You may notice that MIYO feels warm, and this is totally normal. MIYO is a high-performance audio device, and generates a lot of heat with normal operation. The following explains where the heat is coming from, and how we designed MIYO to deal with it.
Perfect Sound is the Highest Priority
We knew that the biggest problem with these kind of devices was driving headphones. There are so many different kinds of headphones, with different impedances and sensitivity. We all had headphones that would sound great with the proper drive, but terrible with other devices – usually computer's headphone jacks, portable players, and other consumer electronic devices. And given our professional audio background, we were targeting the finest professional studio reference headphones available.
So we set out to find out what we had to do to solve that problem, and make sure MIYO sounded amazing with anything. We researched headphones. We researched amplifiers. And when we listened to the TPA6120 headphone amplifier IC, we were blown away – we knew we had to use this device. We also knew we would have one for each of the headphone outputs, so each headphone output could be driven optimally.
This amazing sounding chip gets warm. Even if it’s just sitting there doing nothing. Texas Instruments doesn’t give much detail on the design beyond it being a current feedback amplifier, but one of the ways to get the distortion of a class AB amplifier so low, as this one is, is to bias it hot, so transistors in the topology are always conducting, to eliminate crossover distortion. As if forcing the amplifier into a class-A operation for small signals – that would explain why it’s so hot at rest.
(infrared image of TPA6120 eval board)
Power = Heat
The next question was: can we power this from the USB bus alone? The TPA6120 needs pretty high voltage rails in order to deliver power into high impedance headphones (at least +/-12V, or 24V total), but USB is only 5V. We ended up using a DC-DC converter to give us the high voltage rails we wanted (30 volts total). That circuit is 85% efficient, which means it too is producing some heat.
(Bottom - TPA6120 x 2. Top - XMOS. Below XMOS is the PCM4220.)
(Bottom right - DC-DC converter)
Bringing it to the Surface
Confident that this design would deliver the best possible sound, we went ahead and built it. And as you might guess, all those components crammed together generates lots of heat. The XMOS USB controller generates heat. The PCM4220 generates heat (normally used in mastering A/D converters…again, audio quality comes first). Pretty much every part that is critical to reproducing audio in MIYO generates heat, but once we heard how good this design sounded there was no going back.
With computers, excessive heat generated can cause a failure if the processors get too hot. To avoid this, computer manufacturers use what is called a ‘heat sink’, where a block of heat-conductive metal makes contact with the chip directly to draw heat out of it. Usually, a fan blows across the heat sink to move the hot air out of the computer's case. Rather than use a big bulky case and fans, we decided to design MIYO’s enclosure to be a heat sink itself.
Aluminum is a pretty good heat conductor, and it turns out anodizing aluminum increases the surface area, allowing it to dissipate much more heat. Anodized aluminum also happens to be very durable and looks great. So, we precisely mill the inside of MIYO’s enclosure to make direct contact with the hottest chips on the board, followed by hard anodization. The enclosure is then able to draw heat to the surface of MIYO and into the ambient air.
This design allows MIYO function properly while staying ultra small and portable. Sure, MIYO still gets warm – but once you hear it, you’ll know it’s worth it.