mass-producing a new audio amplifier, the NA1150. This product, the successor to the company’s iconic NJM4558 operational amplifier, is designed for any application that uses a microcontroller to produce sound.

The just-released NA1150 audio amplifier is safe, small in size with a simple design, and suitable for all electronic devices that produce sounds from home appliances to industrial equipment and automobiles. As the direct successor to the NJM4558, an iconic audio operational amplifier (op-amp) that has been in wide use since the 1970s, this model is expected to become the new standard in audio amplifiers.

The NJM4558 was a two-circuit op-amp released in 1977 by New Japan Radio Co., Ltd., one of the predecessors to Nisshinbo Micro Devices. Up until the release of the NJM4558, New Japan Radio had been importing and selling the RC4558 op-amp from Raytheon, in which the company had a stake at the time (New Japan Radio was established in 1961 as a joint venture between Japan Radio and Raytheon). Having built a track record as a semiconductor manufacturer, New Japan Radio decided to make its own high-quality op-amps for the Japanese market. The NJM4558 was developed on the basis of the RC4558 op-amp, an obscure device at the time, as a device that combined top sound quality with overall high quality.

The NJM4558, which added safety, ease of use, and malfunction-free functionality to the RC4558, met with a great public response immediately after its release—1 billion units, including series components, were shipped in 1980. Widely adopted both in the Japanese market and by the rest of the world, the model gained a reputation as the gold standard for audio op-amps. Even today, nearly 50 years after its release, it remains in strong demand, especially with those who place great value on sound quality in audio equipment.

Starting with the NJM4558, New Japan Radio successively developed a series of products for the audio market. In addition to audio amplifiers, such as speaker amplifiers and microphone amplifiers, the company developed a wide lineup of audio signal processing integrated circuits (ICs), such as volume ICs and selectors, solidifying its position as a manufacturer of analog semiconductors with strong presence in the audio field.

Nisshinbo Micro Devices’ main audio IC products. Source: Nisshinbo Micro Devices

Even today, after integrating with RICOH Electronic Devices Co., Ltd. in 2022 to become Nisshinbo Micro Devices, audio ICs remain one of the core products of the company, which is vigorously investing in the development of new products that bring new value to the audio market.

In the field of audio amplifiers in particular, the company developed pop noise reduction technology to reduce the annoying popping noises generated when audio equipment is turned on and off, as well as the technology to suppress the effect of electromagnetic noise from mobile phones and other devices. This earned Nisshinbo Micro Devices high praise for introducing products that meet the needs of the audio market in a timely fashion.

The NA1150, which began to be mass-produced in October 2024, is one of these products. Developed as a new speaker amplifier, the model has the potential to become an iconic product, ideal for any device that produces sound.

The most significant feature of the NA1150 is its ability to directly drive speakers with pulse-width modulated audio signals from microcontrollers and other devices. Until now, in a typical circuit, pulse-width modulated audio signals would be converted to analog signals by a digital-to-analog converter (DAC), then processed by a low-pass filter (LPF) and electronic volume regulator (EVR) before being amplified by an audio amplifier and used to drive the speaker. In more recent times a single IC might sometimes be used to process the signal before the op-amp, but in any case, the PWM signal still needs to be processed before being input into the audio amplifier that drives the speakers.

Diagram of PWM signal-driven speakers. Source: Nisshinbo Micro Devices

Given these circumstances, the increase in output accuracy of PWM signals as a result of high-performance processors like microcontrollers encouraged some companies to try to directly drive speakers with PWM signals. CRI Middleware, an embedded software vendor, is one such company. The CRI D-Amp Driver software, which has been developed by CRI Middleware, can output a PWM signal that reproduces high-quality audio using a hardware configuration that drives speakers with only an H-bridge circuit constructed of four discrete transistors. This solution produces high-quality audio with a simple configuration consisting of a microcontroller, four discrete components, and speakers, and this configuration has been adopted for home appliances, in-vehicle devices, and audio equipment. However, the four discrete components alone lack the features of speaker disconnection detection or overcurrent protection, so separate circuits must be added. Nisshinbo Micro Devices, therefore, joined forces with CRI Middleware to develop a dedicated IC to drive the speaker directly with a PWM signal instead of an H-bridge circuit. The resulting product was the NA1150.

