This week’s EYE ON NPI (video) is in a class of its own, it’s Richtek’s RT9120S Series Class-D Audio Amplifier with DRC Control, a powerful amplifier with a wide operating voltage range, high efficiency, high wattage and many supported input formats.

We love I2S audio amplifiers here, they’re a huge step up from PWM + analog amplifiers in that you will get much higher audio quality due to having true analog output and the DAC being tuned for 16, 24 or 32-bit audio. Most simple PWM is maybe 8 or 10 bits! As long as your microcontroller or microcomputer has I2S output support, you can use 3 or 4 pins to generate a bitstream of audio.

For example, our Stereo Bonnet adds two single-channel I2S amplifiers to the Raspberry Pi so that it can drive 3 Watt speakers.

But what if you need more MOAR power? This is where this I2S amp will shine: it can handle up to 30W per channel, stereo, and best of all it does not require an MCLK input signal so it will work great on Raspberry Pi computers of any size – although of course it will work on other microcontrollers and microcomputers as well. Let’s take a look and some of the features we like in this chip.

First thing to note is that there are a few options in the RT9120 family: the RT9120 is the original, the RT9120S is an upgrade – it can now support 30W instead of 20W thanks to an RDS(on) as low as 90 mΩ and has 94% efficiency over 92%. It does seem to be pin compatible, so if you’re using the non-S version this ought to be pin-compatible for an upgrade.

This amp is Class D, which means if you’re used to class AB amps that require massive heat-sinking, you’ll be please to know you can likely get away without requiring a heat-sink – although you’ll probably want a big back ground plane and 2 oz copper to help dissipate via the thermal-package ground pad.

The trade off is you will need some passive components on the output of the amplifier, because it generates a high frequency – up to 1.5MHz – PWM signal that needs to be filtered down to 10-20KHz. It looks like this amp can use simple ferrite bead + capacitor outputs, but for the best EMI filtering and sound, a 10uH + 0.47uF capacitor is needed for each ‘leg’ of output.

As this is a powerful amp, and since people will want to play it loud, and you’ll want to avoid nasty clipping, the RT9120 also features Dynamic Range Compression which will slowly turn down the gain as the signal gets louder, so that you don’t get square wave distortion.

Lastly, you will need to control the chip over I2C, it isn’t free-running. However, this means you can enable filters, change the Class D frequency, adjust the gain digitally instead of reducing the signal depth, and get an error output report to know if there’s a short or open on the speakers, or missing I2S signal lines.

However, if you’re using a Linux-based system there’s already a Richtek-authored kernel driver you can use.

Sounds good? I think so! And the fact that the Richtek RT9120S Series Class-D Audio Amplifier with DRC Control is in stock right now at DigiKey for immediate shipment at a very nice price is music to my ears.

You can also pick up an eval board for a very reasonable price which will get you started quickly, particularly if you need to implement an I2C driver. Order today and DigiKey will ship your order instantly, you will be able to start adding booming quality sound to your design by tomorrow afternoon.