24-bit DAC + D-class amplifier in singlechip - good solution for pocket MP3 player?

Question

I am doing personal project - portable MP3 player. Goals are sound quality + power efficiency. Price is not limited to a reasonable amount (100-200$ in parts is ok)

Will single-chip DAC+D-class amplifiers give me best possible power efficiency while having excellent audio quality? I've seen 24-bit ones but a little overpowered (like 1.5W per channel). This looks very juicy - No need to design analog part, all sensetive parts are inside the chip. Are there any pitfalls with this approach or with D-class amplifiers with headphones in general?

I afraid that classic op-amp based or AB-class amplifiers will eat too much power + I will need careful design of analog part after DAC.

For example: http://www.cirrus.com/en/products/cs43l22.html Separate output for headphones scares me.

Design targets:

1. 32-50 Ohm headphones support @100mW
2. LiIon power supply, the more play hours I get the better :-)
3. Maximum quality (as long as I can hear the difference)
4. I have 24-bit decoder output, so going down to 16 is possible to not that fun (extra analog filtering and noise shaping).

Answer 1

Your limiting factor is probably voltage swing.

If your driving higher end headphones your probably looking at a 250->300 ohm load, 300ohm for higher end sennheiser cans for instance. But some phones (mostly professional models) can go as high as 600 ohms.

At 300ohms:

For 100mW you need 5.47 Vrms = 15.48 V peak-to-peak ( 18mA RMS)

For 200mW you need 7.75 Vrms = 22 V peak-to-peak ( 25mA RMS )

You probably won't find an integrated part that is specifically designed for portable devices that can achieve those levels of voltage swing. Your best bet is probably an audio op-amp with a high drive current (TI makes several, as does analog devices) or a purpose built headphone driver, for instance the TPA6120A2 would fit your needs.

Your challenge is then to build an efficient power supply to generate the +-12-15V rails needed.

Not sure what your plan it for batteries but one way to make it easier and more efficient is to use two batteries, maybe two 9V batteries, center tap for ground giving you +-9V rails to start with. You end up with bigger/more batteries but you don't lose any power boost converting up to your + rail and then using a charge pump or similar to get your negative rail.

EDIT: I'd target 100mW MAX...200mW would explode your head with most phones. expect 97-102db sensitivity at 1mW for most higher end cans (could be lower for pro models). meaning at 127mW you'd be looking at SPL in roughly the 115dB to 120db range which is more than enough to cause hearing loss if listening for extended periods. Targeting ~63mW would put you at 112db -> 117dB which eases your voltage swing constraints and can still cause plenty of ear damage.

Answer 2

I suggest you use one of the published high quality op-amp headphone driver circuits. The analog design of a class D amplifier is not straightforward. Class-D amps have approximately 0 dB of PSRR.

Have you figured out your power budget? I think you will find that the power savings of Class D is minimal over an AB opamp output stage. You might save more power by using a more appropriate DAC (one with no more bits than your source material.)

Also, a DC/DC converter will waste some power. With appropriate component choices, I think you should be able to power your circuit directly off a LiIon cell.