Wataru Fukase, SP Marketing Section 1, Product Marketing Department, Sales & Marketing Division

The NA1150 is equipped with a load diagnostic feature that can detect speaker disconnection or short circuits and notify the microcontroller. “It is equipped with protective functions that are standard in audio amplifiers, such as overcurrent detection and thermal shutdown, as well as dead-time control circuits that are difficult to design using discrete components. It is now possible to easily achieve direct PWM signal powering, which can significantly reduce the size and number of components while ensuring the same level of safety and security as conventional circuits using DACs and LPFs,” says Wataru Fukase of Nisshinbo Micro Devices’ SP Marketing Section 1, Product Marketing Department, Sales & Marketing Division.

Both differential PWM input and single-ended PWM input are supported as input signals. In addition, the NA1150 can be used with more inexpensive hardware configurations. The NA1150 comes in two sizes, 2.3 mm (leadless DFN package product) and 4.0 x 2.9 mm (leaded VSP package product). “The only external component is a bypass capacitor for the power supply. To send an abnormality notification signal to the microcontroller, all that is required is to add a pull-up resistor. We think that it should be possible to make the circuit about one-tenth the size of a conventional circuit using a DAC and LPF, and about one-third the size of a discrete circuit,” says Fukase.

Main features of the NA1150. Source: Nisshinbo Micro Devices

Sound quality is high even with a simple circuit configuration, and in addition to sound effects voices, it is possible to output clear bell-like voice sounds similar to message readings. “Since speakers are driven directly by PWM signals, everything is transmitted digitally, so sound is lossless and high-quality. Differential PWM input is resistant to noise, which makes it easy to achieve high sound quality, and is an audio output method that also saves power,” adds Fukase.

The operating voltage range is between 2.6 to 5.5 V, and the output voltage at 5 V input is 1.5 W (load = 8 Ω, total harmonic distortion = 10%). Current consumption in standby mode is kept below 1 μA. The operating temperature range is between -40ºC and +125ºC. Mass production of automotive-grade products that comply with the AEC-Q100 quality standard for automotive semiconductors began in October 2024.

Takahiro Ishida, Sales Promotion Section of the Sales & Marketing Management Department of the Sales & Marketing Division

“It has already been considered for many different uses, including in-vehicle devices like automobile meter clusters and ETC systems. We have also received inquiries for a wide range of applications, including home appliances such as air conditioners, rice cookers, water heaters, fire alarms, and vending machines, and we have seen very positive responses. It is broadly bought as an audio amplifier for any device that uses a microcontroller to produce sound,” says Takahiro Ishida of Nisshinbo Micro Devices’ Sales Promotion Section of the Sales & Marketing Management Department of the Sales & Marketing Division.

Nisshinbo Micro Devices currently plans to develop multiple products based on the NA1150. Development of the NA1150B, which has an expanded operating voltage range of between 1.8 to 5.5 V for devices that run on AA batteries, is almost complete. “If there is demand, we would like to start offering it immediately,” adds Ishida. The NA1180 is under development as a unit that supports piezoelectric speakers (piezo sounders), which are cheaper than dynamic speakers. To ensure sufficient volume, piezoelectric speakers, need to be driven at a higher voltage; therefore, the company plans to address this by adding a boost power supply circuit to the NA1150.

Overview of the NA1150 series products under development. Source: Nisshinbo Micro Devices

“Support for battery-operated devices and piezoelectric speakers makes direct PWM powering possible with a resulting reduction in size, costs, and design labor for any application where sound is generated by a microcontroller. I am confident it will become a standard product for new speaker op-amps,” says Ishida. The NA1150, which has inherited from the NJM4558 features that make it safe, easy to use, and malfunction-free is going to get a lot of attention in the future.